---
title: "CVE-2026-54334: UEFI Firmware Parser has a heap out-of-bounds write in tiano decompressor ReadCLen"
url: https://erroneo.us/security/cve/CVE-2026-54334/
description: "CVE-2026-54334 (uefi-firmware (PyPI)): UEFI Firmware Parser parses BIOS, Intel ME, and UEFI firmware structures including volumes, file systems, and files.…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-09-14
date_modified: 2026-09-16
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-54334"
kev: false
cvss: 9.8
epss: 0.00801
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-54334: UEFI Firmware Parser has a heap out-of-bounds write in tiano decompressor ReadCLen

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.8% , percentile 56: FIRST’s model estimates a 0.8% chance of exploitation activity in the next 30 days, higher than 56% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes uefi-firmware (PyPI) <= 1.12, no fixed version listed . Check your lockfile, including transitive dependencies.

## What it is

UEFI Firmware Parser parses BIOS, Intel ME, and UEFI firmware structures including volumes, file systems, and files. Prior to 1.14, ReadCLen() in uefifirmware/compression/Tiano/Decompress.c reads Number from GetBits(Sd, CBIT) with CBIT = 9 and can obtain 511 entries for the 510-element Sd->mCLen heap array because its loop does not enforce Index mCLen[Index++] = 0. The normal CompressedSection.process() to eficompressor.TianoDecompress() to TianoDecompress() to DecodeC() to ReadCLen() parsing path therefore permits crafted Tiano or EFI compressed firmware to corrupt heap memory, deterministically crash the parsing process, and potentially execute code depending on build and runtime details. This issue is fixed in version 1.14.

## Affected packages

- PyPI `uefi-firmware` <= 1.12 → fixed: none listed

## Affected products (CVE record)

- theopolis uefi-firmware-parser: < 1.14

## Severity

- GitHub_M CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
- GitHub CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.80% probability of exploitation activity in the next 30 days; percentile 56.

## Sources

- [GitHub Advisory GHSA-hm2w-vr2p-hq7w](https://github.com/advisories/GHSA-hm2w-vr2p-hq7w)
- [CVE.org record CVE-2026-54334](https://www.cve.org/CVERecord?id=CVE-2026-54334)
- [NVD entry CVE-2026-54334](https://nvd.nist.gov/vuln/detail/CVE-2026-54334)
- [github.com](https://github.com/theopolis/uefi-firmware-parser/security/advisories/GHSA-hm2w-vr2p-hq7w)
- [github.com](https://github.com/theopolis/uefi-firmware-parser/pull/145)
- [github.com](https://github.com/theopolis/uefi-firmware-parser/commit/bf3dfaa8a05675bae6ea0cbfa082ddcebfcde23e)
- [github.com](https://github.com/theopolis/uefi-firmware-parser/releases/tag/v1.14)

---
Source page: https://erroneo.us/security/cve/CVE-2026-54334/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt