CVE-2026-5477: Prefix-substitution forgery via integer overflow in wolfCrypt CMAC

Vulnerability summary

CVE-2026-5477 is a high (CVSS 8.2) Integer Overflow or Wraparound vulnerability in wolfssl. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 32% of scored vulnerabilities.

Timeline

1. 10-04-2026
   PublishedVulnerability first disclosed.
2. 10-04-2026
   Last modifiedVulnerability information updated.

Description

An integer overflow existed in the wolfCrypt CMAC implementation, that could be exploited to forge CMAC tags. The function wc_CmacUpdate used the guard `if (cmac->totalSz != 0)` to skip XOR-chaining on the first block (where digest is all-zeros and the XOR is a no-op). However, totalSz is word32 and wraps to zero after 2^28 block flushes (4 GiB), causing the guard to erroneously discard the live CBC-MAC chain state. Any two messages sharing a common suffix beyond the 4 GiB mark then produce identical CMAC tags, enabling a zero-work prefix-substitution forgery. The fix removes the guard, making the XOR unconditional; the no-op property on the first block is preserved because digest is zero-initialized by wc_InitCmac_ex.

• CWE-190Integer Overflow or WraparoundThe product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.