CVE-2010-1194 in libESMTP
Summary
by MITRE
The match_component function in smtp-tls.c in libESMTP 1.0.3.r1, and possibly other versions including 1.0.4, treats two strings as equal if one is a substring of the other, which allows remote attackers to spoof trusted certificates via a crafted subjectAltName.
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Analysis
by VulDB Data Team • 05/04/2026
The vulnerability identified as CVE-2010-1194 resides within the libESMTP library version 1.0.3.r1 and potentially affects subsequent versions including 1.0.4. This security flaw exists in the smtp-tls.c file within the match_component function, which handles the comparison of strings during TLS certificate validation processes. The issue stems from an improper string comparison algorithm that fails to distinguish between complete string matches and substring relationships, creating a fundamental weakness in the certificate validation mechanism.
The technical flaw manifests when the match_component function evaluates subjectAltName entries in X.509 certificates during SMTP TLS handshakes. Instead of performing strict equality checks between the expected hostname and the certificate's subjectAltName field, the function accepts a match if one string is merely a substring of another. This behavioral deviation creates a critical security gap where an attacker can craft a malicious certificate containing a subjectAltName that is a substring of a legitimate trusted domain name. For instance, if a certificate contains a subjectAltName of "example.com" and the system expects "trusted.example.com", an attacker could potentially exploit this vulnerability by presenting a certificate with "example.com" as the subjectAltName, which would pass validation due to the substring matching logic.
This vulnerability directly impacts the integrity of email security systems that rely on libESMTP for TLS certificate validation. The operational consequences are severe as it allows remote attackers to conduct man-in-the-middle attacks against SMTP communications, potentially intercepting or modifying email traffic between servers. The flaw undermines the trust model of TLS certificates, enabling attackers to spoof certificates for domains they do not control, thereby compromising the authentication mechanisms that protect email communications. The vulnerability is particularly dangerous in environments where email servers use libESMTP for secure mail transmission and rely on certificate validation to ensure server authenticity.
The security implications of this vulnerability align with CWE-254, which addresses security weaknesses related to improper input validation and string comparison functions. This weakness creates a path for attackers to bypass certificate validation entirely, potentially enabling them to establish unauthorized communication channels. According to ATT&CK framework, this vulnerability corresponds to T1566.001 - Phishing: Spearphishing Attachment, as it enables attackers to craft convincing certificate spoofing attacks that can bypass security controls. The vulnerability also relates to T1071.004 - Application Layer Protocol: DNS, as it can be exploited to manipulate DNS resolution through compromised email server communications. Organizations using affected versions of libESMTP should immediately upgrade to patched versions or implement alternative certificate validation mechanisms, while network administrators should monitor for suspicious certificate validation failures and implement additional security controls such as certificate pinning to mitigate potential exploitation.
This vulnerability demonstrates the critical importance of proper string comparison functions in cryptographic security implementations. The flaw represents a classic example of how seemingly minor coding errors in security-sensitive functions can have catastrophic consequences for entire communication systems. The improper substring matching logic essentially creates a backdoor in the certificate validation process, allowing attackers to exploit trust relationships that should remain secure. System administrators and security professionals must understand that certificate validation is a fundamental security control, and any weakness in this area can compromise the entire security posture of email infrastructure and related communication systems.