CVE-2017-17848 in Enigmail
Summary
by MITRE
An issue was discovered in Enigmail before 1.9.9. In a variant of CVE-2017-17847, signature spoofing is possible for multipart/related messages because a signed message part can be referenced with a cid: URI but not actually displayed. In other words, the entire containing message appears to be signed, but the recipient does not see any of the signed text.
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Analysis
by VulDB Data Team • 01/19/2023
The vulnerability described in CVE-2017-17848 represents a sophisticated signature spoofing attack targeting the Enigmail email encryption plugin for Mozilla Thunderbird. This issue affects versions prior to 1.9.9 and demonstrates how cryptographic verification mechanisms can be circumvented through manipulation of multipart email structures. The flaw specifically exploits the handling of multipart/related messages where the cryptographic signature validation process becomes ineffective due to improper content referencing. This vulnerability falls under the broader category of cryptographic weakness and represents a significant security concern for email communication systems that rely on proper signature verification for message authenticity.
The technical implementation of this vulnerability occurs within the message parsing and signature validation logic of Enigmail. When processing multipart/related messages, the plugin allows a signed message part to be referenced using a cid: URI scheme without ensuring that the referenced content is actually displayed to the recipient. This creates a scenario where the entire message appears to carry a valid signature, but the recipient only sees portions of the message that were not actually signed. The underlying mechanism exploits the difference between how the signature verification process operates versus how the message content is rendered to users, creating a deceptive state where cryptographic integrity appears intact while actual content verification is bypassed. This represents a classic case of improper validation of message structure and content display.
The operational impact of CVE-2017-17848 extends beyond simple message integrity concerns to encompass potential deception and trust manipulation in email communications. An attacker could craft messages that appear to be properly signed and verified while actually containing unsigned content that may be malicious or misleading. This vulnerability directly undermines the fundamental security assumptions of email encryption systems, as recipients might trust messages that contain unverified content. The attack vector is particularly dangerous because it leverages legitimate email protocols and standards, making it difficult to detect through conventional security monitoring. This type of vulnerability aligns with attack patterns documented in the ATT&CK framework under credential access and defense evasion techniques, as it enables attackers to bypass security controls while maintaining the appearance of legitimate communication.
Mitigation strategies for CVE-2017-17848 require immediate patching of affected Enigmail versions to 1.9.9 or later, which implements proper validation of multipart/related message structures. System administrators should ensure all email clients using Enigmail are updated to prevent exploitation of this signature spoofing vulnerability. Additional defensive measures include implementing email security policies that mandate signature verification for all received messages, configuring email servers to reject unsigned messages when possible, and educating users about the importance of verifying message authenticity beyond visual appearance. The vulnerability also highlights the need for comprehensive testing of cryptographic implementations against edge cases in message structure handling, particularly for complex multipart email formats. Organizations should consider implementing automated security scanning tools that can detect malformed message structures and signature inconsistencies to provide additional layers of protection against similar vulnerabilities.
This vulnerability demonstrates the complexity of implementing secure cryptographic systems in email applications and the importance of thorough testing of edge cases. The issue connects to CWE-347, which addresses improper verification of cryptographic signatures, and represents a failure in proper input validation and message integrity checking. The attack scenario illustrates how seemingly minor implementation flaws in cryptographic verification can have significant security implications, particularly in systems where user trust relies heavily on visual indicators of security status. Security professionals should consider this vulnerability as part of broader email security assessments and ensure that all cryptographic verification processes properly account for message structure variations and content display mechanisms.