CVE-2017-17847 in Enigmail
Summary
by MITRE
An issue was discovered in Enigmail before 1.9.9. Signature spoofing is possible because the UI does not properly distinguish between an attachment signature, and a signature that applies to the entire containing message, aka TBE-01-021. This is demonstrated by an e-mail message with an attachment that is a signed e-mail message in message/rfc822 format.
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Analysis
by VulDB Data Team • 01/19/2023
The vulnerability identified as CVE-2017-17847 represents a critical signature spoofing flaw in Enigmail versions prior to 1.9.9, which fundamentally undermines the trust model of email encryption systems. This issue stems from a user interface design flaw that fails to properly differentiate between two distinct types of digital signatures within the same email message structure. The vulnerability specifically affects the visualization and interpretation of cryptographic signatures when an email contains both an attachment that is itself a signed email message in message/rfc822 format and the main message body that may or may not be signed. This creates a scenario where users cannot reliably determine whether the signature applies to the entire message or only to the attachment, leading to potential misinterpretation of message authenticity and integrity. The flaw essentially allows attackers to craft malicious emails that appear to contain a valid signature for the entire message while only the attachment portion is actually signed, creating a deceptive user experience that bypasses security expectations.
The technical implementation of this vulnerability resides in the signature verification and display logic within Enigmail's user interface components. When processing email messages with embedded signed attachments, the system fails to properly categorize and display signature information for different message parts. This UI deficiency means that users see a single signature indicator that does not clearly distinguish between a signature covering the entire message context versus one that only applies to a specific attachment. The vulnerability manifests when an email contains an attachment formatted as message/rfc822, which is a standard format for encapsulating email messages within other email messages. The signature verification process should theoretically distinguish between these two contexts, but the flawed implementation allows the signature of the embedded message to be incorrectly interpreted as applying to the parent message. This behavior directly violates security expectations established by cryptographic signature standards where each signature should clearly indicate its scope and validity boundaries.
The operational impact of CVE-2017-17847 extends beyond simple user confusion to represent a significant threat to email security and trust integrity. Attackers can exploit this vulnerability to create deceptive email communications that appear legitimate to users who may believe an entire message is cryptographically verified when only a portion is actually signed. This signature spoofing capability undermines the fundamental security principles of email encryption systems and can enable various attack vectors including social engineering campaigns, phishing attempts, and man-in-the-middle attacks where attackers can manipulate user expectations about message authenticity. The vulnerability particularly affects organizations that rely heavily on encrypted email communications for sensitive data exchange, as users may inadvertently trust messages that contain only partial signatures while believing they have full message integrity verification. The impact is further amplified by the fact that many users may not understand the technical distinction between different signature scopes, making this vulnerability particularly dangerous in operational environments where security awareness may be limited.
This vulnerability aligns with CWE-347, which addresses improper verification of cryptographic signatures, and demonstrates characteristics consistent with attack patterns described in the MITRE ATT&CK framework under the T1566 technique for phishing and T1059 for command and control communications. The flaw represents a UI/UX security issue that violates the principle of least privilege and proper security labeling in cryptographic systems. Organizations should implement immediate mitigation strategies including updating to Enigmail version 1.9.9 or later, which contains the necessary signature verification improvements. Additional defensive measures include user education programs that emphasize the importance of verifying signature scope and context, implementation of automated signature validation checks in email processing pipelines, and regular security audits of email encryption systems. The vulnerability also highlights the importance of proper cryptographic signature handling in email clients and reinforces the need for rigorous testing of security features in cryptographic software implementations. Security teams should monitor for potential exploitation attempts and ensure that email systems properly validate the scope and context of digital signatures before considering messages as fully authenticated.