CVE-2019-8338 in Airmail
Summary
by MITRE
The signature verification routine in the Airmail GPG-PGP Plugin, versions 1.0 (9) and earlier, does not verify the status of the signature at all, which allows remote attackers to spoof arbitrary email signatures by crafting a signed email with an invalid signature. Also, it does not verify the validity of the signing key, which allows remote attackers to spoof arbitrary email signatures by crafting a key with a fake user ID (email address) and injecting it into the user's keyring.
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Analysis
by VulDB Data Team • 09/21/2023
The vulnerability identified as CVE-2019-8338 affects the Airmail GPG-PGP Plugin version 1.0 (9) and earlier, representing a critical flaw in cryptographic signature verification mechanisms. This weakness stems from the complete absence of signature status verification within the plugin's implementation, creating a fundamental security gap that undermines the integrity of email communications. The vulnerability resides in the plugin's failure to properly validate cryptographic signatures, allowing malicious actors to exploit the system's trust model by crafting specially formatted emails with invalid signatures that would be accepted as legitimate by the vulnerable software.
The technical flaw manifests through two distinct attack vectors that together create a comprehensive spoofing capability. First, the plugin completely bypasses signature status verification, meaning that even if an email contains a malformed or forged signature, the system accepts it without validation. This represents a direct violation of cryptographic security principles where signature verification should confirm that the signature matches the content and was generated by the claimed signer. Second, the vulnerability extends to key validation, where the plugin fails to verify the legitimacy of signing keys, allowing attackers to inject malicious keys with fake user identifiers into users' keyrings. This dual failure creates a scenario where attackers can not only forge signatures but can also masquerade as legitimate senders by simply adding crafted keys to the trust chain.
The operational impact of this vulnerability is severe and far-reaching within email security contexts. Organizations relying on Airmail's GPG-PGP plugin for secure email communications face significant risks including man-in-the-middle attacks, email spoofing, and potential data compromise through malicious email content. The vulnerability undermines the core security assumptions of public key cryptography, where users expect that signed emails can be trusted to come from the claimed sender and contain unmodified content. Attackers can exploit this weakness to impersonate legitimate users, send malicious emails that appear to be signed by trusted parties, and potentially gain unauthorized access to systems or information through social engineering attacks that rely on the false appearance of authenticated communications.
From a cybersecurity perspective, this vulnerability aligns with CWE-347, which addresses the lack of proper validation of cryptographic signatures, and represents a failure in the principle of least privilege where the system accepts unverified cryptographic artifacts without proper authentication checks. The attack pattern follows ATT&CK technique T1566, which involves spearphishing with a malicious attachment or link, but in this case the attack leverages the trust model of the email system itself. The vulnerability also relates to CWE-295, which deals with improper certificate validation, as the system fails to properly validate the certificate chain or key legitimacy. Organizations should immediately implement mitigation strategies including updating to patched versions of the plugin, implementing additional email filtering rules, and conducting security awareness training to help users recognize potential spoofing attempts.
The remediation approach requires immediate patching of the Airmail GPG-PGP Plugin to version 1.1 or later, which addresses the signature verification and key validation issues. System administrators should also implement comprehensive monitoring of email traffic for suspicious signature patterns and consider implementing additional layers of email authentication such as DKIM and DMARC. Organizations should review their key management practices to ensure that only legitimate keys are added to user keyrings and implement automated key validation processes. The vulnerability demonstrates the critical importance of proper cryptographic implementation and the necessity of rigorous testing of security features, particularly in email client plugins where user trust and security assumptions are paramount. Without proper verification mechanisms, even well-intentioned security tools can become attack vectors that compromise the entire communication ecosystem.