CVE-2017-17688 in OpenPGP
Summary
by MITRE
** DISPUTED ** The OpenPGP specification allows a Cipher Feedback Mode (CFB) malleability-gadget attack that can indirectly lead to plaintext exfiltration, aka EFAIL. NOTE: third parties report that this is a problem in applications that mishandle the Modification Detection Code (MDC) feature or accept an obsolete packet type, not a problem in the OpenPGP specification.
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Analysis
by VulDB Data Team • 08/06/2024
The vulnerability identified as CVE-2017-17688 represents a significant security flaw in OpenPGP implementations that exploits weaknesses in the Cipher Feedback Mode (CFB) encryption mechanism. This vulnerability, commonly known as EFAIL, demonstrates how cryptographic protocols can be undermined through sophisticated attack vectors that manipulate the underlying encryption modes rather than targeting the cryptographic primitives themselves. The issue arises from the way OpenPGP handles the Modification Detection Code (MDC) feature and its interaction with certain packet types, creating opportunities for attackers to manipulate encrypted content without breaking the cryptographic encryption itself.
The technical flaw manifests in the CFB mode implementation where attackers can exploit malleability properties to manipulate encrypted data in predictable ways. When OpenPGP implementations improperly handle MDC packets or accept obsolete packet types, attackers can craft specific modifications to encrypted messages that, when decrypted, reveal information about the original plaintext. This occurs because the CFB mode's feedback mechanism allows for controlled alterations to the ciphertext that, when processed through the decryption routine, can produce predictable changes in the decrypted output. The vulnerability operates at the protocol level rather than the cryptographic algorithm level, making it particularly insidious as it can bypass traditional cryptographic security measures.
The operational impact of this vulnerability extends beyond simple data corruption or encryption bypass. Attackers can leverage this weakness to perform indirect plaintext exfiltration, where sensitive information is gradually revealed through carefully crafted modifications to encrypted communications. This attack vector is particularly dangerous in email systems where OpenPGP is commonly used for secure communication, as it allows adversaries to extract confidential data from encrypted messages without requiring access to the encryption keys. The vulnerability affects implementations that do not properly validate MDC integrity checks or that continue to support deprecated packet types that were never intended for active use in modern secure communications.
Mitigation strategies for CVE-2017-17688 require comprehensive updates to OpenPGP implementations across affected systems. Organizations must ensure their cryptographic libraries properly validate MDC packets and reject obsolete packet types that contribute to the vulnerability. The remediation process involves implementing strict validation of encryption packet structures and ensuring that applications properly handle the Modification Detection Code feature according to current OpenPGP standards. Security patches should enforce proper MDC verification mechanisms and eliminate support for deprecated cryptographic packet formats that enable this attack. This vulnerability aligns with CWE-310 and follows ATT&CK techniques related to cryptographic attacks and data manipulation, emphasizing the need for robust protocol-level validation rather than relying solely on cryptographic strength alone.
The disputed nature of this vulnerability highlights the distinction between specification weaknesses and implementation flaws. While the OpenPGP specification itself may not contain inherent flaws, the way various implementations handle MDC features and packet validation creates exploitable conditions. This underscores the importance of proper implementation practices and adherence to cryptographic best practices. The vulnerability serves as a critical reminder that even well-established protocols can contain implementation-specific weaknesses that require careful attention to protocol-level security considerations. Security professionals must recognize that the responsibility for preventing such attacks often lies with proper implementation rather than specification flaws, making thorough testing and validation of cryptographic implementations essential for maintaining secure communications.