CVE-2026-52936: Linux Jitterentropy Spinlock Denial of Service – Patch Guide

Technical summary

The jent_kcapi_random() function in the Linux kernel's jitterentropy implementation uses a spinlock (rng->jent_lock) to serialize access to shared entropy generation state. The vulnerability arises because this spinlock is held across the jent_read_entropy() call, which performs computationally expensive jitter collection and SHA3-based entropy conditioning. Under contention, spinning threads consume CPU cycles without yielding, creating lock-hold pathology. The resolution replaces the spinlock with a mutex, enabling blocked callers to sleep rather than spin-wait. This reduces CPU overhead, improves scheduler fairness, and mitigates availability impact from lock contention under high-frequency random number requests.

Business impact

Organizations running affected Linux kernel versions may experience degraded system performance and availability when workloads generate random numbers at scale (e.g., cryptographic operations, TLS handshakes, container initialization). High-contention scenarios can lead to CPU stalls, increased latency in security-critical operations, and in severe cases, denial of service effects. Systems with containerized or virtualized workloads that parallelize entropy requests are particularly vulnerable to noticeable performance regression. This is especially concerning for services requiring frequent cryptographic operations or those deployed on systems with limited CPU resources.

Affected systems

The Linux kernel is affected across versions implementing the jitterentropy RNG with the vulnerable spinlock-based serialization. This impacts all major Linux distributions (Red Hat, Ubuntu, Debian, SUSE, etc.) using affected kernel versions. Virtualized and containerized environments are at higher risk due to multiplexed entropy demand. Kernel versions prior to the fix that use jitterentropy as a random number generator source are in scope; verify your distribution's kernel release notes and vendor advisories for specific affected and patched versions.

Exploitability

This vulnerability does not require privileged access to trigger; any unprivileged user can invoke jitterentropy-based random number generation (via /dev/urandom, OpenSSL, or direct ioctl calls). Exploitation is trivial—simply spawn parallel threads requesting random data—making it easily weaponizable for local denial of service. The CVSS vector AV:L/AC:L/PR:L indicates local attack vector with low complexity and low privilege requirement, confirming ease of trigger. No special tools or sophisticated techniques are needed; standard cryptographic operations under load suffice.

Remediation

Apply kernel updates from your Linux distribution that include the spinlock-to-mutex conversion in jitterentropy. Most major vendors have backported or will backport this fix to supported kernel branches. Verify patch availability through your vendor's security advisory and kernel release notes. No workarounds exist short of disabling jitterentropy as an entropy source (not recommended without understanding your system's RNG stack). Prioritize systems exposed to untrusted users or high-frequency cryptographic workloads for expedited patching.

Patch guidance

Check your distribution's official security advisories and kernel update channels for the jitterentropy mutex patch. Red Hat, Ubuntu, Debian, and other vendors typically publish kernel security updates through their standard mechanisms (yum, apt, zypper). Test kernel updates in a staging environment first, especially on production systems, to confirm compatibility with your workloads. Verify the updated kernel includes the spinlock-to-mutex fix before considering the system remediated. Reboot systems to activate the patched kernel.

Detection guidance

Monitor for symptoms of lock contention on systems running affected kernels: elevated CPU usage during encryption/decryption operations, increased system latency, or processes waiting on jitterentropy locks. Review kernel logs for contention indicators if available. For detection-focused organizations, audit kernel version and jitterentropy implementation details across your fleet; correlate with timing of cryptographic workload spikes. Consider performance regression testing of random number generation under parallel load as a validation step post-patch.

Why prioritize this

Although the CVSS score of 5.5 is moderate, the practical impact is significant: local availability attacks are trivial to execute, affecting any system where unprivileged users can spawn processes. This is particularly urgent for multi-tenant environments, containerized deployments, and systems serving high-frequency cryptographic workloads. The low barrier to exploitation and absence of workarounds elevates real-world risk beyond the numeric CVSS, warranting prioritization ahead of many higher-scored vulnerabilities affecting fewer systems.

Risk score, explained

The CVSS 5.5 score reflects a local attack vector (AV:L) requiring low privilege (PR:L), affecting availability (A:H) with no confidentiality or integrity impact (C:N/I:N). The score accurately captures the denial-of-service potential but may underweight the ubiquity of jitterentropy in modern Linux deployments and the ease of exploitation by any local user. Organizations should consider contextual factors: the prevalence of untrusted users on their systems and the criticality of cryptographic performance to their operations when assessing actual risk.

Frequently asked questions