From 2dae3024897e1898d389835151f4e9606227721d Mon Sep 17 00:00:00 2001
From: Will Cosgrove <will@panic.com>
Date: Fri, 10 Oct 2025 08:26:20 -0700
Subject: [PATCH] Update sftp_symlink to avoid out of bounds read on malformed
 packet #1705 (#1717)

Use buffer struct to guard against out of bounds reads and invalid packets.

Discovery Credit:
Joshua Rogers
---
 src/sftp.c | 66 ++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 47 insertions(+), 19 deletions(-)

Index: libssh2-1.11.1/src/sftp.c
===================================================================
--- libssh2-1.11.1.orig/src/sftp.c
+++ libssh2-1.11.1/src/sftp.c
@@ -3786,6 +3786,8 @@ libssh2_sftp_stat_ex(LIBSSH2_SFTP *sftp,
     return rc;
 }
 
+#define LIBSSH2_UNCONST(p)  ((void *)(uintptr_t)(const void *)(p))
+
 /* sftp_symlink
  * Read or set a symlink
  */
@@ -3795,15 +3797,19 @@ static int sftp_symlink(LIBSSH2_SFTP *sf
 {
     LIBSSH2_CHANNEL *channel = sftp->channel;
     LIBSSH2_SESSION *session = channel->session;
-    size_t data_len = 0, link_len;
+    size_t data_len = 0, lk_len;
     /* 13 = packet_len(4) + packet_type(1) + request_id(4) + path_len(4) */
     ssize_t packet_len =
         path_len + 13 +
         ((link_type == LIBSSH2_SFTP_SYMLINK) ? (4 + target_len) : 0);
     unsigned char *s, *data = NULL;
+    struct string_buf buf;
     static const unsigned char link_responses[2] =
         { SSH_FXP_NAME, SSH_FXP_STATUS };
     int retcode;
+    unsigned char packet_type;
+    uint32_t tmp_u32;
+    unsigned char *lk_target;
 
     if(sftp->symlink_state == libssh2_NB_state_idle) {
         sftp->last_errno = LIBSSH2_FX_OK;
@@ -3891,8 +3897,25 @@ static int sftp_symlink(LIBSSH2_SFTP *sf
 
     sftp->symlink_state = libssh2_NB_state_idle;
 
-    if(data[0] == SSH_FXP_STATUS) {
-        retcode = _libssh2_ntohu32(data + 5);
+    buf.data = (unsigned char *)LIBSSH2_UNCONST(data);
+    buf.dataptr = buf.data;
+    buf.len = data_len;
+
+    if(_libssh2_get_byte(&buf, &packet_type)) {
+        LIBSSH2_FREE(session, data);
+        return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,
+                              "SFTP Protocol Error (type)");
+    }
+
+    if(packet_type == SSH_FXP_STATUS) {
+        if(_libssh2_get_u32(&buf, &tmp_u32)) {
+            LIBSSH2_FREE(session, data);
+            return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,
+                                  "SFTP Protocol Error (code)");
+        }
+
+        retcode = (int)tmp_u32;
+
         LIBSSH2_FREE(session, data);
         if(retcode == LIBSSH2_FX_OK)
             return LIBSSH2_ERROR_NONE;
@@ -3903,30 +3926,37 @@ static int sftp_symlink(LIBSSH2_SFTP *sf
         }
     }
 
-    if(_libssh2_ntohu32(data + 5) < 1) {
+    /* advance past id */
+    if(_libssh2_get_u32(&buf, &tmp_u32)) {
         LIBSSH2_FREE(session, data);
         return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,
-                              "Invalid READLINK/REALPATH response, "
-                              "no name entries");
+                              "SFTP Protocol Error (id)");
     }
 
-    if(data_len < 13) {
-        if(data_len > 0) {
-            LIBSSH2_FREE(session, data);
-        }
+    /* look for at least one link */
+    if(_libssh2_get_u32(&buf, &tmp_u32) || tmp_u32 < 1) {
+        LIBSSH2_FREE(session, data);
         return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,
-                              "SFTP stat packet too short");
+                                     "Invalid READLINK/REALPATH response, "
+                                     "no name entries");
     }
 
-    /* this reads a u32 and stores it into a signed 32bit value */
-    link_len = _libssh2_ntohu32(data + 9);
-    if(link_len < target_len) {
-        memcpy(target, data + 13, link_len);
-        target[link_len] = 0;
-        retcode = (int)link_len;
+    if(_libssh2_get_string(&buf, &lk_target, &lk_len) == LIBSSH2_ERROR_NONE) {
+        if(lk_len < target_len) {
+            memcpy(target, lk_target, lk_len);
+            target[lk_len] = '\0';
+            retcode = (int)lk_len;
+        }
+        else {
+            retcode = LIBSSH2_ERROR_BUFFER_TOO_SMALL;
+        }
     }
-    else
-        retcode = LIBSSH2_ERROR_BUFFER_TOO_SMALL;
+    else {
+        LIBSSH2_FREE(session, data);
+        return _libssh2_error(session, LIBSSH2_ERROR_SFTP_PROTOCOL,
+                              "SFTP Protocol Error (filename)");
+    }
+
     LIBSSH2_FREE(session, data);
 
     return retcode;