CVE-2009-2666 in fetchmail
Summary
by MITRE
socket.c in fetchmail before 6.3.11 does not properly handle a \0 character in a domain name in the subject s Common Name (CN) field of an X.509 certificate, which allows man-in-the-middle attackers to spoof arbitrary SSL servers via a crafted certificate issued by a legitimate Certification Authority, a related issue to CVE-2009-2408.
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Analysis
by VulDB Data Team • 05/17/2025
The vulnerability described in CVE-2009-2666 affects the fetchmail email retrieval utility version 6.3.10 and earlier, specifically within the socket.c component that handles SSL/TLS certificate validation. This flaw represents a critical security weakness in the certificate verification process that can be exploited to bypass SSL certificate validation mechanisms. The issue stems from improper handling of null characters within domain names contained in the Common Name field of X.509 certificates, creating a potential pathway for attackers to establish fraudulent SSL connections that appear legitimate to the vulnerable client.
The technical implementation flaw occurs when fetchmail processes X.509 certificates during SSL/TLS connections, where the software fails to properly sanitize or validate domain name strings that contain null characters. This weakness allows malicious actors to craft certificates with null characters embedded within the Common Name field, specifically in the domain name portion, which can cause the certificate validation logic to incorrectly accept certificates that should be rejected. The vulnerability is particularly concerning because it enables man-in-the-middle attacks where attackers can present certificates that appear valid to the client, even though they are issued by unauthorized or malicious entities.
From an operational perspective, this vulnerability compromises the fundamental security guarantee of SSL/TLS encryption by allowing attackers to spoof arbitrary SSL servers. When a user connects to a legitimate server using fetchmail, an attacker who has compromised a certificate authority or obtained a valid certificate can manipulate the certificate to include null characters in the domain name, thereby bypassing the certificate validation process. This creates a scenario where users believe they are connecting securely to their intended destination while actually communicating with an attacker's system, potentially exposing sensitive email communications, authentication credentials, and other confidential data.
The impact of this vulnerability aligns with attack patterns documented in the MITRE ATT&CK framework under the T1573.001 technique for "Encrypted Channel" and T1071.004 for "Application Layer Protocol: DNS," as it enables attackers to establish fraudulent secure communication channels. This weakness can be exploited in various attack scenarios including email interception, credential harvesting, and data exfiltration from compromised email accounts. The vulnerability also relates to CWE-20, "Improper Input Validation," and CWE-310, "Cryptographic Issues," as it represents both improper validation of certificate data and cryptographic weakness in SSL/TLS certificate validation.
Organizations should immediately upgrade to fetchmail version 6.3.11 or later, which contains the necessary patches to properly handle null characters in certificate domain names. Additional mitigations include implementing certificate pinning for critical connections, monitoring for unusual SSL certificate validation failures, and ensuring that network traffic is inspected for potential man-in-the-middle attacks. System administrators should also consider implementing additional security controls such as certificate transparency monitoring and regular security audits of SSL/TLS configurations to detect and prevent exploitation of similar vulnerabilities in other components of the email infrastructure.