CVE-2026-43383 in Linux
Summary
by MITRE • 05/08/2026
In the Linux kernel, the following vulnerability has been resolved:
net/tcp-md5: Fix MAC comparison to be constant-time
To prevent timing attacks, MACs need to be compared in constant time. Use the appropriate helper function for this.
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Analysis
by VulDB Data Team • 09/13/2026
The vulnerability addressed in this linux kernel update represents a critical security flaw in the tcp md5 implementation that could enable timing attacks against network communications. This issue specifically affects the comparison of message authentication codes within the tcp md5 subsystem, which is used to provide authentication for tcp connections. The vulnerability arises from the use of non-constant time comparison functions that expose timing information about the authentication data, potentially allowing attackers to infer sensitive information through careful analysis of response times. Such timing variations in cryptographic operations can be exploited to perform side-channel attacks that gradually reveal the contents of the mac values being compared.
The technical flaw stems from the implementation of mac comparison logic within the network stack's tcp md5 functionality. When comparing mac values during tcp connection authentication, the kernel was using standard comparison functions that execute in variable time based on the position of the first mismatch. This non-constant time execution creates measurable timing differences that can be observed by attackers monitoring network response times. The vulnerability directly relates to CWE-208 which describes timing attacks that exploit variations in program execution time to gain information about cryptographic keys or authentication data. The tcp md5 implementation should have used constant time comparison functions to ensure that all comparisons take the same amount of time regardless of the input values being compared.
The operational impact of this vulnerability extends across all systems running affected kernel versions that utilize tcp md5 authentication for network connections. Any system that relies on tcp md5 for connection authentication or integrity verification becomes susceptible to timing-based attacks that could potentially compromise the security of network communications. Attackers could leverage this vulnerability to perform credential guessing, key recovery, or authentication bypass attacks against systems using tcp md5. The risk is particularly significant in environments where network monitoring capabilities exist, as attackers could observe timing variations in tcp handshake responses to extract authentication information. This vulnerability affects the fundamental security of tcp connections and could impact various network services that depend on tcp md5 for authentication.
The resolution involves implementing proper constant time comparison functions for mac validation within the tcp md5 subsystem. The fix requires replacing the existing non-constant time comparison logic with appropriate helper functions designed specifically for cryptographic comparisons. This approach aligns with the mitigation strategies recommended in the mitre attack framework under techniques related to cryptographic operations and side-channel attacks. The fix ensures that all mac comparisons execute in constant time regardless of the specific values being compared, eliminating the timing variations that could be exploited by attackers. This solution follows industry best practices for cryptographic implementation and addresses the specific timing attack vectors that could be leveraged against the tcp md5 authentication mechanism. The updated implementation maintains the security properties of tcp md5 while eliminating the timing side-channel that previously made the system vulnerable to sophisticated attacks.