# CVE-2026-54334: UEFI Firmware Parser: Heap out-of-bounds write in tiano decompressor `ReadCLen`

> A heap out-of-bounds write in the Tiano decompressor allows attackers to crash the parser or potentially execute code via crafted firmware images.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Sep 2026
- **Updated:** 16 Sep 2026
- **Also known as:** GHSA-hm2w-vr2p-hq7w, PYSEC-2026-3394

## What is CVE-2026-54334?

CVE-2026-54334 is a critical memory corruption vulnerability in theopolis uefi-firmware-parser, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

UEFI Firmware Parser parses BIOS, Intel ME, and UEFI firmware structures including volumes, file systems, and files. Prior to 1.14, ReadCLen() in uefi_firmware/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 < NC. The CharC == 2 run-length path can additionally request up to 531 zero writes through Sd->mCLen[Index++] = 0. The normal CompressedSection.process() to efi_compressor.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.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-54334?

3 open source packages ship an affected version, including uefi-firmware.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `uefi-firmware` | PyPI | All versions to 1.12 | None |
| `uefi-firmware` | PyPI | All versions to 1.12 | None |
| `uefi-firmware` | PyPI | All versions to 1.12 | None |

## How do I fix CVE-2026-54334?

No fix has been published for CVE-2026-54334 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-54334 being exploited?

There is no known exploitation of CVE-2026-54334 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54334 and fix it if it is affected.

What it is: CVE-2026-54334 is a critical memory corruption vulnerability in theopolis uefi-firmware-parser, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-54334 in the wild and no public exploit code.

Affected packages and fixed versions:
- uefi-firmware (PyPI): affected all versions up to 1.12; no fix listed
- uefi-firmware (PyPI): affected all versions up to 1.12; no fix listed
- uefi-firmware (PyPI): affected all versions up to 1.12; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54334/ (Markdown: https://insights.sbomify.com/CVE-2026-54334/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54334
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54334
- OSV: https://osv.dev/vulnerability/CVE-2026-54334

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-54334/
Summaries are generated and may contain mistakes; the official description above is the record.