CVE-2026-31421
Null Pointer Dereference in Linux Kernel net/sched cls_fw Module
Publication date: 2026-04-13
Last updated on: 2026-04-18
Assigner: kernel.org
Description
Description
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| linux | linux_kernel | * |
| linux_kernel | linux_kernel | * |
Helpful Resources
Exploitability
| CWE ID | Description |
|---|---|
| CWE-UNKNOWN |
Attack-Flow Graph
AI Powered Q&A
How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:
The provided information about CVE-2026-31421 does not include any details regarding its impact on compliance with common standards and regulations such as GDPR or HIPAA.
Can you explain this vulnerability to me?
This vulnerability exists in the Linux kernel's network scheduler, specifically in the cls_fw classifier. The issue is a NULL pointer dereference that occurs when using the old method in the fw_classify() function on shared blocks. Shared blocks have a NULL pointer for block->q, but the old method dereferences q->handle, leading to a NULL pointer dereference when an empty cls_fw filter is attached to a shared block and a packet with a nonzero major skb mark is classified.
The fix involves rejecting configurations that use the old method without TCA_OPTIONS on shared blocks, preventing the NULL pointer dereference by ensuring the code does not attempt to access block->q when it is NULL.
How can this vulnerability impact me? :
This vulnerability can cause a NULL pointer dereference in the Linux kernel's network scheduler, which may lead to a kernel crash or denial of service. Such a crash can disrupt network traffic processing and potentially affect system stability or availability.
How can this vulnerability be detected on my network or system? Can you suggest some commands?
This vulnerability involves a NULL pointer dereference in the Linux kernel's cls_fw classifier when using shared blocks with the old method in fw_classify(). Detection would involve checking for the presence of empty cls_fw filters attached to shared blocks and monitoring for kernel crashes or KASAN reports indicating null pointer dereferences in fw_classify.
Since the vulnerability is triggered by specific configurations in the traffic control (tc) subsystem, you can inspect your traffic control filters using commands like:
- tc filter show dev <interface>
- Look specifically for cls_fw filters attached to shared blocks without TCA_OPTIONS.
Additionally, monitoring kernel logs for messages related to KASAN null pointer dereferences in fw_classify can help detect exploitation attempts.
What immediate steps should I take to mitigate this vulnerability?
The vulnerability is fixed by rejecting configurations that use the old method (no TCA_OPTIONS) on shared blocks in fw_change(). Therefore, immediate mitigation steps include:
- Avoid using empty cls_fw filters attached to shared blocks with the old method.
- Update your Linux kernel to a version that includes the fix for this vulnerability.
- Review and modify any traffic control configurations that may use the old method on shared blocks to ensure they comply with the fixed behavior.