CVE-2017-17844 in Enigmail
Summary
by MITRE
An issue was discovered in Enigmail before 1.9.9. A remote attacker can obtain cleartext content by sending an encrypted data block (that the attacker cannot directly decrypt) to a victim, and relying on the victim to automatically decrypt that block and then send it back to the attacker as quoted text, aka the TBE-01-005 "replay" issue.
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Analysis
by VulDB Data Team • 01/19/2023
The vulnerability identified as CVE-2017-17844 represents a sophisticated attack vector within the Enigmail email encryption plugin for Mozilla Thunderbird. This security flaw exists in versions prior to 1.9.9 and demonstrates a critical weakness in how encrypted email content is handled during the decryption process. The vulnerability stems from the improper handling of encrypted data blocks that are automatically decrypted by the victim's email client, creating an unintended information leakage channel that attackers can exploit to recover cleartext content from encrypted messages.
The technical mechanism behind this vulnerability involves a specific interaction between the Enigmail plugin and the underlying email client's automatic decryption features. When a remote attacker sends an encrypted data block to a victim, the victim's Enigmail plugin automatically attempts to decrypt the content. However, due to flawed processing logic, the decrypted cleartext content can be inadvertently exposed when the victim replies to the message or forwards it as quoted text. This occurs because the plugin does not properly sanitize or isolate the decrypted content during the quoting process, allowing the attacker to reconstruct the original cleartext message from the quoted response. The vulnerability is classified as a type of text-based exploitation where the attacker leverages the legitimate decryption functionality to gain unauthorized access to encrypted information.
This flaw has significant operational impact on users of the affected Enigmail versions, as it undermines the fundamental security promise of email encryption. The vulnerability allows attackers to bypass encryption protections without needing to possess the private keys or overcome cryptographic barriers that would normally be required to decrypt messages. The attack requires minimal technical expertise and can be executed remotely, making it particularly dangerous for users who may not be aware of the risk. The issue affects any user who receives encrypted messages and subsequently replies to them, creating a persistent threat vector that can be exploited repeatedly against unsuspecting victims. Organizations relying on encrypted email communications for sensitive data protection face substantial risk of information disclosure, potentially compromising confidential business communications, personal data, or classified information.
The vulnerability aligns with several cybersecurity frameworks and threat models, including CWE-200, which addresses "Information Exposure," and relates to ATT&CK technique T1566, "Phishing," as it exploits user trust in legitimate email interactions. Additionally, this issue demonstrates characteristics of a privilege escalation attack where the attacker gains unauthorized access to information through legitimate user actions rather than direct system compromise. The security implications extend beyond individual users to organizational security posture, as the vulnerability can be exploited in targeted attacks against high-value targets. Mitigation strategies include immediate upgrading to Enigmail version 1.9.9 or later, implementing additional email filtering rules to detect and block suspicious quoted content patterns, and educating users about the risks of replying to encrypted messages without proper verification. Organizations should also consider implementing additional layers of security monitoring to detect potential exploitation attempts and establish incident response procedures specifically addressing this type of information leakage vulnerability.