CVE-2015-8313 in GnuTLS
Summary
by MITRE
GnuTLS incorrectly validates the first byte of padding in CBC modes
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Analysis
by VulDB Data Team • 11/11/2024
The vulnerability identified as CVE-2015-8313 represents a critical flaw in the GnuTLS cryptographic library that affects the validation of padding in cipher block chaining modes. This issue specifically targets the implementation of padding validation logic within the TLS protocol stack, where the library fails to properly verify the initial byte of padding data during decryption operations. The flaw exists in the way GnuTLS processes padding bytes when decrypting data encrypted using CBC mode algorithms, creating a potential avenue for attackers to exploit the cryptographic implementation.
The technical root cause of this vulnerability stems from improper handling of padding validation in CBC decryption routines. When GnuTLS processes encrypted data in CBC mode, it must verify that the padding bytes conform to the expected format according to PKCS#7 padding standards. The vulnerability manifests when the library only checks certain padding bytes while neglecting to properly validate the first byte of the padding structure, which is critical for ensuring the integrity of the decrypted plaintext. This incomplete validation allows attackers to manipulate the padding bytes in a way that could potentially lead to padding oracle attacks, where the attacker can infer information about the plaintext through careful analysis of the cryptographic responses.
The operational impact of CVE-2015-8313 extends beyond simple cryptographic weakness to encompass broader security implications for systems relying on GnuTLS for secure communications. Systems utilizing vulnerable versions of GnuTLS are at risk of exposure to padding oracle attacks that can gradually reveal sensitive information from encrypted communications, including session tokens, authentication credentials, or confidential data. The vulnerability affects any application or service that uses GnuTLS in its TLS implementation, particularly those employing CBC cipher suites, making it a widespread concern across numerous networked applications and infrastructure components. This weakness can be exploited by remote attackers without requiring authentication, potentially compromising the confidentiality and integrity of protected communications.
Mitigation strategies for this vulnerability require immediate patching of affected GnuTLS installations to the latest secure versions that contain corrected padding validation logic. Organizations should prioritize updating their GnuTLS implementations and ensuring that all systems using the library are running patched versions. Additionally, administrators should consider disabling CBC cipher suites in favor of more secure alternatives such as those based on authenticated encryption modes like GCM or CCM. The vulnerability aligns with CWE-209, which addresses improper handling of padding in cryptographic operations, and can be categorized under ATT&CK technique T1552.001 for unsecured credentials and T1552.004 for credentials in files, as the weakness could potentially enable attackers to extract sensitive information from encrypted communications. System administrators should also implement monitoring for unusual padding validation patterns and consider deploying intrusion detection systems that can identify potential exploitation attempts targeting this specific vulnerability.