Vulnerability: In libssh2 v1.9.0 and earlier versions, the SSH_MSG_DISCONNECT logic in packet.c has an integer overflow in a bounds check, enabling an attacker to specify an arbitrary (out-of-bounds) offset for a subsequent memory read. A crafted SSH server may be able to disclose sensitive information or cause a denial of service condition on the client system when a user connects to the server.
Vulnerability ID: CVE-2019-17498
CWE: CWE-190
Severity score: 4.9
Project: https://github.com/libssh2/libssh2
Published: 2019-10-21
File: src/packet.c
Commit message: packet.c: improve message parsing (#402)

* packet.c: improve parsing of packets

file: packet.c

notes:
Use _libssh2_get_string API in SSH_MSG_DEBUG/SSH_MSG_DISCONNECT. Additional uint32 bounds check in SSH_MSG_GLOBAL_REQUEST.
Commit author: Will Cosgrove

Vulnerable version (before fix):
 _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,                     size_t datalen, int macstate) {     int rc = 0;     char *message = NULL;     char *language = NULL;     size_t message_len = 0;     size_t language_len = 0;     LIBSSH2_CHANNEL *channelp = NULL; _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,          case SSH_MSG_DISCONNECT:             if(datalen >= 5) {                 size_t reason = _libssh2_ntohu32(data + 1);                  if(datalen >= 9) {                     message_len = _libssh2_ntohu32(data + 5);                      if(message_len < datalen-13) {                         /* 9 = packet_type(1) + reason(4) + message_len(4) */                         message = (char *) data + 9;                          language_len =                             _libssh2_ntohu32(data + 9 + message_len);                         language = (char *) data + 9 + message_len + 4;                          if(language_len > (datalen-13-message_len)) {                             /* bad input, clear info */                             language = message = NULL;                             language_len = message_len = 0;                         }                     }                     else                         /* bad size, clear it */                         message_len = 0;                 }                 if(session->ssh_msg_disconnect) {                     LIBSSH2_DISCONNECT(session, reason, message,                                        message_len, language, language_len);                 }                 _libssh2_debug(session, LIBSSH2_TRACE_TRANS,                                "Disconnect(%d): %s(%s)", reason,                                message, language); _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,                 int always_display = data[1];                  if(datalen >= 6) {                     message_len = _libssh2_ntohu32(data + 2);                      if(message_len <= (datalen - 10)) {                         /* 6 = packet_type(1) + display(1) + message_len(4) */                         message = (char *) data + 6;                         language_len = _libssh2_ntohu32(data + 6 +                                                         message_len);                          if(language_len <= (datalen - 10 - message_len))                             language = (char *) data + 10 + message_len;                     }                 }                  if(session->ssh_msg_debug) {                     LIBSSH2_DEBUG(session, always_display, message,                                   message_len, language, language_len);                 }             }             /*              * _libssh2_debug will actually truncate this for us so              * that it's not an inordinate about of data _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,                 uint32_t len = 0;                 unsigned char want_reply = 0;                 len = _libssh2_ntohu32(data + 1);                 if(datalen >= (6 + len)) {                     want_reply = data[5 + len];                     _libssh2_debug(session,                                    LIBSSH2_TRACE_CONN,

Fixed version (after fix):
 _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,                     size_t datalen, int macstate) {     int rc = 0;     unsigned char *message = NULL;     unsigned char *language = NULL;     size_t message_len = 0;     size_t language_len = 0;     LIBSSH2_CHANNEL *channelp = NULL; _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,          case SSH_MSG_DISCONNECT:             if(datalen >= 5) {                 uint32_t reason = 0;                 struct string_buf buf;                 buf.data = (unsigned char *)data;                 buf.dataptr = buf.data;                 buf.len = datalen;                 buf.dataptr++; /* advance past type */                  _libssh2_get_u32(&buf, &reason);                 _libssh2_get_string(&buf, &message, &message_len);                 _libssh2_get_string(&buf, &language, &language_len);                  if(session->ssh_msg_disconnect) {                     LIBSSH2_DISCONNECT(session, reason, (const char *)message,                                        message_len, (const char *)language,                                        language_len);                 }                  _libssh2_debug(session, LIBSSH2_TRACE_TRANS,                                "Disconnect(%d): %s(%s)", reason,                                message, language); _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,                 int always_display = data[1];                  if(datalen >= 6) {                     struct string_buf buf;                     buf.data = (unsigned char *)data;                     buf.dataptr = buf.data;                     buf.len = datalen;                     buf.dataptr += 2; /* advance past type & always display */                      _libssh2_get_string(&buf, &message, &message_len);                     _libssh2_get_string(&buf, &language, &language_len);                 }                  if(session->ssh_msg_debug) {                     LIBSSH2_DEBUG(session, always_display,                                   (const char *)message,                                   message_len, (const char *)language,                                   language_len);                 }             }              /*              * _libssh2_debug will actually truncate this for us so              * that it's not an inordinate about of data _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,                 uint32_t len = 0;                 unsigned char want_reply = 0;                 len = _libssh2_ntohu32(data + 1);                 if((len <= (UINT_MAX - 6)) && (datalen >= (6 + len))) {                     want_reply = data[5 + len];                     _libssh2_debug(session,                                    LIBSSH2_TRACE_CONN,

Patch:
patch: @@ -419,8 +419,8 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
                     size_t datalen, int macstate)
 {
     int rc = 0;
-    char *message = NULL;
-    char *language = NULL;
+    unsigned char *message = NULL;
+    unsigned char *language = NULL;
     size_t message_len = 0;
     size_t language_len = 0;
     LIBSSH2_CHANNEL *channelp = NULL;
@@ -472,33 +472,23 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
 
         case SSH_MSG_DISCONNECT:
             if(datalen >= 5) {
-                size_t reason = _libssh2_ntohu32(data + 1);
+                uint32_t reason = 0;
+                struct string_buf buf;
+                buf.data = (unsigned char *)data;
+                buf.dataptr = buf.data;
+                buf.len = datalen;
+                buf.dataptr++; /* advance past type */
 
-                if(datalen >= 9) {
-                    message_len = _libssh2_ntohu32(data + 5);
+                _libssh2_get_u32(&buf, &reason);
+                _libssh2_get_string(&buf, &message, &message_len);
+                _libssh2_get_string(&buf, &language, &language_len);
 
-                    if(message_len < datalen-13) {
-                        /* 9 = packet_type(1) + reason(4) + message_len(4) */
-                        message = (char *) data + 9;
-
-                        language_len =
-                            _libssh2_ntohu32(data + 9 + message_len);
-                        language = (char *) data + 9 + message_len + 4;
-
-                        if(language_len > (datalen-13-message_len)) {
-                            /* bad input, clear info */
-                            language = message = NULL;
-                            language_len = message_len = 0;
-                        }
-                    }
-                    else
-                        /* bad size, clear it */
-                        message_len = 0;
-                }
                 if(session->ssh_msg_disconnect) {
-                    LIBSSH2_DISCONNECT(session, reason, message,
-                                       message_len, language, language_len);
+                    LIBSSH2_DISCONNECT(session, reason, (const char *)message,
+                                       message_len, (const char *)language,
+                                       language_len);
                 }
+
                 _libssh2_debug(session, LIBSSH2_TRACE_TRANS,
                                "Disconnect(%d): %s(%s)", reason,
                                message, language);
@@ -539,24 +529,24 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
                 int always_display = data[1];
 
                 if(datalen >= 6) {
-                    message_len = _libssh2_ntohu32(data + 2);
-
-                    if(message_len <= (datalen - 10)) {
-                        /* 6 = packet_type(1) + display(1) + message_len(4) */
-                        message = (char *) data + 6;
-                        language_len = _libssh2_ntohu32(data + 6 +
-                                                        message_len);
-
-                        if(language_len <= (datalen - 10 - message_len))
-                            language = (char *) data + 10 + message_len;
-                    }
+                    struct string_buf buf;
+                    buf.data = (unsigned char *)data;
+                    buf.dataptr = buf.data;
+                    buf.len = datalen;
+                    buf.dataptr += 2; /* advance past type & always display */
+
+                    _libssh2_get_string(&buf, &message, &message_len);
+                    _libssh2_get_string(&buf, &language, &language_len);
                 }
 
                 if(session->ssh_msg_debug) {
-                    LIBSSH2_DEBUG(session, always_display, message,
-                                  message_len, language, language_len);
+                    LIBSSH2_DEBUG(session, always_display,
+                                  (const char *)message,
+                                  message_len, (const char *)language,
+                                  language_len);
                 }
             }
+
             /*
              * _libssh2_debug will actually truncate this for us so
              * that it's not an inordinate about of data
@@ -579,7 +569,7 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
                 uint32_t len = 0;
                 unsigned char want_reply = 0;
                 len = _libssh2_ntohu32(data + 1);
-                if(datalen >= (6 + len)) {
+                if((len <= (UINT_MAX - 6)) && (datalen >= (6 + len))) {
                     want_reply = data[5 + len];
                     _libssh2_debug(session,
                                    LIBSSH2_TRACE_CONN,
path: src/packet.c