Advisory Text :  

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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)
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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
execution.

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

p=buf;
sk=s->session->ciphers;
for (i=0; i<sk_SSL_CIPHER_num(sk); i++)
{
/* Decrement for either the ':' or a '&#65533;' */
len--; [4]
c=sk_SSL_CIPHER_value(sk,i);
for (cp=c->name; *cp; )
{
if (len-- <= 0) [1]
{
*p='&#65533;'; [5]
return(buf);
}
else
*(p++)= *(cp++); [2]
}
*(p++)=':'; [3]
}
p[-1]='&#65533;';
return(buf);

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 '&#65533;' 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 concealed]
2007/07/06 Response received by Ben Laurie <ben (at) links (dot) org
[email concealed]>
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.