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CVE Record

CVE-2021-27851: Local privilege escalation in GNU Guix via guix-daemon and '--keep-failed'

A security vulnerability that can lead to local privilege escalation has been found in ’guix-daemon’. It affects multi-user setups in which ’guix-daemon’ runs locally. The attack consists in having an unprivileged user spawn a build process, for instance with `guix build`, that makes its build directory world-writable. The user then creates a hardlink to a root-owned file such as /etc/shadow in that build directory. If the user passed the --keep-failed option and the build eventually fails, the daemon changes ownership of the whole build tree, including the hardlink, to the user. At that point, the user has write access to the target file. Versions after and including v0.11.0-3298-g2608e40988, and versions prior to v1.2.0-75109-g94f0312546 are vulnerable.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2021-27851 lets a local unprivileged user on a vulnerable multi-user GNU Guix system gain write access to root-owned files. This can become full local privilege escalation. It does not describe a remote internet-facing attack.

Executive priority

Treat this as high priority for shared Linux systems using GNU Guix. It is not remotely exploitable from the provided evidence, but local compromise can undermine root-owned files and system integrity.

Technical view

The issue is in guix-daemon on local multi-user setups. A failed build retained by --keep-failed can cause the daemon to change ownership across a build tree in a way that affects a hardlinked root-owned file. Vulnerable range is stated as v0.11.0-3298-g2608e40988 through before v1.2.0-75109-g94f0312546.

Likely exposure

Exposure is limited to systems running vulnerable GNU Guix guix-daemon versions in multi-user mode with local unprivileged users able to start builds.

Exploitation context

No KEV listing or provided source states active exploitation. The public advisory describes the local privilege escalation condition, but exploitation requires local user access and affected daemon behavior.

Researcher notes

CWE is listed as CWE-264. The key research focus is daemon ownership handling on retained failed build directories and hardlink behavior. Evidence is source-grounded but lacks CVSS scoring and active exploitation confirmation.

Mitigation direction

  • Upgrade GNU Guix to a non-vulnerable version per the GNU advisory.
  • Prioritize multi-user Guix hosts with untrusted or shared local accounts.
  • Review whether guix-daemon runs locally in multi-user mode.
  • Check vendor guidance before relying on configuration workarounds.
  • Reduce unnecessary local build access on affected systems until upgraded.

Validation and detection

  • Inventory hosts running GNU Guix and guix-daemon.
  • Confirm guix-daemon version against the vulnerable revision range.
  • Identify systems allowing unprivileged users to run Guix builds.
  • Check whether affected systems are multi-user rather than single-user setups.
  • Review patch status against the GNU Guix advisory.
Prepared
Confidence
high
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-264: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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description · low confidence lookup

Privilege behavior lookup

The CVE wording references privilege impact, so privilege escalation and authorization behavior review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2021-27851 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
GNU Guixguix-daemonv0.11.0-3298-g2608e40988, unspecifiedListed
Weakness

CWE details

CWE-264

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-264 · source CWE mapping

Permissions, Privileges, and Access Controls

Permissions, Privileges, and Access Controls represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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