Missing Required Cryptographic Step Affecting mbedtls package, versions *

Note: Versions mentioned in the description apply only to the upstream mbedtls package and not the mbedtls package as distributed by Ubuntu. See How to fix? for Ubuntu:25.04 relevant fixed versions and status.

In MbedTLS 3.3.0 before 3.6.4, mbedtls_lms_verify may accept invalid signatures if hash computation fails and internal errors go unchecked, enabling LMS (Leighton-Micali Signature) forgery in a fault scenario. Specifically, unchecked return values in mbedtls_lms_verify allow an attacker (who can induce a hardware hash accelerator fault) to bypass LMS signature verification by reusing stale stack data, resulting in acceptance of an invalid signature. In mbedtls_lms_verify, the return values of the internal Merkle tree functions create_merkle_leaf_value and create_merkle_internal_value are not checked. These functions return an integer that indicates whether the call succeeded or not. If a failure occurs, the output buffer (Tc_candidate_root_node) may remain uninitialized, and the result of the signature verification is unpredictable. When the software implementation of SHA-256 is used, these functions will not fail. However, with hardware-accelerated hashing, an attacker could use fault injection against the accelerator to bypass verification.

• http://people.ubuntu.com/~ubuntu-security/cve/CVE-2025-49600
• https://github.com/Mbed-TLS/mbedtls-docs/blob/main/security-advisories/mbedtls-security-advisory-2025-06-3.md