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[security-alerts] FW: OpenSSL SSL_Get_Shared_Ciphers Off-by-One Buffer Overflow

> 07.40.17 CVE: Not Available
> Platform: Linux
> Title: OpenSSL SSL_Get_Shared_Ciphers Off-by-One Buffer Overflow
> Description: OpenSSL is an open-source implementation of the SSL
> protocol that is used by a number of other projects, including but not
> restricted to Apache, Sendmail and Bind. It is commonly found on
> Linux and UNIX systems. The application is exposed to an off-by-one
> buffer overflow issue because the library fails to properly bounds
> check user-supplied input before copying it to an insufficiently sized
> memory buffer.
> Ref: http://www.securityfocus.com/archive/1/480855
> ______________________________________________________________________

OpenSSL SSL_get_shared_ciphers() off-by-one buffer overflow

Copyright (c) 2007 Moritz Jodeit <moritz (at) jodeit (dot) org [email 
concealed]> (2007/09/27)

Application details:

OpenSSL is a widely used open source implementation of the
SSL v2/v3 and TLS v1 protocols.

Vulnerability description:

OpenSSL 0.9.7l and 0.9.8d fixed a buffer overflow found in
the SSL_get_shared_ciphers() function reported by Tavis
Ormandy and Will Drewry of the Google Security Team.

Although this fix prevented the unlimited overflow of the
buffer, it still allowed an off-by-one buffer overflow to
happen, which could potentially still result in remote code

Here is an excerpt of the function from ssl/ssl_lib.c:

for (i=0; i<sk_SSL_CIPHER_num(sk); i++)
/* Decrement for either the ':' or a '\0' */
len--; [4]
for (cp=c->name; *cp; )
if (len-- <= 0) [1]
*p='\0'; [5]
*(p++)= *(cp++); [2]
*(p++)=':'; [3]

The old vulnerability got fixed at [1] by comparing 'len'
against <= 0 instead of == 0 to detect the possible
underflow of 'len'.

To trigger the off-by-one, you'd just fill the buffer
with cipher strings up to the point, where 'len' == 1 and
'cp' pointing to the last character of the current cipher
string. The last round of the inner for() loop would then
decrement 'len' to 0 at [1] and write the last byte of the
current cipher string into the buffer [2], increasing 'p'
to point to the last free byte of the buffer.
The last free byte is then filled by the ':' separator and
'p' is increased to point one byte behind the buffer.
Now if there are still ciphers remaining, we enter the
outer loop again, decrease 'len' to -1 at [4] and then
hit the check at [1] again. This time it's true and the
terminating '\0' byte is written one byte behind the
buffer [5] before returning.

Vendor response:

2007/06/06 Initial contact with openssl-security (at) openssl (dot) org [email 
2007/07/06 Response received by Ben Laurie <ben (at) links (dot) org [email 
regarding a proposed fix.
2007/09/19 Fix committed to the OpenSSL_0_9_8-stable branch
in CVS.

Vulnerable packages:

All applications using the SSL_get_shared_ciphers() function from
the OpenSSL library up to 0.9.7m and 0.9.8e.


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