CVE-2025-40206: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40206 is a vulnerability discovered in the Linux kernel's netfilter component, specifically affecting the nft_objref validation mechanism. The vulnerability was disclosed on November 11, 2025, and involves issues with synproxy stateful object references from the OUTPUT hook (NVD, Ubuntu).

The vulnerability occurs when referencing a synproxy stateful object from the OUTPUT hook, which causes a kernel crash due to infinite recursive calls. The issue manifests as a stack guard page hit at specific memory addresses (000000008bda5b8c, with stack range 000000003ab1c4a5..00000000494d8b12). The vulnerability affects the netfilter subsystem's nft_objref and objrefmap expressions validation functionality (NVD).

When exploited, the vulnerability results in a kernel crash, effectively creating a denial of service condition. This occurs specifically when attempting to use synproxy objects in the OUTPUT hook, impacting system stability and availability (NVD).

The issue has been resolved through the implementation of objref and objrefmap expression validate functions. The fix specifically addresses the NFTOBJECTSYNPROXY object type validation and handles jumps to chains using synproxy objects from the OUTPUT hook. After applying the fix, attempts to reference a synproxy object in the OUTPUT hook will be blocked with an appropriate error message (NVD).