# CVE-2026-121520: Malicious client could set xattr value outside of BuildKit state directory

Status: published. Published: 2026-11-03. Updated: 2026-11-06.

In BuildKit versions prior to v0.34.1, a malicious client with access to the BuildKit control APIs can submit a build request that sets an extended attribute on a file outside the BuildKit state directory. Extended attributes in the trusted. or security. namespaces may also be set, potentially giving a program on the host elevated privileges. This issue affects build services that accept build requests from untrusted users. The issue is fixed in v0.34.1.

## Remediation

- fix: Upgrade BuildKit to v0.34.1 or later. (source: CNA: Docker)
- workaround: Restrict access to BuildKit control APIs to trusted clients and do not accept build requests from untrusted users. (source: CNA: Docker)

## Affected products and versions

- Moby / BuildKit: ≥ 0; < 0.34.1 (affected, semver). Source: CNA: Docker

## Severity and prioritization

- CVSS 4.0: 4.5 (medium); CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:N/VC:N/VI:N/VA:N/SC:N/SI:H/SA:H; source: CNA: Docker

- CVSS 4.0: 4.5 (medium); CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:N/VC:N/VI:N/VA:N/SC:N/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: security@docker.com

- CISA KEV: No listing in collected sources

- EPSS: 0.14% as of 2026-11-06; percentile 3.28%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-61

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/moby/buildkit/security/advisories/GHSA-w26x-xg27-7qjc (CNA: Docker; vendor-advisory)

- https://github.com/moby/buildkit/security/advisories/GHSA-w26x-xg27-7qjc (security@docker.com)

## Provenance

- CNA: Docker: https://www.cve.org/CVERecord?id=CVE-2026-121520; retrieved 2026-11-06; source updated 2026-11-06

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-121520; retrieved 2026-11-07; source updated 2026-11-06

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-121520; retrieved 2026-11-06; source updated Not reported

Revision: dc1065e20ed713e5483a1aa7f2daa490e0133e894171dab7f33ea0ceff882cbb

Canonical: https://y2.dev/cve/CVE-2026-121520

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