CVE-2025-39802: 
Linux Debian vulnerability analysis and mitigation

CVE-2025-39802 is a vulnerability discovered in the Linux kernel affecting the lib/crypto ARM/Poly1305 implementation. The vulnerability was disclosed on September 15, 2025, and involves register corruption issues in no-SIMD contexts (NVD).

The vulnerability stems from the removal of a SIMD usability check that was originally removed by commit 773426f4771b. When Poly1305 functions are called when SIMD registers are unusable, it leads to corruption of random tasks' registers and/or computation of incorrect MACs. The fix involves restoring the SIMD usability check using mayusesimd() instead of the original cryptosimdusable() (NVD).

The vulnerability can result in register corruption and incorrect MAC (Message Authentication Code) computations when Poly1305 functions are called in contexts where SIMD registers are unusable. This could potentially affect the security and integrity of cryptographic operations in the Linux kernel (NVD).

The vulnerability has been resolved by restoring the SIMD usability check in the Linux kernel. The fix implements a safety check using mayusesimd() function, which is described as cheap and effective at eliminating the potential security risk (NVD).