CVE-2026-31495
MediumCVSS 5.5Exploitation Probability (EPSS)
Low risk2th percentile - higher than 2% of all known CVEs
Summary
In the Linux kernel netfilter subsystem, a vulnerability in ctnetlink allowed accepting invalid values for TCP protocol attributes, potentially causing undefined behavior when used as shift counts. The fix replaces manual range and mask validations with netlink policy annotations, rejecting invalid values early.
Risk Assessment
An attacker can send a crafted netlink packet with invalid values (e.g., TCP window scale 0-255), causing undefined kernel behavior, potentially leading to system crash or privilege escalation.
Recommendation
Immediately update the Linux kernel to a version containing the fix (commit in the netfilter tree). Monitor distributions for the patch and apply it urgently.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: netfilter: ctnetlink: use netlink policy range checks Replace manual range and mask validations with netlink policy annotations in ctnetlink code paths, so that the netlink core rejects invalid values early and can generate extack errors. - CTA_PROTOINFO_TCP_STATE: reject values > TCP_CONNTRACK_SYN_SENT2 at policy level, removing the manual >= TCP_CONNTRACK_MAX check. - CTA_PROTOINFO_TCP_WSCALE_ORIGINAL/REPLY: reject values > TCP_MAX_WSCALE (14). The normal TCP option parsing path already clamps to this value, but the ctnetlink path accepted 0-255, causing undefined behavior when used as a u32 shift count. - CTA_FILTER_ORIG_FLAGS/REPLY_FLAGS: use NLA_POLICY_MASK with CTA_FILTER_F_ALL, removing the manual mask checks. - CTA_EXPECT_FLAGS: use NLA_POLICY_MASK with NF_CT_EXPECT_MASK, adding a new mask define grouping all valid expect flags. Extracted from a broader nf-next patch by Florian Westphal, scoped to ctnetlink for the fixes tree.