CVE-2007-1558

Vulnerability

The APOP (Authenticated Post Office Protocol) authentication mechanism contains a design flaw that allows remote attackers to guess the first three characters of a user's password. The attack exploits man-in-the-middle (MITM) techniques using crafted message IDs and MD5 collisions. This design-level issue affects all products implementing APOP, including Thunderbird 1.x before 1.5.0.12 and 2.x before 2.0.0.4, Evolution, mutt, fetchmail before 6.3.8, SeaMonkey 1.0.x before 1.0.9 and 1.1.x before 1.1.2, Balsa 2.3.16 and earlier, Mailfilter before 0.8.2, and possibly others [2][4].

Exploitation

An attacker must be in a position to perform a man-in-the-middle attack on the network connection between the client and the APOP server. The attacker sends crafted message IDs to the client and observes the MD5 hash responses. By leveraging MD5 collision techniques, the attacker can iteratively guess the first three characters of the password. No authentication or user interaction beyond normal email retrieval is required; the attack is passive after initial MITM setup.

Impact

Successful exploitation allows the attacker to recover the first three characters of the user's APOP password. While this is a partial disclosure, it significantly reduces the password entropy and can be combined with other attacks or brute-force to recover the full password. The compromise affects confidentiality of credentials.

Mitigation

The vulnerability is design-level and cannot be fixed by individual patches; however, affected products have released updates to disable or mitigate APOP usage. For example, Red Hat issued RHSA-2007:0402 addressing this issue [4]. Users should upgrade to fixed versions: Thunderbird 1.5.0.12/2.0.0.4, fetchmail 6.3.8, SeaMonkey 1.0.9/1.1.2, and others as listed. Alternatively, switch to more secure authentication methods like CRAM-MD5 or SSL/TLS. No workaround exists if APOP must be used.