EUVD-2026-21305
GHSA-grqc-3vmg-p68x
CVSS Vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Lifecycle Timeline
3Description
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.
Analysis
Integer overflow in wolfSSL CMAC implementation (versions ≤5.9.0) enables zero-effort cryptographic forgery. The wc_CmacUpdate function uses a 32-bit counter (totalSz) that wraps to zero after processing 4 GiB of data, erroneously discarding live CBC-MAC chain state. …
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Remediation
Within 24 hours: Inventory all applications and systems using wolfSSL ≤5.9.0, prioritizing those handling CMAC authentication or processing large files/streams >4 GiB. Within 7 days: Implement technical compensating controls (see below) and restrict CMAC usage to messages <4 GiB or implement external message authentication. …
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External POC / Exploit Code
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