Core Security Technologies reported that previous upstream fixes addressing insufficient input validation flaw in pidgin / libpurple in function msn_slplink_process_msg() are inefficient and can be bypassed. This flaw allows an attacker to overwrite pidgin's memory and possibly execute arbitrary code with the privileges of the user running application using libpurple.
This issue was previously tracked as CVE-2008-2927 (bug #453764) and CVE-2009-1376 (bug #500493, incomplete fix).
Users can lower the impact of this flaw by making sure their privacy settings only allow Pidgin to accept messages from the users on their buddy list. This will prevent exploitation of this flaw by other random MSN users.
Technically, this is not really an incomplete fix of the previous integer overflow issues, rather a new issue affecting same code part as previous issues.
In the new attack, attacker aims to exploit a NULL pointer dereference flaw. This is achieved by sending message with non-0 offset. When such message is processed in msn_slplink_process_msg(), msn_slplink_message_find() is called to find previous parts of the message sent within the same session. With specially crafted previous messages, msn_slplink_message_find() may return a structure for ACK message, rather than request message, that later triggers NULL pointer dereference in:
memcpy(slpmsg->buffer + offset, data, len);
In ACK message, slpmsg->buffer is NULL and attacker supplied offset can be used to control what memory area will be overwritten.
This is now public:
This issue has been addressed in following products:
Red Hat Enterprise Linux 3
Red Hat Enterprise Linux 4
Red Hat Enterprise Linux 5
Via RHSA-2009:1218 https://rhn.redhat.com/errata/RHSA-2009-1218.html
MITRE's CVE-2009-2694 record:
The msn_slplink_process_msg function in
libpurple/protocols/msn/slplink.c in libpurple, as used in Pidgin
(formerly Gaim) before 2.5.9 and Adium 1.3.5 and earlier, allows
remote attackers to execute arbitrary code or cause a denial of
service (memory corruption and application crash) by sending multiple
crafted SLP (aka MSNSLP) messages to trigger an overwrite of an
arbitrary memory location. NOTE: this issue reportedly exists because
of an incomplete fix for CVE-2009-1376.
All current Fedora versions are now updated to 2.6.0+ too.