CVE-2026-54333: UEFI Firmware Parser has a stack out-of-bounds write in tiano decompressor MakeTable

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• 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, MakeTable() in uefifirmware/compression/Tiano/Decompress.c does not validate that bit-length values read from a crafted Tiano or EFI compressed firmware bitstream remain within the expected range from 0 through 16. The normal CompressedSection.process() to eficompressor.TianoDecompress() to TianoDecompress() to ReadPTLen() to MakeTable() parsing path can consequently write beyond the stack-allocated Count[17] array and related decode tables. The resulting stack corruption deterministically crashes the parsing process and may permit code execution depending on build and runtime details. This issue is fixed in version 1.14.

From the GitHub advisory

uefi-firmware contains a stack out-of-bounds write vulnerability in the native tiano/EFI decompressor. in uefifirmware/compression/Tiano/Decompress.c, MakeTable() does not validate that bit-length values read from the compressed bitstream are within the expected range (0..16). a crafted firmware blob can supply bit lengths greater than 16, causing out-of-bounds writes to the stack-allocated Count[17] array and related decode tables.

reachability is through the normal parsing path: CompressedSection.process() -> eficompressor.TianoDecompress() -> TianoDecompress() -> ReadPTLen() -> MakeTable().

Minimum impact is a deterministic crash; depending on build/runtime details, the stack memory corruption may be exploitable for code execution in the context of the parsing process. this project shipped its own copy of the decompressor without the upstream EDK2 hardening for this bug class.

References:

- PR: - fix commit: - upstream related fixes: CVE-2017-5731, CVE-2017-5732, CVE-2017-5733, CVE-2017-5734, CVE-2017-5735

Weakness type: CWE-787.

Product: uefi-firmware (PyPI) · Assigned by: GitHub_M · Published: 2026-09-14

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• 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.

Timeline

• 2026-04-16 — GitHub-reviewed advisory GHSA-2689-5p89-6j3j published (critical)
• 2026-09-14 — CVE record published by GitHub_M
• 2026-09-15 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-2689-5p89-6j3j github.com
2. CVE.org record CVE-2026-54333 cve.org
3. NVD entry CVE-2026-54333 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

Critical PyPI advisories · Today’s Security Digest