CVE-2026-5419 Overview
CVE-2026-5419 is a timing side-channel vulnerability in GnuTLS, a widely deployed implementation of the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. The flaw resides in the PKCS#7 padding check performed during decryption, which executes in non-constant time. A remote attacker can measure observable timing differences to infer information about padding bytes. This places the issue in the padding oracle class of attacks, classified under [CWE-208] (Observable Timing Discrepancy). The vulnerability enables information disclosure but does not affect integrity or availability of the affected service.
Critical Impact
Remote attackers may leak sensitive padding information from GnuTLS decryption routines through statistical timing analysis, potentially aiding plaintext recovery in cryptographic operations.
Affected Products
- GnuTLS library (versions prior to the Red Hat patched releases)
- Red Hat Enterprise Linux distributions referenced in RHSA-2026:20612
- Red Hat Enterprise Linux distributions referenced in RHSA-2026:20613
Discovery Timeline
- 2026-06-01 - CVE-2026-5419 published to the National Vulnerability Database (NVD)
- 2026-06-02 - Last updated in NVD database
Technical Details for CVE-2026-5419
Vulnerability Analysis
GnuTLS performs PKCS#7 padding validation as part of block cipher decryption in Cipher Block Chaining (CBC) mode. The defective implementation processes padding bytes using branching logic whose execution time depends on padding validity. An attacker who can submit ciphertexts and measure response times can distinguish valid from invalid padding. Repeated measurements over many requests allow statistical recovery of padding structure, the classic precondition for a padding oracle attack. This issue is an information disclosure flaw scoped to confidentiality. The high attack complexity reflects the network noise and large sample sizes required to extract reliable timing signals.
Root Cause
The root cause is the absence of constant-time execution in the PKCS#7 padding validation routine within GnuTLS decryption. Conditional branches and early-exit paths produce measurable timing variance correlated with the contents of decrypted padding bytes. Cryptographic best practice requires all padding-check operations to execute in time independent of secret data. The vulnerable code path violates this requirement, yielding an observable side channel as described in [CWE-208].
Attack Vector
The attack vector is network-based and requires no authentication or user interaction. An adversary positioned to send crafted ciphertexts to a GnuTLS-backed service collects timing samples for each response. Statistical analysis of response latencies reveals padding validity, which can be chained with adaptive chosen-ciphertext queries to recover plaintext bytes. Exploitation complexity is high due to network jitter, but co-located attackers or those on adjacent networks face fewer obstacles.
No verified public exploit code or proof-of-concept is available at the time of publication. The vulnerability manifests in the PKCS#7 padding verification routine; refer to the Red Hat CVE Analysis CVE-2026-5419 for technical details.
Detection Methods for CVE-2026-5419
Indicators of Compromise
- High volumes of TLS or PKCS#7 decryption requests originating from a single source within short time windows, consistent with timing measurement collection.
- Repeated malformed or unusual ciphertext submissions to services that invoke GnuTLS decryption.
- Anomalous client behavior characterized by precise, regular request intervals targeting cryptographic endpoints.
Detection Strategies
- Inventory all systems running GnuTLS using package management queries such as rpm -q gnutls or dpkg -l gnutls and correlate versions against Red Hat advisories RHSA-2026:20612 and RHSA-2026:20613.
- Deploy network monitoring rules that flag clients issuing high-frequency cryptographic requests with statistically uniform timing patterns.
- Use vulnerability scanners with updated signatures referencing CVE-2026-5419 to identify unpatched library versions across the environment.
Monitoring Recommendations
- Log and review TLS handshake failures and decryption errors at scale to surface padding-oracle probing.
- Track request-per-second baselines for cryptographic services and alert on sustained deviations.
- Forward GnuTLS-linked application logs to a centralized analytics platform for cross-host correlation of suspicious decryption activity.
How to Mitigate CVE-2026-5419
Immediate Actions Required
- Apply the patched GnuTLS packages distributed through Red Hat Security Advisory RHSA-2026:20612 and Red Hat Security Advisory RHSA-2026:20613 on all affected hosts.
- Restart services that link against GnuTLS so updated library code is loaded into running processes.
- Audit dependent applications that perform PKCS#7 decryption to confirm they consume the patched library.
Patch Information
Red Hat has released updated GnuTLS packages that implement constant-time PKCS#7 padding verification. Customers should consult Red Hat CVE Analysis CVE-2026-5419 and the corresponding Red Hat Bug Report #2467686 for package versions and downstream tracking. Upstream GnuTLS users should apply the equivalent fix from their distribution maintainer.
Workarounds
- Restrict network exposure of services that invoke GnuTLS PKCS#7 decryption by placing them behind authenticated gateways or trusted network segments.
- Apply rate limiting and request throttling to cryptographic endpoints to raise the cost of timing measurement collection.
- Where feasible, disable CBC-mode cipher suites in favor of authenticated encryption modes such as AES-GCM until patches are deployed.
# Verify installed GnuTLS version and apply vendor updates
rpm -q gnutls
sudo dnf update gnutls
sudo systemctl restart <service-using-gnutls>
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
