From SquirrelMail vulnerability report:
An issue was fixed that allowed an attacker to possibly steal user data by hijacking the SquirrelMail login session.
Credits: Tomas Hoger
squirrelmail-1.4.18-1.fc9 has been submitted as an update for Fedora 9.
This issue does not affect current squirrelmail packages shipped in Red Hat Enterprise Linux 3, 4 and 5. Change similar to the upstream fix mentioned in comment #0 (upstream SVN r13676) was introduced via RHSA-2009:0057 (http://rhn.redhat.com/errata/RHSA-2009-0057.html) to mitigate the impact of the Red Hat-specific session management regression introduced in RHSA-2009:0010, bug #480224. The change ensured new session id was regenerated after each successful login, and also provides protection against session fixation class of attacks. The change has now been included in upstream SquirrelMail too.
The impact of the flaw was limited, as it required an attacker to be able to set malicious cookie on the victim's system. Successful exploitation did not allow an attacker to get full access to the victim's mail.
squirrelmail-1.4.18-1.fc9 has been pushed to the Fedora 9 stable repository. If problems still persist, please make note of it in this bug report.
squirrelmail-1.4.18-1.fc11 has been pushed to the Fedora 11 stable repository. If problems still persist, please make note of it in this bug report.
squirrelmail-1.4.18-1.fc10 has been pushed to the Fedora 10 stable repository. If problems still persist, please make note of it in this bug report.
Session fixation vulnerability in SquirrelMail before 1.4.18 allows
remote attackers to hijack web sessions via a crafted cookie.
Fedora updated now to 1.4.18, no further action required for Red Hat Enterprise Linux packages (see comment #2). Closing.