Prefix-substitution forgery via integer overflow in wolfCrypt CMAC
Summary
Prefix-substitution forgery via integer overflow in wolfCrypt CMAC
Original description
The description as published by the source.
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.
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-5477 wolfssl
- Ubuntu UBUNTU-CVE-2026-5477 wolfssl
References
- advisory github.com
- advisory nvd.nist.gov
- patch github.com