CVE-2026-8720 in wolfSSL
Summary
by MITRE • 06/26/2026
wc_Blake2bHmacFinal and wc_Blake2sHmacFinal discard the message when the key length exceeds the block size, producing a MAC that is independent of the input. When the supplied key is longer than the BLAKE2 block size the key-hashing branch reinitialized the running hash state, discarding the accumulated message data, so the resulting MAC depended only on the key and not on the message being authenticated. This bug is specific to the HMAC-BLAKE2 APIs that were added in wolfSSL version 5.9.0.
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Analysis
by VulDB Data Team • 06/26/2026
The vulnerability described represents a critical cryptographic flaw in the wolfSSL library affecting the BLAKE2 HMAC implementation. This issue manifests in the wc_Blake2bHmacFinal and wc_Blake2sHmacFinal functions where the message authentication code generation becomes fundamentally compromised when keys exceed the block size threshold. The technical root cause lies in the improper handling of key lengths that surpass the BLAKE2 block size requirements, specifically causing the cryptographic implementation to discard all accumulated message data during the key-hashing process. This design flaw creates a severe security vulnerability where the resulting MAC output becomes completely independent of the actual message content, effectively rendering the authentication mechanism useless for its intended purpose.
The operational impact of this vulnerability is catastrophic for any system relying on wolfSSL's BLAKE2 HMAC functionality for message authentication. When a key longer than the block size is provided, the cryptographic library incorrectly reinitializes the running hash state, thereby eliminating all message data from consideration in the final MAC computation. This means that even if an attacker could somehow observe or manipulate the MAC output, they would have no way of verifying the authenticity of the actual message content since the MAC would be computed solely based on the key material rather than incorporating the message data. The vulnerability specifically affects wolfSSL versions 5.9.0 and later where these HMAC-BLAKE2 APIs were introduced, making it a regression issue that fundamentally breaks the security guarantees provided by HMAC authentication.
This cryptographic weakness aligns with CWE-327, which addresses the use of weak or broken cryptographic algorithms, and more specifically relates to improper implementation of cryptographic primitives where the fundamental security properties are violated. The vulnerability also maps to ATT&CK technique T1552.001, which covers unsecured credentials, as it effectively renders the authentication mechanism unreliable for protecting sensitive data communications. The flaw represents a critical failure in the HMAC construction process where the integrity protection mechanism is entirely bypassed when keys exceed size limits. This type of vulnerability is particularly dangerous because it silently undermines security without raising any obvious errors or warnings during normal operation, making it extremely difficult to detect and verify the proper functioning of the cryptographic implementation.
The recommended mitigations involve immediate upgrading to wolfSSL versions that contain fixes for this specific vulnerability, ensuring that all systems using BLAKE2 HMAC functionality are updated to prevent potential exploitation. Organizations should also implement comprehensive testing procedures to validate that their cryptographic implementations properly handle key sizes exceeding block boundaries. Additionally, security teams should conduct thorough audits of all cryptographic libraries in use to identify similar patterns where message data might be inadvertently discarded during hash computations, particularly in HMAC implementations that involve key stretching or hashing operations. The vulnerability highlights the critical importance of rigorous testing and validation of cryptographic implementations, especially when introducing new APIs or modifying existing ones that handle sensitive security parameters such as key lengths and message authentication codes.