CVE-2025-39931 – A use-after-free vulnerability in the Linux ker... (How to Fix)

Q: What is the severity of CVE-2025-39931?

CVE-2025-39931 has a CVSS score of 5.5 (MEDIUM). A use-after-free vulnerability in the Linux kernel's cryptographic subsystem (af_alg) could cause kernel crashes or potential privilege escalation. The vulnerability occurs when an error in af_alg_sendmsg leaves a garbage value in ctx->merge, which may trigger a crash on subsequent operations. This affects systems using the kernel's cryptographic API for socket-based operations.

📋 TL;DR

A use-after-free vulnerability in the Linux kernel's cryptographic subsystem (af_alg) could cause kernel crashes or potential privilege escalation. The vulnerability occurs when an error in af_alg_sendmsg leaves a garbage value in ctx->merge, which may trigger a crash on subsequent operations. This affects systems using the kernel's cryptographic API for socket-based operations.

💻 Affected Systems

Products:

Linux kernel

Versions: Specific affected versions not specified in CVE, but patches exist for multiple stable branches

Operating Systems: Linux distributions using vulnerable kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects systems using the kernel's AF_ALG cryptographic socket interface. Many systems may not use this feature by default.

📦 What is this software?

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Kernel panic leading to system crash and denial of service, or potential privilege escalation if the garbage value manipulation leads to memory corruption exploitable for code execution.

🟠

Likely Case

System crash or kernel panic causing denial of service, requiring system reboot to restore functionality.

🟢

If Mitigated

Minimal impact if systems are patched or don't use the affected cryptographic socket operations.

🌐 Internet-Facing: LOW - This vulnerability requires local access or ability to trigger specific cryptographic operations, not directly exploitable over network.

🏢 Internal Only: MEDIUM - Local users or processes could trigger the vulnerability, potentially causing system instability or crashes.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires local access and ability to trigger specific error conditions in the cryptographic subsystem. No public exploits known at this time.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Multiple stable kernel versions with commit hashes provided in references

Vendor Advisory: https://git.kernel.org/stable/c/045ee26aa3920a47ec46d7fcb302420bf01fd753

Restart Required: Yes

Instructions:

1. Update Linux kernel to patched version. 2. Check distribution-specific security advisories. 3. Reboot system after kernel update. 4. Verify kernel version matches patched release.

🔧 Temporary Workarounds

Disable AF_ALG module

linux

Prevent loading of the affected cryptographic socket module

echo 'install af_alg /bin/true' >> /etc/modprobe.d/disable-af_alg.conf
rmmod af_alg

🧯 If You Can't Patch

Restrict access to cryptographic socket operations to trusted users only
Monitor system logs for kernel panic or crash events related to cryptographic operations

🔍 How to Verify

Check if Vulnerable:

Check kernel version and compare with patched versions in git commits. Use 'uname -r' and verify against distribution security advisories.

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version after update matches patched release. Check that system no longer crashes when performing cryptographic socket operations.

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages
System crash dumps
Cryptographic subsystem error logs

Network Indicators:

None - local vulnerability only

SIEM Query:

source="kernel" AND ("panic" OR "Oops" OR "BUG") AND ("af_alg" OR "crypto")

📊 Metadata

CVE ID: CVE-2025-39931

CVSS v3 Score: 5.5 (MEDIUM)

CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CWE: CWE-908

EPSS Score: 0.03% exploit probability (9.1th percentile)

Published: October 4, 2025

Last Updated: January 27, 2026

🔗 References

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