CVE-2018-16868 in GnuTLS
Summary
by MITRE
A Bleichenbacher type side-channel based padding oracle attack was found in the way gnutls handles verification of RSA decrypted PKCS#1 v1.5 data. An attacker who is able to run process on the same physical core as the victim process, could use this to extract plaintext or in some cases downgrade any TLS connections to a vulnerable server.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 04/16/2020
The vulnerability identified as CVE-2018-16868 represents a critical side-channel attack surface within the GNU TLS library's implementation of RSA padding verification. This flaw specifically targets the handling of PKCS#1 v1.5 formatted data during RSA decryption operations, creating a padding oracle condition that adversaries can exploit through sophisticated timing-based side-channel analysis. The vulnerability stems from the library's inconsistent response timing when processing invalid padding structures, allowing attackers to infer information about the decrypted data through careful measurement of cryptographic operation delays.
The technical implementation of this vulnerability resides in the cryptographic verification process where GNU TLS performs RSA decryption followed by PKCS#1 v1.5 padding validation. When the library encounters malformed padding, it does not execute the validation with constant time operations, instead exhibiting different execution times based on the specific padding error encountered. This timing variation creates a predictable oracle that reveals information about the plaintext, enabling attackers to perform iterative padding oracle attacks similar to the classic Bleichenbacher attack methodology. The vulnerability is particularly dangerous because it operates at the protocol level rather than requiring direct access to cryptographic keys or plaintext data.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables sophisticated man-in-the-middle attacks that can compromise entire TLS connections. An attacker positioned on the same physical core as a victim process can leverage cache timing attacks to extract sensitive information from the cryptographic operations, potentially leading to complete session compromise. The vulnerability is especially concerning for TLS servers that use RSA key exchange, as it allows attackers to downgrade connections to less secure cipher suites or extract session keys. This attack vector directly violates the fundamental security principles of TLS encryption and can result in complete loss of confidentiality and integrity for affected communications.
Mitigation strategies for CVE-2018-16868 require immediate patching of affected GNU TLS versions, with organizations prioritizing updates to versions that implement constant-time padding validation. System administrators should also consider implementing additional security controls such as CPU isolation to prevent co-located malicious processes and monitoring for unusual timing patterns in cryptographic operations. The vulnerability aligns with CWE-203 - Observable Behavioral Vulnerability and maps to ATT&CK technique T1059.001 - Command and Scripting Interpreter, specifically through the exploitation of timing-based side channels. Organizations should also consider transitioning away from RSA key exchange mechanisms toward more secure alternatives like ECDHE, and implement proper cryptographic protocol configuration to avoid falling victim to downgrade attacks that this vulnerability enables.