CVE-2018-18509 in Thunderbird
Summary
by MITRE
A flaw during verification of certain S/MIME signatures causes emails to be shown in Thunderbird as having a valid digital signature, even if the shown message contents aren't covered by the signature. The flaw allows an attacker to reuse a valid S/MIME signature to craft an email message with arbitrary content. This vulnerability affects Thunderbird < 60.5.1.
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Analysis
by VulDB Data Team • 09/07/2023
This vulnerability represents a critical cryptographic flaw in Mozilla Thunderbird's S/MIME signature verification implementation that undermines the fundamental security guarantees of digital signatures. The issue stems from how Thunderbird processes S/MIME signatures when certain cryptographic parameters are present, specifically when the signature covers only a portion of the message content while appearing to validate the entire message. This creates a false positive scenario where users are deceived into believing that the complete email content has been authenticated by a valid digital signature, when in reality only specific elements may be covered by the cryptographic verification. The flaw directly violates the principles of data integrity and authentication that digital signatures are designed to provide, creating a significant attack surface for malicious actors seeking to compromise email communications.
The technical root cause of this vulnerability lies in Thunderbird's improper handling of S/MIME signature verification algorithms, particularly when dealing with signed message parts that contain a signature covering only specific portions of the email. This behavior can be traced to CWE-310, which addresses cryptographic weaknesses in signature verification processes, and more specifically to CWE-327, which deals with the use of weak or broken cryptographic algorithms. The vulnerability manifests when Thunderbird encounters S/MIME messages where the signature covers elements such as the message headers or specific content sections while omitting other parts that may have been modified by an attacker. This misimplementation allows for signature reuse attacks where an attacker can take a valid S/MIME signature from one message and apply it to a different message content, creating a false impression of authenticity while maintaining the cryptographic appearance of legitimate verification.
The operational impact of this vulnerability extends beyond simple deception, as it enables sophisticated social engineering and phishing attacks where attackers can craft convincing fraudulent emails that appear to be from legitimate sources. According to ATT&CK framework category T1566, this vulnerability facilitates social engineering techniques by exploiting user trust in digital signatures, while also aligning with T1071 which covers application layer protocol usage. Attackers can leverage this weakness to create emails that appear to be signed by trusted organizations or individuals, potentially bypassing security controls and user skepticism that would normally be present when encountering unsigned messages. The vulnerability affects users across various threat scenarios including corporate espionage, identity theft, and information warfare, as the false authentication can be particularly effective in environments where S/MIME signatures are routinely expected and trusted. Organizations relying on S/MIME for email security may experience significant operational disruption if attackers successfully exploit this vulnerability to compromise their email communications.
The remediation approach for this vulnerability requires immediate patching of Thunderbird installations to version 60.5.1 or later, which contains the necessary cryptographic verification fixes. Security administrators should implement comprehensive email security monitoring to detect potential exploitation attempts and establish proper incident response procedures for signature-related security events. Organizations should consider implementing additional email security layers such as DKIM and DMARC validation to provide defense-in-depth against signature manipulation attacks. The vulnerability highlights the importance of proper cryptographic implementation and the necessity of thorough security testing for cryptographic components, particularly in email clients where users place significant trust in digital signature verification. Regular security assessments of email systems and user education about the limitations of digital signatures, including awareness of this specific vulnerability, should be part of ongoing security program initiatives to prevent exploitation of similar cryptographic weaknesses in other email platforms and security tools.