CVE-2013-4351 in GnuPG
Summary
by MITRE
GnuPG 1.4.x, 2.0.x, and 2.1.x treats a key flags subpacket with all bits cleared (no usage permitted) as if it has all bits set (all usage permitted), which might allow remote attackers to bypass intended cryptographic protection mechanisms by leveraging the subkey.
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Analysis
by VulDB Data Team • 05/25/2021
The vulnerability identified as CVE-2013-4351 represents a critical flaw in the GNU Privacy Guard implementation that affects versions 1.4.x, 2.0.x, and 2.1.x of the software. This issue stems from an improper handling of key flags subpackets within OpenPGP key structures, specifically when all bits in the subpacket are cleared. The vulnerability operates at the cryptographic protocol level where GnuPG incorrectly interprets a key flags subpacket with all bits set to zero as if all bits were set to one, fundamentally altering the security posture of encrypted communications. This misinterpretation occurs during the key validation process when GnuPG processes subkey information, creating a scenario where keys that should be restricted from certain cryptographic operations are treated as fully permitted.
The technical implementation flaw manifests in the key validation logic where GnuPG fails to properly distinguish between a key flags subpacket that explicitly indicates no usage permissions and one that might be malformed or improperly processed. This condition creates a bypass mechanism where attackers can manipulate key structures to appear as if they have unrestricted cryptographic capabilities. The vulnerability specifically impacts the key flags subpacket format which is defined in the OpenPGP standard and contains information about how a key should be used for encryption, signing, or certification operations. When all bits are cleared in this subpacket, the standard interpretation should be that no usage is permitted, but GnuPG's implementation incorrectly treats this as full permission, effectively disabling the intended cryptographic restrictions.
The operational impact of this vulnerability is significant as it allows remote attackers to bypass cryptographic protection mechanisms that are designed to prevent unauthorized use of keys. An attacker could potentially exploit this by crafting malicious key structures that appear to have full usage permissions when they should be restricted, thereby enabling unauthorized encryption or signing operations. This vulnerability particularly affects scenarios where subkeys are used with specific usage restrictions, such as when a master key is used for certification only while subkeys handle encryption and signing operations. The flaw creates a scenario where an attacker could leverage a compromised or malicious subkey to perform operations that should be restricted by the key flags, potentially leading to data compromise or unauthorized cryptographic operations. This issue has direct implications for the security of email communications, file encryption, and digital signatures that rely on proper key flag enforcement.
Mitigation strategies for CVE-2013-4351 require immediate software updates to patched versions of GnuPG that properly implement key flags subpacket validation according to the OpenPGP specification. System administrators should ensure all instances of GnuPG are updated to versions that address this specific parsing error, as the vulnerability exists in multiple major releases and affects the fundamental cryptographic operations of the software. Additionally, organizations should implement key validation procedures that explicitly check for malformed key flags subpackets and consider implementing monitoring systems that can detect unusual key usage patterns that might indicate exploitation attempts. The vulnerability aligns with CWE-254 in that it represents a security flaw in the handling of cryptographic key information, and from an ATT&CK perspective, it maps to techniques involving privilege escalation and credential access through manipulation of cryptographic protocols, specifically targeting the execution of unauthorized cryptographic operations that should be restricted by key flags.