CVE-2007-1558 in APOP protocol
Summary
by MITRE
The APOP protocol allows remote attackers to guess the first 3 characters of a password via man-in-the-middle (MITM) attacks that use crafted message IDs and MD5 collisions. NOTE: this design-level issue potentially affects all products that use APOP, including (1) Thunderbird 1.x before 1.5.0.12 and 2.x before 2.0.0.4, (2) Evolution, (3) mutt, (4) fetchmail before 6.3.8, (5) SeaMonkey 1.0.x before 1.0.9 and 1.1.x before 1.1.2, (6) Balsa 2.3.16 and earlier, (7) Mailfilter before 0.8.2, and possibly other products.
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Analysis
by VulDB Data Team • 07/18/2019
The vulnerability described in CVE-2007-1558 represents a fundamental design flaw in the Authentication and Authorization Protocol (APOP) that exposes systems to sophisticated man-in-the-middle attacks. This weakness specifically targets the authentication mechanism used by email clients and servers, creating a pathway for attackers to compromise user credentials through carefully crafted network interference. The vulnerability operates at the protocol level, affecting the core authentication process rather than application-specific implementations, making it particularly dangerous as it impacts multiple software products that rely on APOP for email authentication. The issue stems from the predictable nature of APOP's message ID generation and the mathematical properties of MD5 hashing that can be exploited to infer partial password information.
The technical exploitation of this vulnerability relies on the attacker's ability to intercept and manipulate network traffic between email clients and servers while simultaneously leveraging MD5 collision properties to deduce the first three characters of user passwords. When APOP is used for authentication, the protocol generates a message ID that incorporates a timestamp, making it susceptible to prediction attacks. The attacker can craft specific message IDs that, when processed through the MD5 hashing algorithm, reveal patterns that allow password inference. This attack vector specifically targets the authentication handshake process where the client sends a challenge response based on the MD5 hash of the message ID concatenated with the user's password. The vulnerability is classified under CWE-310 as "Cryptographic Issues" and represents a significant weakness in the cryptographic implementation of the APOP protocol.
The operational impact of this vulnerability extends far beyond individual user accounts, as it affects a wide range of email client applications that have implemented APOP authentication. The affected software includes popular email clients such as Thunderbird, Evolution, mutt, and fetchmail, as well as web browsers and email servers like SeaMonkey and Balsa. This widespread impact means that organizations relying on these tools for email communication face substantial risk of credential compromise, potentially leading to unauthorized access to email accounts, data breaches, and further exploitation opportunities. The vulnerability's persistence across multiple versions and products indicates a systemic issue with the protocol's design that requires fundamental changes to address properly. Attackers can systematically target users by monitoring network traffic and using the MD5 collision techniques to gradually uncover password information, making this a particularly insidious threat that can be exploited over extended periods.
Mitigation strategies for this vulnerability require both immediate and long-term approaches to address the underlying protocol weakness. Organizations should immediately disable APOP authentication in favor of more secure alternatives such as SASL (Simple Authentication and Security Layer) or TLS/SSL encrypted connections that provide proper authentication and encryption. Software vendors must implement patches that either disable APOP functionality or strengthen the cryptographic implementation to prevent predictable message ID generation. The recommended approach aligns with ATT&CK technique T1566 for credential access through network sniffing and manipulation, emphasizing the need for secure authentication protocols. System administrators should also implement network monitoring to detect suspicious traffic patterns and ensure that all email clients are updated to versions that either eliminate APOP support or properly mitigate the vulnerability. The long-term solution involves transitioning away from APOP entirely in favor of modern authentication protocols that do not suffer from these cryptographic weaknesses, ensuring that organizations are protected against both current and future variants of similar attacks.