Advisory: Websockify: Remote Code Execution via Buffer Overflow

RedTeam Pentesting discovered a buffer overflow vulnerability in the C
implementation of Websockify, which allows attackers to execute
arbitrary code.


Details
=======

Product: Websockify C implementation
Affected Versions: all versions <= 0.8.0
Fixed Versions: versions since commit 192ec6f (2016-04-22) [0]
Vulnerability Type: Buffer Overflow
Security Risk: high
Vendor URL: https://github.com/kanaka/websockify
Vendor Status: fixed
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2016-004
Advisory Status: published
CVE: GENERIC-MAP-NOMATCH
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=GENERIC-MAP-NOMATCH


Introduction
============

"websockify was formerly named wsproxy and was part of the noVNC
project.

At the most basic level, websockify just translates WebSockets traffic
to normal TCP socket traffic. Websockify accepts the WebSockets
handshake, parses it, and then begins forwarding traffic between the
client and the target in both directions."

(from the project's readme)


More Details
============

For each new connection, websockify forks and calls the function
do_handshake() to receive a client's WebSocket handshake. The
following excerpt shows some of the source code responsible for
receiving the client's data from the socket file descriptor:

------------------------------------------------------------------------

ws_ctx_t *do_handshake(int sock) {
    char handshake[4096], response[4096], sha1[29], trailer[17];
    [...]
    offset = 0;
    for (i = 0; i < 10; i++) {
        len = ws_recv(ws_ctx, handshake+offset, 4096);
        if (len == 0) {
            handler_emsg("Client closed during handshake\n");
            return NULL;
        }
        offset += len;
        handshake[offset] = 0;
        if (strstr(handshake, "\r\n\r\n")) {
            break;
        }
        usleep(10);
    }
    [...]

------------------------------------------------------------------------

As can be seen in the listing, the function ws_recv() is called in a
loop to read data from the client's socket into the stack-allocated
buffer 'handshake'. Each time ws_recv() is called, a maximum of 4096
bytes are read from the socket and stored in the handshake buffer.
The variable 'offset' determines the position in the buffer at which
the received data is written. In each iteration, the value of 'offset'
is increased by the amount of bytes received. If the received data
contains the string "\r\n\r\n", which marks the end of the WebSocket
handshake data, the loop is terminated. Otherwise, the loop is
terminated after a maximum of 10 iterations. The do_handshake()
function returns early if no more data can be received from the
socket.

By forcing websockify to iterate multiple times, attackers can
exploit this behaviour to write data past the space allocated for the
handshake buffer, thereby corrupting adjacent memory.


Proof of Concept
================

The following curl command can be used to trigger the buffer overflow:

$ curl https://example.com/$(python -c 'print "A"*5000')

Providing a generic exploit for this vulnerability is not feasible, as
it depends on the server side environment websockify is used in as well
as the used compiler and its flags. However, during a penetration test
it was possible to successfully exploit this buffer overflow
vulnerability and to execute arbitrary commands on the server.

Workaround
==========

Use the Python implementation of websockify.


Fix
===

The vulnerability has been fixed in commit 192ec6f [0].


Security Risk
=============

Successful exploitation of the vulnerability allows attackers to execute
arbitrary code on the affected system. It is therefore rated as a high
risk.


Timeline
========

2016-04-14 Vulnerability identified
2016-05-03 Advisory provided to customer
2016-05-06 Customer provided updated firmware, notified users
2016-05-23 Customer notified users again
2016-05-31 Advisory published


References
==========

[0] https://github.com/kanaka/websockify/commit/192ec6f5f9bf9c80a089ca020d05ad4bd9e7bcd9


RedTeam Pentesting GmbH
=======================

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