CVE-2026-5419

What it actually does

The vulnerability is a timing side channel in GnuTLS during PKCS#7 padding validation performed as part of decryption. Because the padding check is not implemented in constant time, different padding byte patterns can produce measurably different response times. In a suitable protocol or application context where an attacker can repeatedly submit chosen ciphertexts and observe precise timing behavior, the attacker may infer information about decrypted padding and potentially recover limited plaintext information. The CVSS score is low because exploitation requires high attack complexity, reliable timing measurements, many repeated attempts, and a vulnerable application behavior that exposes the decryption oracle over the network.

Why it matters to the business

Systems and applications that rely on affected GnuTLS versions for decrypting PKCS#7-padded data may expose sensitive information under specific conditions. The most likely impact is confidentiality loss rather than service disruption or data modification. Although there is no known public exploit, no CISA KEV listing, low EPSS probability, and no known ransomware association, organizations using GnuTLS in internet-facing or high-sensitivity cryptographic workflows should still patch during normal maintenance because cryptographic side-channel issues can become more practical when combined with application-specific behavior.

How an attack unfolds

An attacker first identifies a service or application that uses a vulnerable GnuTLS build to decrypt attacker-controlled ciphertext using PKCS#7 padding. The attacker then sends many carefully crafted encrypted messages and measures the time taken for each response or failure condition. Because the padding verification path takes slightly different amounts of time depending on the decrypted padding bytes, the attacker statistically analyzes response timing over repeated trials to distinguish valid and invalid padding-related states. If network jitter is low enough and the application exposes enough repeated decryption attempts, the attacker may gradually infer information about the protected plaintext. Exploitation is expected to be difficult in typical internet conditions and depends heavily on the surrounding protocol, error handling, rate limits, and timing noise.

How to fix it

Upgrade GnuTLS to a vendor-provided fixed release when available for the affected operating system, distribution, appliance, container image, or embedded product. Because no specific fixed versions were provided in the input, use the security advisory from the relevant Linux distribution, package vendor, or product supplier as the source of truth. Rebuild or restart applications that dynamically link to GnuTLS after applying updates so the patched library is loaded. For statically linked software, rebuild the application against a corrected GnuTLS version. As interim mitigation, reduce exposure of services that decrypt attacker-controlled PKCS#7 data, require authentication before decryption where possible, apply rate limiting to repeated malformed ciphertext submissions, normalize error responses, and avoid exposing precise timing differences through application behavior.

How to confirm the fix worked

Confirm the installed GnuTLS package version is at or above the fixed version published by the relevant vendor. Validate that running processes have been restarted and are using the patched shared library rather than an older in-memory copy. On Linux systems, package checks may include commands such as `rpm -q gnutls`, `dpkg -l | grep gnutls`, or distribution-specific vulnerability scanners. For applications packaged in containers, verify the base image and all runtime layers include the updated GnuTLS package. For statically linked binaries, confirm the binary was rebuilt against the corrected library. Exposure validation should focus on whether any externally reachable service accepts attacker-controlled encrypted PKCS#7 content and uses GnuTLS for decryption.

Typical attack path

1. Identify a network-accessible application or service linked against a vulnerable GnuTLS version and performing PKCS#7-padded decryption on attacker-supplied data.
2. Confirm that the application allows repeated ciphertext submissions and produces observable timing differences or consistent failure behavior.
3. Generate crafted ciphertext variants designed to influence decrypted padding bytes.
4. Send large numbers of requests while collecting high-resolution response timing data.
5. Use statistical analysis to separate timing differences caused by padding-check behavior from normal network and system noise.
6. Iteratively infer padding-related information and attempt to recover limited sensitive plaintext, depending on the application protocol and cryptographic construction.

How to detect it

Detecting successful exploitation is difficult because timing side-channel attacks often leave no definitive artifact and may look like normal failed decryption attempts. Focus on identifying probing behavior: unusually high volumes of malformed encrypted messages, repeated PKCS#7 or CMS parsing/decryption failures from the same source, high-frequency requests with similar payload sizes, and requests designed to trigger cryptographic error paths. Relevant log sources include application logs, reverse proxy and load balancer logs, TLS-terminating service logs where applicable, mail or CMS-processing service logs, WAF logs, network flow records, and host telemetry showing applications repeatedly invoking GnuTLS-backed decryption routines. Investigations should look for repeated failures over long periods, low-and-slow request patterns intended to gather timing data, and source IPs that send many near-identical encrypted payloads while causing padding or decryption errors. There are no public exploit-specific indicators or known threat actor campaigns in the provided data, so detection should be behavioral rather than signature-based.

What your security team should do

• Inventory servers, containers, appliances, and applications that include or link against GnuTLS, especially those exposed to untrusted networks.
• Prioritize patching internet-facing services, cryptographic gateways, mail/security tooling, VPN-related components, and applications that decrypt attacker-supplied PKCS#7/CMS data.
• Apply vendor security updates for GnuTLS and restart dependent services to ensure the patched library is loaded.
• For containerized workloads, rebuild and redeploy images from patched base images rather than only patching running containers.
• Apply rate limiting and authentication controls around endpoints that process encrypted blobs or malformed cryptographic input.
• Review application behavior to ensure cryptographic failures return uniform errors and do not expose detailed timing or padding failure distinctions.
• Monitor for unusual volumes of malformed encrypted requests, repeated decryption failures, or timing-probing behavior against services using GnuTLS.
• Track vendor advisories for exact fixed versions because fixed-version data was not provided in the CVE input.