CVE-2026-0965 config: Do not attempt to read non-regular and too large configuration files

Changes also the reading of known_hosts to use the new helper function

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/misc.h

@@ -36,6 +36,7 @@
 #include <sys/types.h>
 #include <stdbool.h>
 #endif /* _WIN32 */
+#include <stdio.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -137,6 +138,8 @@ int ssh_check_username_syntax(const char *username);
 void ssh_proxyjumps_free(struct ssh_list *proxy_jump_list);
 bool ssh_libssh_proxy_jumps(void);
 
+FILE *ssh_strict_fopen(const char *filename, size_t max_file_size);
+
 #ifdef __cplusplus
 }
 #endif

include/libssh/priv.h

@@ -508,6 +508,9 @@ char *ssh_strerror(int err_num, char *buf, size_t buflen);
 #define SSH_TTY_MODES_MAX_BUFSIZE   (55 * 5 + 1)
 int encode_current_tty_opts(unsigned char *buf, size_t buflen);
 
+/** The default maximum file size for a configuration file */
+#define SSH_MAX_CONFIG_FILE_SIZE 16 * 1024 * 1024
+
 #ifdef __cplusplus
 }
 #endif

src/bind_config.c

@@ -217,7 +217,7 @@ local_parse_file(ssh_bind bind,
         return;
     }
 
-    f = fopen(filename, "r");
+    f = ssh_strict_fopen(filename, SSH_MAX_CONFIG_FILE_SIZE);
     if (f == NULL) {
         SSH_LOG(SSH_LOG_RARE, "Cannot find file %s to load",
                 filename);
@@ -655,7 +655,7 @@ int ssh_bind_config_parse_file(ssh_bind bind, const char *filename)
      * option to be redefined later by another file. */
     uint8_t seen[BIND_CFG_MAX] = {0};
 
-    f = fopen(filename, "r");
+    f = ssh_strict_fopen(filename, SSH_MAX_CONFIG_FILE_SIZE);
     if (f == NULL) {
         return 0;
     }

src/config.c

@@ -256,10 +256,9 @@ local_parse_file(ssh_session session,
         return;
     }
 
-    f = fopen(filename, "r");
+    f = ssh_strict_fopen(filename, SSH_MAX_CONFIG_FILE_SIZE);
     if (f == NULL) {
-        SSH_LOG(SSH_LOG_RARE, "Cannot find file %s to load",
-                filename);
+        /* The underlying function logs the reasons */
         return;
     }
 
@@ -1708,8 +1707,9 @@ int ssh_config_parse_file(ssh_session session, const char *filename)
     int rv;
     bool global = 0;
 
-    fp = fopen(filename, "r");
+    fp = ssh_strict_fopen(filename, SSH_MAX_CONFIG_FILE_SIZE);
     if (fp == NULL) {
+        /* The underlying function logs the reasons */
         return 0;
     }
 

src/dh-gex.c

@@ -526,9 +526,9 @@ static int ssh_retrieve_dhgroup(char *moduli_file,
     }
 
     if (moduli_file != NULL)
-        moduli = fopen(moduli_file, "r");
+        moduli = ssh_strict_fopen(moduli_file, SSH_MAX_CONFIG_FILE_SIZE);
     else
-        moduli = fopen(MODULI_FILE, "r");
+        moduli = ssh_strict_fopen(MODULI_FILE, SSH_MAX_CONFIG_FILE_SIZE);
 
     if (moduli == NULL) {
         char err_msg[SSH_ERRNO_MSG_MAX] = {0};

src/known_hosts.c

@@ -83,7 +83,7 @@ static struct ssh_tokens_st *ssh_get_knownhost_line(FILE **file,
     struct ssh_tokens_st *tokens = NULL;
 
     if (*file == NULL) {
-        *file = fopen(filename,"r");
+        *file = ssh_strict_fopen(filename, SSH_MAX_CONFIG_FILE_SIZE);
         if (*file == NULL) {
             return NULL;
         }

src/knownhosts.c

@@ -243,7 +243,7 @@ static int ssh_known_hosts_read_entries(const char *match,
     FILE *fp = NULL;
     int rc;
 
-    fp = fopen(filename, "r");
+    fp = ssh_strict_fopen(filename, SSH_MAX_CONFIG_FILE_SIZE);
     if (fp == NULL) {
         char err_msg[SSH_ERRNO_MSG_MAX] = {0};
         SSH_LOG(SSH_LOG_TRACE, "Failed to open the known_hosts file '%s': %s",

src/misc.c

@@ -37,6 +37,7 @@
 #endif /* _WIN32 */
 
 #include <errno.h>
+#include <fcntl.h>
 #include <limits.h>
 #include <stdio.h>
 #include <string.h>
@@ -2416,4 +2417,77 @@ ssh_libssh_proxy_jumps(void)
     return !(t != NULL && t[0] == '1');
 }
 
+/**
+ * @internal
+ *
+ * @brief Safely open a file containing some configuration.
+ *
+ * Runs checks if the file can be used as some configuration file (is regular
+ * file and is not too large). If so, returns the opened file (for reading).
+ * Otherwise logs error and returns `NULL`.
+ *
+ * @param filename      The path to the file to open.
+ * @param max_file_size Maximum file size that is accepted.
+ *
+ * @returns the opened file or `NULL` on error.
+ */
+FILE *ssh_strict_fopen(const char *filename, size_t max_file_size)
+{
+    FILE *f = NULL;
+    struct stat sb;
+    char err_msg[SSH_ERRNO_MSG_MAX] = {0};
+    int r, fd;
+
+    /* open first to avoid TOCTOU */
+    fd = open(filename, O_RDONLY);
+    if (fd == -1) {
+        SSH_LOG(SSH_LOG_RARE,
+                "Failed to open a file %s for reading: %s",
+                filename,
+                ssh_strerror(errno, err_msg, SSH_ERRNO_MSG_MAX));
+        return NULL;
+    }
+
+    /* Check the file is sensible for a configuration file */
+    r = fstat(fd, &sb);
+    if (r != 0) {
+        SSH_LOG(SSH_LOG_RARE,
+                "Failed to stat %s: %s",
+                filename,
+                ssh_strerror(errno, err_msg, SSH_ERRNO_MSG_MAX));
+        close(fd);
+        return NULL;
+    }
+    if ((sb.st_mode & S_IFMT) != S_IFREG) {
+        SSH_LOG(SSH_LOG_RARE,
+                "The file %s is not a regular file: skipping",
+                filename);
+        close(fd);
+        return NULL;
+    }
+
+    if ((size_t)sb.st_size > max_file_size) {
+        SSH_LOG(SSH_LOG_RARE,
+                "The file %s is too large (%jd MB > %zu MB): skipping",
+                filename,
+                (intmax_t)sb.st_size / 1024 / 1024,
+                max_file_size / 1024 / 1024);
+        close(fd);
+        return NULL;
+    }
+
+    f = fdopen(fd, "r");
+    if (f == NULL) {
+        SSH_LOG(SSH_LOG_RARE,
+                "Failed to open a file %s for reading: %s",
+                filename,
+                ssh_strerror(r, err_msg, SSH_ERRNO_MSG_MAX));
+        close(fd);
+        return NULL;
+    }
+
+    /* the flcose() will close also the underlying fd */
+    return f;
+}
+
 /** @} */

src/options.c

@@ -2008,11 +2008,16 @@ int ssh_options_parse_config(ssh_session session, const char *filename)
         goto out;
     }
     if (filename == NULL) {
-        if ((fp = fopen(GLOBAL_CLIENT_CONFIG, "r")) != NULL) {
+        fp = ssh_strict_fopen(GLOBAL_CLIENT_CONFIG, SSH_MAX_CONFIG_FILE_SIZE);
+        if (fp != NULL) {
             filename = GLOBAL_CLIENT_CONFIG;
 #ifdef USR_GLOBAL_CLIENT_CONFIG
-        } else if ((fp = fopen(USR_GLOBAL_CLIENT_CONFIG, "r")) != NULL) {
-            filename = USR_GLOBAL_CLIENT_CONFIG;
+        } else {
+            fp = ssh_strict_fopen(USR_GLOBAL_CLIENT_CONFIG,
+                                  SSH_MAX_CONFIG_FILE_SIZE);
+            if (fp != NULL) {
+                filename = USR_GLOBAL_CLIENT_CONFIG;
+            }
 #endif
         }
 

tests/unittests/torture_config.c

@@ -2933,6 +2933,23 @@ static void torture_config_jump(void **state)
     printf("%s: EOF\n", __func__);
 }
 
+/* Invalid configuration files
+ */
+static void torture_config_invalid(void **state)
+{
+    ssh_session session = *state;
+
+    ssh_options_set(session, SSH_OPTIONS_HOST, "Bar");
+
+    /* non-regular file -- ignored (or missing on non-unix) so OK */
+    _parse_config(session, "/dev/random", NULL, SSH_OK);
+
+#ifndef _WIN32
+    /* huge file -- ignored (or missing on non-unix) so OK */
+    _parse_config(session, "/proc/kcore", NULL, SSH_OK);
+#endif
+}
+
 int torture_run_tests(void)
 {
     int rc;
@@ -3087,6 +3104,9 @@ int torture_run_tests(void)
         cmocka_unit_test_setup_teardown(torture_config_jump,
                                         setup,
                                         teardown),
+        cmocka_unit_test_setup_teardown(torture_config_invalid,
+                                        setup,
+                                        teardown),
     };
 
     ssh_init();