Security-Assessment.com Advisory: Asterisk remote heap overflow

2006-10-19T00:00:00
ID SECURITYVULNS:DOC:14751
Type securityvulns
Reporter Securityvulns
Modified 2006-10-19T00:00:00

Description

========================================================================

= Asterisk - chan_skinny Remote Unauthenticated Heap Overflow

= Vendor Website: = http://www.asterisk.org = = Affected Version: = All 1.2-branch releases prior to and including 1.2.12.1 = All 1.0-branch releases prior to and including 1.0.12 = = Not Affected: = All 1.4-branch beta releases (1.4.0-beta1, 1.4.0-beta2) = = Public disclosure on Oct 19, 2006 ========================================================================

== Overview ==

Asterisk is "The Opensource PBX", a popular software telephony server.

The Asterisk Skinny channel driver for Cisco SCCP phones chan_skinny.so) incorrectly validates a length value in the packet header. An integer wrap-around leads to heap overwrite, and arbitrary remote code execution as root.

== Details ==

The function 'static int get_input(struct skinnysession *s)' in chan_skinny.c incorrectly validates a user supplied length in the packet header. In the code below, four bytes of data are read from the socket, cast to a signed integer, and assigned to dlen. If dlen is between -1 and -8 then (dlen + 8) will integer wrap to be greater than zero, but less than sizeof(s->inbuf) for the purposes of this comparison.

Next, dlen + 4 is passed to read() as the maximum number of bytes to write to s->inbuf+4. Read() takes an unsigned value, so dlen is interpreted as a very large number. For example, a value of -6 is interpreted as 0xfffffffa bytes. This instructs read() to write beyond the allocated 1000 byte length of the buffer s->inbuf.

Code asterisk-1.2.12.1/channels/chan_skinny.c lines 2860-2870

res = read(s->fd, s->inbuf, 4); // <- integer read from attacker if (res != 4) { ast_log(LOG_WARNING, "Skinny Client sent less data than expected.\n"); return -1; } dlen = letohl((int )s->inbuf); // <- input 0xfffffffa // interpreted as signed if (dlen+8 > sizeof(s->inbuf)) // <- integer wrap to +2

dlen = sizeof(s->inbuf) - 8; // bypasses this check } (int )s->inbuf = htolel(dlen); // casting just for amusement res = read(s->fd, s->inbuf+4, dlen+4); / <- dlen now unsigned again * permitting read() to write * up to 0xfffffffa bytes off * the end of s->inbuf /


== Exploitation ==

An attacker who can connect to the Asterisk server SCCP "Skinny" port (by default 2000/tcp) can attack the vulnerable function prior to registering as a configured Skinny phone, permitting pre-authentication remote compromise.

Once the initial length header value in the packet performs an integer-wraparound an attacker can overflow off the end of the malloc()ed input buffer, and into heap space above it. Exploitation is possible via standard heap-overflow malloc-unlink-macro technique[1] on glibc versions prior to 2.3.5. On systems with newer glibc, a more sophisticated exploitation method is necessary due to the improved validation of malloc's internal heap management linked lists. Brett Moore's work[2] on bypassing similar restrictions in WinXPSP2 is instructive.

Our proof-of-concept exploit uses vanilla malloc-unlink() to overwrite a GOT entry to point execution back into our buffer, and executes Metasploit port-binding shellcode.

== Solutions ==

  • Disable the chan_skinny module if it is not required.
  • Firewall port 2000/tcp from untrusted networks.
  • Install the vendor supplied upgrades: 1.0-branch: Upgrade to 1.0.12 or later 1.2-branch: Upgrade to 1.2.13 or later

== Credit ==

Discovered and advised to Digium 17th October, 2006 by Adam Boileau of Security-Assessment.com.

Security-Assessment.com commends Digium on their extremely rapid response, releasing an updated version within two days of receiving our vulnerability report.

== References ==

[1] "Advanced Doug Lea's Malloc Exploits" by jp http://doc.bughunter.net/buffer-overflow/advanced-malloc-exploits.html [2] "Exploiting Freelist[0] On Windows XP Service Pack 2" by Brett Moore http://www.security-assessment.com/technical/

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