CVE-2026-5419: 
GnuTLS vulnerability analysis and mitigation

CVE-2026-5419 is an information disclosure vulnerability in GnuTLS caused by a non-constant-time PKCS#7 padding check during decryption. The timing side-channel allows a remote, unauthenticated attacker to infer sensitive information about padding bytes by observing measurable differences in decryption response times. Affected software includes GnuTLS as shipped with Red Hat Enterprise Linux 9 and 10 (patched in gnutls-3.8.10-4.el9_8 and gnutls-3.8.10-4.el10_2), as well as upstream GnuTLS versions prior to 3.8.13. The vulnerability was first reported on May 7, 2026, and publicly disclosed on June 1, 2026. It carries a CVSS v3.1 base score of 3.7 (Low) (Github Advisory, Red Hat RHSA-2026:20613).

The root cause is classified as CWE-208 (Observable Timing Discrepancy): the PKCS#7 padding removal routine in GnuTLS's decryption path used conditional branching logic rather than branch-free, constant-time operations, causing execution time to vary based on the content of padding bytes. An attacker positioned on the network can repeatedly submit crafted ciphertext and measure response latency to statistically infer padding byte values — a classic padding oracle-style timing attack. No authentication or user interaction is required, but exploitation demands high attack complexity due to the need for precise, repeated timing measurements across a network channel. The fix rewrites the padding removal to use a branch-free approach, eliminating the observable timing difference (Red Hat Bugzilla, Github Advisory).

Successful exploitation results in a limited confidentiality breach: an attacker can gradually leak information about PKCS#7 padding bytes from decrypted data, potentially enabling partial recovery of plaintext from encrypted communications protected by GnuTLS. There is no impact on integrity or availability. The scope is limited to the vulnerable component, and the attack does not enable code execution, privilege escalation, or lateral movement. The practical risk is constrained by the high attack complexity and the incremental nature of information leakage (Github Advisory, Red Hat Bugzilla).

The primary remediation is to update GnuTLS to a patched version. Red Hat has released fixes via:

• RHEL 9: gnutls-3.8.10-4.el9_8 (RHSA-2026:20612)
• RHEL 10: gnutls-3.8.10-4.el10_2 (RHSA-2026:20613, RHSA-2026:26409)
• Upstream: GnuTLS 3.8.13 addresses this and 11 other vulnerabilities

Patches are also available for Amazon Linux 2023, Ubuntu (USN-8284-1), openSUSE/SUSE (SUSE-SU-2026:2115-1), and Debian. As a network-level mitigation, adding random delays to decryption responses can reduce timing side-channel exposure, though patching is strongly preferred. Monitor for unusual volumes of repeated decryption requests that could indicate timing analysis attempts (Red Hat RHSA-2026:20613, Red Hat RHSA-2026:20612, Github Advisory).

The vulnerability was disclosed as part of a broader GnuTLS 3.8.13 release that addressed 12 security issues, which received coverage from security news outlets including CyberSecurityNews and Cryptika. Community discussion noted the release on platforms such as the Solus Linux forums. The fix was described technically in the Red Hat Bugzilla as rewriting the padding removal to use a branch-free manner, eliminating the timing discrepancy. Overall community reaction has been measured, consistent with the low CVSS score and absence of active exploitation (cybersecuritynews.com, Red Hat Bugzilla).