CVE-2026-43383

This vulnerability in the Linux kernel's TCP MD5 signature handling involves a non-constant-time comparison of Message Authentication Codes (MACs). The kernel function responsible for validating the MAC used in TCP MD5 (unnecessary quote) did not use a constant-time comparison primitive, making it susceptible to timing side-channel attacks where an attacker can deduce the MAC value by observing timing variations.

The attack surface is exposed to any unauthenticated network-level adversary who can send crafted TCP segments to a host that uses TCP MD5 signatures (commonly deployed in BGP peering sessions). By measuring the time taken for the kernel to compare a supplied MAC against the expected value, the attacker can iteratively leak the correct MAC byte by byte, without needing to know the shared secret. No authentication is no authentication prerequisite beyond network connectivity; the attacker must be able to deliver packets to the target and observe response latencies.

A successful exploit allows an attacker to forge TCP segments with a valid MD5 signature, leading to impersonation of legitimate TCP peers. This can enable session hijacking, man-in-the-middle attacks, and disruption of BGP routing sessions, with potentially severe consequences for network infrastructure. Since the vulnerability is in the common Linux kernel networking stack, systems running affected kernels (before the fix) that use TCP MD5 are at risk.

The fix is already committed to the Linux kernel stable tree [1], and distribution vendors have backported the patch. Users are strongly encouraged to apply the latest kernel updates or set the TCP MD5 usage as soon as possible. The patch replaces the non-constant memory comparison with a constant-time helper, eliminating the timing side channel. No workaround exists other than disabling TCP MD5 or applying the fix.