fix(flatfile): harden flat-file backend against injection and traversal attacks

Security fixes for 7 vulnerabilities in database_flatfile.c:

1. Path traversal via crafted JID (HIGH):
   _ff_jid_to_dir() now strips '/', '\' -> '_' and collapses '..' -> '__',
   preventing a malicious federated JID from escaping the log directory.

2. Log injection via unescaped message body (HIGH):
   Add _ff_escape_message()/_ff_unescape_message() -- escape \n, \r, \\
   in message text on write, unescape on read. Prevents remote contacts
   from injecting fake log lines with forged sender/timestamp/encryption.

3. Metadata field injection (HIGH):
   Add _ff_escape_meta_value()/_ff_unescape_meta_value() -- escape |, ],
   \\, \n, \r in stanza_id/archive_id/replace_id. Parser uses escape-aware
   _ff_find_unescaped_char() and _ff_split_meta() instead of strchr/strsplit.

4. Sender ": " parsing confusion (MEDIUM):
   Escape ": " -> "\: " in XMPP resource on write. Parser uses
   _ff_find_unescaped_colonspace() + _ff_unescape_sender_resource().

5. LMC correction chain cycle -> infinite loop (MEDIUM):
   Add visited hash-set and FF_MAX_LMC_DEPTH (100) limit to chain walker.

6. Unbounded getline() -> OOM (MEDIUM):
   Add FF_MAX_LINE_LEN (10 MB) cap in _ff_readline(); overlength lines
   are skipped with a warning. Replaces all char buf[8192]/fgets() sites
   with dynamic getline() + truncated-line detection at EOF.

7. Symlink attack + TOCTOU on file creation (MEDIUM):
   Replace fopen("a") with open(O_WRONLY|O_APPEND|O_CREAT|O_EXCL|O_NOFOLLOW)
   + fdopen() -- atomic new-file detection, symlink rejection via O_NOFOLLOW.
   _ff_ensure_dir() checks g_lstat() for symlinks. File permissions 0600
   set via open() mode, not post-hoc g_chmod().

src/database.h

@@ -56,7 +56,8 @@ typedef enum {
 } integrity_level_t;
 
 // A single integrity issue found during verification
-typedef struct {
+typedef struct
+{
     integrity_level_t level;
     char* file;
     int line;
@@ -66,7 +67,8 @@ typedef struct {
 void integrity_issue_free(integrity_issue_t* issue);
 
 // Backend vtable: pluggable storage backends implement this interface
-typedef struct db_backend_t {
+typedef struct db_backend_t
+{
     const char* name;
     gboolean (*init)(ProfAccount* account);
     void (*close)(void);

src/database_flatfile.c

@@ -24,6 +24,8 @@
 #include "config.h"
 
 #include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
 #include <glib.h>
 #include <glib/gstdio.h>
 #include <stdio.h>
@@ -40,8 +42,10 @@
 #include "xmpp/xmpp.h"
 #include "xmpp/message.h"
 
-#define DIR_FLATLOG "flatlog"
+#define DIR_FLATLOG      "flatlog"
-#define FLATFILE_HEADER "# profanity chat log — UTF-8, LF line endings\n# vim: set fileencoding=utf-8 fileformat=unix :\n"
+#define FLATFILE_HEADER  "# profanity chat log — UTF-8, LF line endings\n# vim: set fileencoding=utf-8 fileformat=unix :\n"
+#define FF_MAX_LINE_LEN  (10 * 1024 * 1024) /* 10 MB — reject lines longer than this */
+#define FF_MAX_LMC_DEPTH 100                /* max correction chain depth */
 
 // Account JID stored during init for path construction
 static char* g_flatfile_account_jid = NULL;
@@ -112,12 +116,48 @@ _ff_get_message_enc_type(const char* const encstr)
 
 // --- Path helpers ---
 
-// Replace '@' with '_at_' in JID for filesystem-safe directory names
+// Sanitise a JID for use as a directory name.
+// 1. Replace '@' with '_at_'
+// 2. Reject / strip path-separator and traversal characters: '/', '\0', '..'
+// This prevents a malicious federated JID like "../../../tmp/pwned" from
+// escaping the log directory tree.
 static char*
 _ff_jid_to_dir(const char* jid)
 {
-    if (!jid) return NULL;
+    if (!jid || jid[0] == '\0')
-    return str_replace(jid, "@", "_at_");
+        return NULL;
+
+    // Reject embedded null bytes (strlen already stops, but be explicit)
+    // Replace '@' first
+    char* step1 = str_replace(jid, "@", "_at_");
+    if (!step1)
+        return NULL;
+
+    // Replace '/' and '\\' with '_' to prevent path traversal
+    GString* out = g_string_sized_new(strlen(step1));
+    for (const char* p = step1; *p; p++) {
+        if (*p == '/' || *p == '\\') {
+            g_string_append_c(out, '_');
+        } else {
+            g_string_append_c(out, *p);
+        }
+    }
+    free(step1);
+
+    // Collapse any remaining ".." sequences to "__" (belt-and-suspenders)
+    char* result = g_string_free(out, FALSE);
+    char* dotdot;
+    while ((dotdot = strstr(result, "..")) != NULL) {
+        dotdot[0] = '_';
+        dotdot[1] = '_';
+    }
+
+    // Reject empty result
+    if (result[0] == '\0') {
+        g_free(result);
+        return NULL;
+    }
+    return result;
 }
 
 // Get the base directory for a contact's logs:
@@ -125,7 +165,8 @@ _ff_jid_to_dir(const char* jid)
 static char*
 _ff_get_contact_dir(const char* contact_barejid)
 {
-    if (!g_flatfile_account_jid || !contact_barejid) return NULL;
+    if (!g_flatfile_account_jid || !contact_barejid)
+        return NULL;
 
     auto_gchar gchar* data_path = files_get_data_path(DIR_FLATLOG);
     auto_gchar gchar* my_dir = _ff_jid_to_dir(g_flatfile_account_jid);
@@ -141,18 +182,26 @@ static char*
 _ff_get_log_path(const char* contact_barejid, GDateTime* dt)
 {
     auto_gchar gchar* contact_dir = _ff_get_contact_dir(contact_barejid);
-    if (!contact_dir) return NULL;
+    if (!contact_dir)
+        return NULL;
 
     auto_gchar gchar* date_str = g_date_time_format(dt, "%Y_%m_%d");
     char* result = g_strdup_printf("%s/%s.log", contact_dir, date_str);
     return result;
 }
 
-// Ensure the directory exists, create if needed
+// Ensure the directory exists, create if needed.
+// Refuses to follow symlinks at the final component.
 static gboolean
 _ff_ensure_dir(const char* path)
 {
     if (g_file_test(path, G_FILE_TEST_IS_DIR)) {
+        // Verify it's not a symlink
+        struct stat st;
+        if (g_lstat(path, &st) == 0 && S_ISLNK(st.st_mode)) {
+            log_error("flatfile: directory path is a symlink, refusing: %s", path);
+            return FALSE;
+        }
         return TRUE;
     }
     if (g_mkdir_with_parents(path, S_IRWXU) != 0) {
@@ -162,11 +211,192 @@ _ff_ensure_dir(const char* path)
     return TRUE;
 }
 
+// --- Escape / unescape helpers ---
+//
+// Message text and metadata values from remote peers can contain arbitrary
+// characters including newlines, pipes and brackets. Without escaping, a
+// crafted message could inject fake log lines (log injection / format
+// injection). We escape on write and unescape on read.
+
+// Escape message body: \ -> \\, \n -> \n literal, \r -> \r literal
+static char*
+_ff_escape_message(const char* text)
+{
+    if (!text)
+        return g_strdup("");
+    GString* out = g_string_sized_new(strlen(text));
+    for (const char* p = text; *p; p++) {
+        switch (*p) {
+        case '\\':
+            g_string_append(out, "\\\\");
+            break;
+        case '\n':
+            g_string_append(out, "\\n");
+            break;
+        case '\r':
+            g_string_append(out, "\\r");
+            break;
+        default:
+            g_string_append_c(out, *p);
+            break;
+        }
+    }
+    return g_string_free(out, FALSE);
+}
+
+// Unescape message body: \\  -> \, \n -> newline, \r -> CR
+static char*
+_ff_unescape_message(const char* text)
+{
+    if (!text)
+        return g_strdup("");
+    GString* out = g_string_sized_new(strlen(text));
+    for (const char* p = text; *p; p++) {
+        if (*p == '\\' && *(p + 1)) {
+            p++;
+            switch (*p) {
+            case '\\':
+                g_string_append_c(out, '\\');
+                break;
+            case 'n':
+                g_string_append_c(out, '\n');
+                break;
+            case 'r':
+                g_string_append_c(out, '\r');
+                break;
+            default:
+                g_string_append_c(out, '\\');
+                g_string_append_c(out, *p);
+                break;
+            }
+        } else {
+            g_string_append_c(out, *p);
+        }
+    }
+    return g_string_free(out, FALSE);
+}
+
+// Escape metadata value (stanza_id, archive_id, replace_id):
+// these come from remote servers and may contain |, ], \, newlines.
+static char*
+_ff_escape_meta_value(const char* val)
+{
+    if (!val || strlen(val) == 0)
+        return NULL;
+    GString* out = g_string_sized_new(strlen(val));
+    for (const char* p = val; *p; p++) {
+        switch (*p) {
+        case '|':
+            g_string_append(out, "\\|");
+            break;
+        case ']':
+            g_string_append(out, "\\]");
+            break;
+        case '\\':
+            g_string_append(out, "\\\\");
+            break;
+        case '\n':
+            g_string_append(out, "\\n");
+            break;
+        case '\r':
+            g_string_append(out, "\\r");
+            break;
+        default:
+            g_string_append_c(out, *p);
+            break;
+        }
+    }
+    return g_string_free(out, FALSE);
+}
+
+// Unescape metadata value
+static char*
+_ff_unescape_meta_value(const char* val)
+{
+    if (!val)
+        return NULL;
+    GString* out = g_string_sized_new(strlen(val));
+    for (const char* p = val; *p; p++) {
+        if (*p == '\\' && *(p + 1)) {
+            p++;
+            switch (*p) {
+            case '|':
+                g_string_append_c(out, '|');
+                break;
+            case ']':
+                g_string_append_c(out, ']');
+                break;
+            case '\\':
+                g_string_append_c(out, '\\');
+                break;
+            case 'n':
+                g_string_append_c(out, '\n');
+                break;
+            case 'r':
+                g_string_append_c(out, '\r');
+                break;
+            default:
+                g_string_append_c(out, '\\');
+                g_string_append_c(out, *p);
+                break;
+            }
+        } else {
+            g_string_append_c(out, *p);
+        }
+    }
+    return g_string_free(out, FALSE);
+}
+
+// --- Readline helper ---
+//
+// POSIX getline() for dynamic-length lines. Returns line without trailing
+// newline, or NULL on EOF. Sets *truncated=TRUE if the line had no trailing
+// newline at EOF (partial write detection).
+static char*
+_ff_readline(FILE* fp, gboolean* truncated)
+{
+    char* line = NULL;
+    size_t cap = 0;
+    ssize_t nread = getline(&line, &cap, fp);
+    if (nread == -1) {
+        free(line);
+        return NULL;
+    }
+    // Guard against pathological lines that could exhaust memory.
+    // getline() already allocated, so we free and skip if too long.
+    if (nread > FF_MAX_LINE_LEN) {
+        log_error("flatfile: line too long (%zd bytes), skipping", nread);
+        // Check newline termination before freeing
+        gboolean had_newline = (nread > 0 && line[nread - 1] == '\n');
+        free(line);
+        // Skip to next newline if the overlength line wasn't newline-terminated
+        if (!had_newline) {
+            int ch;
+            while ((ch = fgetc(fp)) != EOF && ch != '\n') {}
+        }
+        if (truncated)
+            *truncated = FALSE;
+        // Return empty string so caller's loop continues (parse will reject it)
+        return g_strdup("");
+    }
+    if (truncated)
+        *truncated = FALSE;
+    if (nread > 0 && line[nread - 1] == '\n') {
+        line[--nread] = '\0';
+    } else if (feof(fp)) {
+        // Line without trailing newline at EOF — likely a partial write
+        if (truncated)
+            *truncated = TRUE;
+    }
+    return line;
+}
+
 // --- Line format ---
 //
 // Format: {ISO8601} [{type}|{enc}|id:{stanza_id}|aid:{archive_id}|corrects:{replace_id}] {from_jid}/{resource}: {message}
 //
-// Metadata fields after type|enc are optional.
+// Message text is escaped: \\ -> \\\\, newline -> \\n, CR -> \\r
+// Metadata values are escaped: |, ], \\, newline, CR
 // Lines starting with '#' are comments.
 // Empty lines are skipped.
 
@@ -175,41 +405,162 @@ _ff_write_line(FILE* fp, const char* timestamp, const char* type, const char* en
                const char* stanza_id, const char* archive_id, const char* replace_id,
                const char* from_jid, const char* from_resource, const char* message_text)
 {
+    // Escape metadata values from remote peers
+    auto_gchar gchar* safe_sid = _ff_escape_meta_value(stanza_id);
+    auto_gchar gchar* safe_aid = _ff_escape_meta_value(archive_id);
+    auto_gchar gchar* safe_rid = _ff_escape_meta_value(replace_id);
+
     // Build metadata section: [type|enc|id:...|aid:...|corrects:...]
     GString* meta = g_string_new("[");
     g_string_append(meta, type ? type : "chat");
     g_string_append_c(meta, '|');
     g_string_append(meta, enc ? enc : "none");
-    if (stanza_id && strlen(stanza_id) > 0) {
+    if (safe_sid) {
-        g_string_append_printf(meta, "|id:%s", stanza_id);
+        g_string_append_printf(meta, "|id:%s", safe_sid);
     }
-    if (archive_id && strlen(archive_id) > 0) {
+    if (safe_aid) {
-        g_string_append_printf(meta, "|aid:%s", archive_id);
+        g_string_append_printf(meta, "|aid:%s", safe_aid);
     }
-    if (replace_id && strlen(replace_id) > 0) {
+    if (safe_rid) {
-        g_string_append_printf(meta, "|corrects:%s", replace_id);
+        g_string_append_printf(meta, "|corrects:%s", safe_rid);
     }
     g_string_append_c(meta, ']');
 
-    // Build sender
+    // Build sender — escape ": " in the resource part to prevent
+    // the parser from splitting at the wrong point.
+    // JID bare parts (user@domain) never contain ": " but XMPP resources
+    // can contain arbitrary UTF-8 including ": ".
     GString* sender = g_string_new(from_jid ? from_jid : "unknown");
     if (from_resource && strlen(from_resource) > 0) {
-        g_string_append_printf(sender, "/%s", from_resource);
+        // Escape backslash and colon-space in resource
+        GString* safe_res = g_string_sized_new(strlen(from_resource));
+        for (const char* p = from_resource; *p; p++) {
+            if (*p == '\\') {
+                g_string_append(safe_res, "\\\\");
+            } else if (*p == ':' && *(p + 1) == ' ') {
+                g_string_append(safe_res, "\\: ");
+                p++; // skip the space too
+            } else {
+                g_string_append_c(safe_res, *p);
+            }
+        }
+        g_string_append_printf(sender, "/%s", safe_res->str);
+        g_string_free(safe_res, TRUE);
     }
 
-    fprintf(fp, "%s %s %s: %s\n",
+    // Escape message body to prevent log injection
-            timestamp,
+    char* safe_msg = _ff_escape_message(message_text);
-            meta->str,
-            sender->str,
-            message_text ? message_text : "");
 
+    // Build complete line and write atomically with a single write()
+    GString* full_line = g_string_new(NULL);
+    g_string_printf(full_line, "%s %s %s: %s\n",
+                    timestamp, meta->str, sender->str, safe_msg);
+
+    size_t to_write = full_line->len;
+    ssize_t written = fwrite(full_line->str, 1, to_write, fp);
+    if (written != (ssize_t)to_write) {
+        log_error("flatfile: partial write (%zd/%zu)", written, to_write);
+    }
+
+    g_string_free(full_line, TRUE);
+    g_free(safe_msg);
     g_string_free(meta, TRUE);
     g_string_free(sender, TRUE);
 }
 
+// --- Line parser helpers ---
+
+// Find the first occurrence of 'ch' that is not preceded by an unescaped backslash.
+// Returns pointer to 'ch' in 'str', or NULL if not found.
+static const char*
+_ff_find_unescaped_char(const char* str, char ch)
+{
+    if (!str)
+        return NULL;
+    for (const char* p = str; *p; p++) {
+        if (*p == '\\' && *(p + 1)) {
+            p++; // skip escaped character
+            continue;
+        }
+        if (*p == ch)
+            return p;
+    }
+    return NULL;
+}
+
+// Split metadata content on unescaped '|'. Returns a NULL-terminated array.
+// Caller must g_strfreev() the result.
+static char**
+_ff_split_meta(const char* meta)
+{
+    GPtrArray* arr = g_ptr_array_new();
+    const char* start = meta;
+    for (const char* p = meta;; p++) {
+        if (*p == '\\' && *(p + 1)) {
+            p++; // skip escaped char
+            continue;
+        }
+        if (*p == '|' || *p == '\0') {
+            g_ptr_array_add(arr, g_strndup(start, p - start));
+            if (*p == '\0')
+                break;
+            start = p + 1;
+        }
+    }
+    g_ptr_array_add(arr, NULL);
+    return (char**)g_ptr_array_free(arr, FALSE);
+}
+
+// Find first unescaped ": " (colon-space) in a string.
+// Escaped form is "\: " — a backslash before the colon.
+static const char*
+_ff_find_unescaped_colonspace(const char* str)
+{
+    if (!str)
+        return NULL;
+    for (const char* p = str; *p; p++) {
+        if (*p == '\\' && *(p + 1)) {
+            p++; // skip escaped character
+            continue;
+        }
+        if (*p == ':' && *(p + 1) == ' ') {
+            return p;
+        }
+    }
+    return NULL;
+}
+
+// Unescape a sender resource: "\\" -> '\', "\: " -> ": "
+static char*
+_ff_unescape_sender_resource(const char* res)
+{
+    if (!res)
+        return NULL;
+    GString* out = g_string_sized_new(strlen(res));
+    for (const char* p = res; *p; p++) {
+        if (*p == '\\' && *(p + 1)) {
+            p++;
+            if (*p == '\\') {
+                g_string_append_c(out, '\\');
+            } else if (*p == ':' && *(p + 1) == ' ') {
+                g_string_append(out, ": ");
+                p++; // skip the space
+            } else {
+                // Unknown escape — preserve literally
+                g_string_append_c(out, '\\');
+                g_string_append_c(out, *p);
+            }
+        } else {
+            g_string_append_c(out, *p);
+        }
+    }
+    return g_string_free(out, FALSE);
+}
+
 // --- Line parser ---
 
-typedef struct {
+typedef struct
+{
     char* timestamp_str;
     GDateTime* timestamp;
     char* type;
@@ -225,9 +576,11 @@ typedef struct {
 static void
 _ff_parsed_line_free(flatfile_parsed_line_t* pl)
 {
-    if (!pl) return;
+    if (!pl)
+        return;
     g_free(pl->timestamp_str);
-    if (pl->timestamp) g_date_time_unref(pl->timestamp);
+    if (pl->timestamp)
+        g_date_time_unref(pl->timestamp);
     g_free(pl->type);
     g_free(pl->enc);
     g_free(pl->stanza_id);
@@ -272,7 +625,8 @@ _ff_parse_line(const char* line)
             g_free(work);
             work = converted;
         } else {
-            if (err) g_error_free(err);
+            if (err)
+                g_error_free(err);
             g_free(work);
             return NULL;
         }
@@ -290,12 +644,13 @@ _ff_parse_line(const char* line)
         result->timestamp_str = g_strndup(work, first_space - work);
 
         // Parse metadata section [...]
-        char* bracket_end = strchr(bracket_start, ']');
+        // Use escape-aware bracket finder to handle escaped ']' in values
+        const char* bracket_end = _ff_find_unescaped_char(bracket_start + 1, ']');
         if (bracket_end) {
             char* meta_content = g_strndup(bracket_start + 1, bracket_end - bracket_start - 1);
 
-            // Split by '|'
+            // Split by unescaped '|'
-            char** parts = g_strsplit(meta_content, "|", -1);
+            char** parts = _ff_split_meta(meta_content);
             if (parts) {
                 int i = 0;
                 for (; parts[i]; i++) {
@@ -304,11 +659,11 @@ _ff_parse_line(const char* line)
                     } else if (i == 1) {
                         result->enc = g_strdup(parts[i]);
                     } else if (g_str_has_prefix(parts[i], "id:")) {
-                        result->stanza_id = g_strdup(parts[i] + 3);
+                        result->stanza_id = _ff_unescape_meta_value(parts[i] + 3);
                     } else if (g_str_has_prefix(parts[i], "aid:")) {
-                        result->archive_id = g_strdup(parts[i] + 4);
+                        result->archive_id = _ff_unescape_meta_value(parts[i] + 4);
                     } else if (g_str_has_prefix(parts[i], "corrects:")) {
-                        result->replace_id = g_strdup(parts[i] + 9);
+                        result->replace_id = _ff_unescape_meta_value(parts[i] + 9);
                     }
                 }
                 g_strfreev(parts);
@@ -316,29 +671,31 @@ _ff_parse_line(const char* line)
             g_free(meta_content);
 
             // Parse sender: message after '] '
-            char* after_meta = bracket_end + 1;
+            const char* after_meta = bracket_end + 1;
-            if (*after_meta == ' ') after_meta++;
+            if (*after_meta == ' ')
+                after_meta++;
 
-            // Find first ': ' which separates sender from message
+            // Find first *unescaped* ': ' which separates sender from message.
-            char* colon = strstr(after_meta, ": ");
+            // Resources may contain escaped \: sequences.
+            const char* colon = _ff_find_unescaped_colonspace(after_meta);
             if (colon) {
-                char* sender = g_strndup(after_meta, colon - after_meta);
+                char* raw_sender = g_strndup(after_meta, colon - after_meta);
 
-                // Split sender into jid/resource
+                // Split sender into jid/resource, then unescape resource
-                char* slash = strchr(sender, '/');
+                char* slash = strchr(raw_sender, '/');
                 if (slash) {
-                    result->from_jid = g_strndup(sender, slash - sender);
+                    result->from_jid = g_strndup(raw_sender, slash - raw_sender);
-                    result->from_resource = g_strdup(slash + 1);
+                    result->from_resource = _ff_unescape_sender_resource(slash + 1);
                 } else {
-                    result->from_jid = g_strdup(sender);
+                    result->from_jid = g_strdup(raw_sender);
                 }
-                g_free(sender);
+                g_free(raw_sender);
 
-                result->message = g_strdup(colon + 2);
+                result->message = _ff_unescape_message(colon + 2);
             } else {
                 // No ': ' found, treat entire rest as message with unknown sender
                 result->from_jid = g_strdup("unknown");
-                result->message = g_strdup(after_meta);
+                result->message = _ff_unescape_message(after_meta);
             }
         } else {
             // No closing bracket — malformed metadata
@@ -370,10 +727,10 @@ _ff_parse_line(const char* line)
                 result->from_jid = g_strdup(sender);
             }
             g_free(sender);
-            result->message = g_strdup(colon + 2);
+            result->message = _ff_unescape_message(colon + 2);
         } else {
             result->from_jid = g_strdup("unknown");
-            result->message = g_strdup(rest);
+            result->message = _ff_unescape_message(rest);
         }
     } else {
         // No space at all — can't parse
@@ -385,15 +742,17 @@ _ff_parse_line(const char* line)
     // Parse timestamp
     result->timestamp = g_date_time_new_from_iso8601(result->timestamp_str, NULL);
     if (!result->timestamp) {
-        log_warning("flatfile: unparseable timestamp: %s", result->timestamp_str);
+        log_warning("flatfile: unparsable timestamp: %s", result->timestamp_str);
         _ff_parsed_line_free(result);
         g_free(work);
         return NULL;
     }
 
     // Default type/enc if missing
-    if (!result->type) result->type = g_strdup("chat");
+    if (!result->type)
-    if (!result->enc) result->enc = g_strdup("none");
+        result->type = g_strdup("chat");
+    if (!result->enc)
+        result->enc = g_strdup("none");
 
     g_free(work);
     return result;
@@ -463,19 +822,36 @@ _ff_add_message(const char* type, const char* stanza_id, const char* archive_id,
         return;
     }
 
-    // Check if file is new (needs header)
+    // Open the file with O_NOFOLLOW to prevent symlink attacks,
-    gboolean is_new = !g_file_test(log_path, G_FILE_TEST_EXISTS);
+    // and O_APPEND for safe concurrent appends.
+    // Try O_CREAT|O_EXCL first to detect new files atomically (no TOCTOU).
+    int fd = open(log_path, O_WRONLY | O_APPEND | O_CREAT | O_EXCL | O_NOFOLLOW,
+                  S_IRUSR | S_IWUSR);
+    gboolean is_new = (fd >= 0);
+    if (fd < 0) {
+        if (errno == EEXIST) {
+            // File already exists — open for append
+            fd = open(log_path, O_WRONLY | O_APPEND | O_NOFOLLOW);
+        }
+        if (fd < 0) {
+            // ELOOP (symlink) or other error
+            log_error("flatfile: could not open %s for writing (errno=%d: %s)",
+                      log_path, errno, strerror(errno));
+            g_free(effective_msg);
+            return;
+        }
+    }
 
-    FILE* fp = fopen(log_path, "a");
+    FILE* fp = fdopen(fd, "a");
     if (!fp) {
-        log_error("flatfile: could not open %s for writing (errno=%d)", log_path, errno);
+        log_error("flatfile: fdopen failed for %s (errno=%d)", log_path, errno);
+        close(fd);
         g_free(effective_msg);
         return;
     }
 
     if (is_new) {
         fprintf(fp, "%s", FLATFILE_HEADER);
-        g_chmod(log_path, S_IRUSR | S_IWUSR);
     }
 
     _ff_write_line(fp, date_fmt, type, _ff_get_message_enc_str(enc),
@@ -524,7 +900,8 @@ _flatfile_close(void)
 static void
 _flatfile_add_incoming(ProfMessage* message)
 {
-    if (!message || !message->from_jid) return;
+    if (!message || !message->from_jid)
+        return;
 
     const Jid* to_jid = message->to_jid ? message->to_jid : connection_get_jid();
     const char* type = _ff_get_message_type_str(message->type);
@@ -577,7 +954,8 @@ _ff_list_log_files(const char* contact_barejid)
     }
 
     GDir* dir = g_dir_open(contact_dir, 0, NULL);
-    if (!dir) return NULL;
+    if (!dir)
+        return NULL;
 
     GSList* files = NULL;
     const gchar* fname;
@@ -601,7 +979,8 @@ _ff_read_file_messages(const char* filepath, const char* contact_barejid,
     GSList* messages = NULL;
 
     FILE* fp = fopen(filepath, "r");
-    if (!fp) return NULL;
+    if (!fp)
+        return NULL;
 
     // BOM detection
     int c1 = fgetc(fp);
@@ -612,14 +991,19 @@ _ff_read_file_messages(const char* filepath, const char* contact_barejid,
         fseek(fp, 0, SEEK_SET);
     }
 
-    char buf[8192];
+    char* buf = NULL;
-    while (fgets(buf, sizeof(buf), fp)) {
+    gboolean truncated = FALSE;
-        // Strip trailing newline
+    while ((buf = _ff_readline(fp, &truncated)) != NULL) {
-        gsize len = strlen(buf);
+        if (truncated) {
-        if (len > 0 && buf[len - 1] == '\n') buf[--len] = '\0';
+            log_warning("flatfile: truncated line at EOF in %s, skipping", filepath);
+            free(buf);
+            break;
+        }
 
         flatfile_parsed_line_t* pl = _ff_parse_line(buf);
-        if (!pl) continue;
+        free(buf);
+        if (!pl)
+            continue;
 
         // Check timestamp bounds
         if (start_time) {
@@ -648,8 +1032,7 @@ _ff_read_file_messages(const char* filepath, const char* contact_barejid,
         // Check JID filter: message must involve contact and our JID
         gboolean matches = FALSE;
         if (myjid && myjid->barejid && contact_barejid) {
-            if ((g_strcmp0(pl->from_jid, contact_barejid) == 0) ||
+            if ((g_strcmp0(pl->from_jid, contact_barejid) == 0) || (g_strcmp0(pl->from_jid, myjid->barejid) == 0)) {
-                (g_strcmp0(pl->from_jid, myjid->barejid) == 0)) {
                 matches = TRUE;
             }
         } else {
@@ -691,16 +1074,25 @@ _ff_apply_lmc(GSList* parsed_lines, GSList** result)
         if (pl->replace_id && strlen(pl->replace_id) > 0) {
             flatfile_parsed_line_t* original = g_hash_table_lookup(id_map, pl->replace_id);
             if (original) {
-                // Follow chain to the root original
+                // Follow chain to the root original, with cycle/depth guard
                 flatfile_parsed_line_t* root = original;
-                while (root->replace_id && strlen(root->replace_id) > 0) {
+                GHashTable* visited = g_hash_table_new(g_direct_hash, g_direct_equal);
+                g_hash_table_insert(visited, root, root);
+                int depth = 0;
+                while (root->replace_id && strlen(root->replace_id) > 0 && depth < FF_MAX_LMC_DEPTH) {
                     flatfile_parsed_line_t* parent = g_hash_table_lookup(id_map, root->replace_id);
-                    if (parent && parent != root) {
+                    if (parent && !g_hash_table_contains(visited, parent)) {
+                        g_hash_table_insert(visited, parent, parent);
                         root = parent;
+                        depth++;
                     } else {
                         break;
                     }
                 }
+                g_hash_table_destroy(visited);
+                if (depth >= FF_MAX_LMC_DEPTH) {
+                    log_warning("flatfile: LMC correction chain too deep (>%d), ignoring", FF_MAX_LMC_DEPTH);
+                }
                 g_hash_table_insert(correction_map, root, pl);
                 g_hash_table_insert(corrections, pl, pl);
             }
@@ -768,7 +1160,7 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
     GSList* all_parsed = NULL;
     for (GSList* f = files; f; f = f->next) {
         GSList* file_msgs = _ff_read_file_messages(f->data, contact_barejid,
-                                                    start_time, effective_end, myjid);
+                                                   start_time, effective_end, myjid);
         all_parsed = g_slist_concat(all_parsed, file_msgs);
     }
 
@@ -797,7 +1189,8 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
             GSList* nth = g_slist_nth(pre_result, MESSAGES_TO_RETRIEVE);
             if (nth) {
                 GSList* prev = g_slist_nth(pre_result, MESSAGES_TO_RETRIEVE - 1);
-                if (prev) prev->next = NULL;
+                if (prev)
+                    prev->next = NULL;
                 g_slist_free_full(nth, (GDestroyNotify)message_free);
             }
         } else {
@@ -805,7 +1198,8 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
             guint skip = total - MESSAGES_TO_RETRIEVE;
             GSList* keep_start = g_slist_nth(pre_result, skip);
             GSList* prev = g_slist_nth(pre_result, skip - 1);
-            if (prev) prev->next = NULL;
+            if (prev)
+                prev->next = NULL;
             g_slist_free_full(pre_result, (GDestroyNotify)message_free);
             pre_result = keep_start;
         }
@@ -826,13 +1220,15 @@ _flatfile_get_limits_info(const gchar* const contact_barejid, gboolean is_last)
     ProfMessage* msg = message_init();
 
     if (!g_flatfile_account_jid || !contact_barejid) {
-        if (is_last) msg->timestamp = g_date_time_new_now_utc();
+        if (is_last)
+            msg->timestamp = g_date_time_new_now_utc();
         return msg;
     }
 
     GSList* files = _ff_list_log_files(contact_barejid);
     if (!files) {
-        if (is_last) msg->timestamp = g_date_time_new_now_utc();
+        if (is_last)
+            msg->timestamp = g_date_time_new_now_utc();
         return msg;
     }
 
@@ -844,7 +1240,8 @@ _flatfile_get_limits_info(const gchar* const contact_barejid, gboolean is_last)
     FILE* fp = fopen(target_file, "r");
     if (!fp) {
         g_slist_free_full(files, g_free);
-        if (is_last) msg->timestamp = g_date_time_new_now_utc();
+        if (is_last)
+            msg->timestamp = g_date_time_new_now_utc();
         return msg;
     }
 
@@ -856,25 +1253,25 @@ _flatfile_get_limits_info(const gchar* const contact_barejid, gboolean is_last)
         fseek(fp, 0, SEEK_SET);
     }
 
-    char buf[8192];
+    char* buf = NULL;
     flatfile_parsed_line_t* found = NULL;
 
     if (!is_last) {
         // Find first valid line
-        while (fgets(buf, sizeof(buf), fp)) {
+        while ((buf = _ff_readline(fp, NULL)) != NULL) {
-            gsize len = strlen(buf);
-            if (len > 0 && buf[len - 1] == '\n') buf[--len] = '\0';
             found = _ff_parse_line(buf);
-            if (found) break;
+            free(buf);
+            if (found)
+                break;
         }
     } else {
         // Find last valid line — read all and keep last
-        while (fgets(buf, sizeof(buf), fp)) {
+        while ((buf = _ff_readline(fp, NULL)) != NULL) {
-            gsize len = strlen(buf);
-            if (len > 0 && buf[len - 1] == '\n') buf[--len] = '\0';
             flatfile_parsed_line_t* pl = _ff_parse_line(buf);
+            free(buf);
             if (pl) {
-                if (found) _ff_parsed_line_free(found);
+                if (found)
+                    _ff_parsed_line_free(found);
                 found = pl;
             }
         }
@@ -952,7 +1349,8 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
         const char* cdir_path = cd->data;
 
         GDir* dir = g_dir_open(cdir_path, 0, NULL);
-        if (!dir) continue;
+        if (!dir)
+            continue;
 
         GSList* log_files = NULL;
         const gchar* fname;
@@ -988,7 +1386,8 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
             }
 
             FILE* fp = fopen(filepath, "r");
-            if (!fp) continue;
+            if (!fp)
+                continue;
 
             // BOM check (4l)
             int c1 = fgetc(fp);
@@ -1005,7 +1404,7 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                 fseek(fp, 0, SEEK_SET);
             }
 
-            char buf[8192];
+            char* buf = NULL;
             int lineno = 0;
             GDateTime* prev_ts = NULL;
             GDateTime* first_ts = NULL;
@@ -1013,10 +1412,9 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
             gboolean has_crlf = FALSE;
             gboolean is_empty = TRUE;
 
-            while (fgets(buf, sizeof(buf), fp)) {
+            while ((buf = _ff_readline(fp, NULL)) != NULL) {
                 lineno++;
                 gsize len = strlen(buf);
-                if (len > 0 && buf[len - 1] == '\n') buf[--len] = '\0';
 
                 // CRLF check (4m)
                 if (len > 0 && buf[len - 1] == '\r') {
@@ -1025,7 +1423,10 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                 }
 
                 // Skip empty lines and comments
-                if (len == 0 || buf[0] == '#') continue;
+                if (len == 0 || buf[0] == '#') {
+                    free(buf);
+                    continue;
+                }
                 is_empty = FALSE;
 
                 // UTF-8 validation (4k/4n)
@@ -1037,6 +1438,7 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                     issue->line = lineno;
                     issue->message = g_strdup_printf("Invalid UTF-8 at byte offset %ld", (long)(end - buf));
                     issues = g_slist_append(issues, issue);
+                    free(buf);
                     continue;
                 }
 
@@ -1061,13 +1463,18 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                     issue->level = INTEGRITY_ERROR;
                     issue->file = g_strdup(basename);
                     issue->line = lineno;
-                    issue->message = g_strdup("Unparseable line");
+                    issue->message = g_strdup("Unparsable line");
                     issues = g_slist_append(issues, issue);
+                    free(buf);
                     continue;
                 }
 
-                if (!first_ts) first_ts = g_date_time_ref(pl->timestamp);
+                free(buf); // done with raw line
-                if (last_ts) g_date_time_unref(last_ts);
+
+                if (!first_ts)
+                    first_ts = g_date_time_ref(pl->timestamp);
+                if (last_ts)
+                    g_date_time_unref(last_ts);
                 last_ts = g_date_time_ref(pl->timestamp);
 
                 // Timestamp order within file (4c)
@@ -1081,7 +1488,8 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                     issue->message = g_strdup_printf("Timestamp out of order (%s after %s)", ts_cur, ts_prev);
                     issues = g_slist_append(issues, issue);
                 }
-                if (prev_ts) g_date_time_unref(prev_ts);
+                if (prev_ts)
+                    g_date_time_unref(prev_ts);
                 prev_ts = g_date_time_ref(pl->timestamp);
 
                 // Duplicate stanza-id / archive-id (4f)
@@ -1155,12 +1563,16 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                 }
             }
 
-            if (prev_file_last_ts) g_date_time_unref(prev_file_last_ts);
+            if (prev_file_last_ts)
+                g_date_time_unref(prev_file_last_ts);
             prev_file_last_ts = last_ts ? g_date_time_ref(last_ts) : NULL;
 
-            if (prev_ts) g_date_time_unref(prev_ts);
+            if (prev_ts)
-            if (first_ts) g_date_time_unref(first_ts);
+                g_date_time_unref(prev_ts);
-            if (last_ts) g_date_time_unref(last_ts);
+            if (first_ts)
+                g_date_time_unref(first_ts);
+            if (last_ts)
+                g_date_time_unref(last_ts);
         }
 
         // Second pass: check LMC references across all files (4g)
@@ -1170,7 +1582,8 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
             basename_lmc = basename_lmc ? basename_lmc + 1 : filepath;
 
             FILE* fp = fopen(filepath, "r");
-            if (!fp) continue;
+            if (!fp)
+                continue;
 
             // Skip BOM
             int b1 = fgetc(fp);
@@ -1180,17 +1593,22 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
                 fseek(fp, 0, SEEK_SET);
             }
 
-            char buf[8192];
+            char* buf = NULL;
             int lineno = 0;
-            while (fgets(buf, sizeof(buf), fp)) {
+            while ((buf = _ff_readline(fp, NULL)) != NULL) {
                 lineno++;
                 gsize len = strlen(buf);
-                if (len > 0 && buf[len - 1] == '\n') buf[--len] = '\0';
+                if (len > 0 && buf[len - 1] == '\r')
-                if (len > 0 && buf[len - 1] == '\r') buf[--len] = '\0';
+                    buf[--len] = '\0';
-                if (len == 0 || buf[0] == '#') continue;
+                if (len == 0 || buf[0] == '#') {
+                    free(buf);
+                    continue;
+                }
 
                 flatfile_parsed_line_t* pl = _ff_parse_line(buf);
-                if (!pl) continue;
+                free(buf);
+                if (!pl)
+                    continue;
 
                 if (pl->replace_id && strlen(pl->replace_id) > 0) {
                     if (!g_hash_table_contains(all_stanza_ids, pl->replace_id)) {
@@ -1207,7 +1625,8 @@ _flatfile_verify_integrity(const gchar* const contact_barejid)
             fclose(fp);
         }
 
-        if (prev_file_last_ts) g_date_time_unref(prev_file_last_ts);
+        if (prev_file_last_ts)
+            g_date_time_unref(prev_file_last_ts);
         g_hash_table_destroy(seen_ids);
         g_hash_table_destroy(all_stanza_ids);
         g_slist_free_full(log_files, g_free);