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The CVE-2025-39926 vulnerability is a logic flaw within the `genl_bind()` function of the Linux kernel’s genetlink subsystem. Genetlink provides a communication framework between kernel components and user-space applications, using families and multicast groups to manage events. Per-family `bind()` and `unbind()` callbacks were introduced to let these kernel families track when user-space processes start or stop listening to multicast groups, allowing for efficient resource management.
The vulnerability occurs due to an error in the sequence of operations within genl_bind(). When a user-space process attempts to bind to a genetlink multicast group, the kernel performs capability checks to verify the caller has the necessary permissions. If these checks fail, the function should immediately return an `-EPERM` (operation not permitted) error to user space.
However, the flawed code invoked the family’s `bind()` callback before evaluating the result of the permission checks. This means that even when the capability check failed (ret was set to -EPERM), the `bind()` callback was still executed on behalf of the unauthorized caller. The syscall ultimately returns a failure (-EPERM) to the user-space application, but the unauthorized kernel callback has already been triggered. This creates a window where a low-privileged local attacker can induce the kernel to execute callbacks without proper authorization, potentially disrupting internal kernel state tracking for multicast groups.
Platform: Linux Kernel
Version: Up to 6.12.48/6.16.8
Vulnerability: Unauthorized Callback Execution
Severity: Medium
date: 2025-10-01
Prediction: 2025-10-08
What Undercode Say:
Check current kernel version uname -r Update kernel on Ubuntu/Debian systems sudo apt update && sudo apt upgrade linux-image-$(uname -r) Update kernel on RHEL-based systems sudo yum update kernel Check system logs for genetlink activity sudo dmesg | grep -i genetlink sudo journalctl -k --grep="genetlink" Verify if a specific kernel version contains the fix The fix was applied in mainline commit(s) after the vulnerability .
How Exploit:
An attacker with low-privileged local access can attempt to bind to genetlink multicast groups without proper capabilities. By exploiting the flawed sequence, they can cause the kernel to execute `bind()` callbacks associated with various kernel families. This could be used to manipulate kernel state, disrupt event tracking, or probe for further exploitation opportunities, though direct confidentiality/integrity loss is limited. No public proof-of-concept exists, and there is no evidence of active exploitation.
Protection from this CVE
Update the Linux kernel to a patched version (e.g., 6.12.48, 6.16.8, or later as provided by your distribution). Implement strict access controls to limit local user privileges. Monitor system logs for unexpected genetlink-related activities or errors. Apply patches from official vendor security advisories (e.g., ELSA, SUSE updates).
Impact:
The impact is primarily on system availability, with a potential high availability impact (A:H) according to the CVSS 3.1 vector. Successful exploitation could allow a local attacker to trigger unauthorized kernel callbacks, potentially disrupting operations or causing denial-of-service conditions within the genetlink subsystem. There is no impact on confidentiality (C:N) or integrity (I:N).
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Sources:
Reported By: nvd.nist.gov
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