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virtualenv Has TOCTOU Vulnerabilities in Directory Creation

Moderate severity GitHub Reviewed Published Jan 9, 2026 in pypa/virtualenv • Updated Jan 13, 2026

Package

pip virtualenv (pip)

Affected versions

< 20.36.1

Patched versions

20.36.1

Description

Impact

TOCTOU (Time-of-Check-Time-of-Use) vulnerabilities in virtualenv allow local attackers to perform symlink-based attacks on directory creation operations. An attacker with local access can exploit a race condition between directory existence checks and creation to redirect virtualenv's app_data and lock file operations to attacker-controlled locations.

Affected versions: All versions up to and including 20.36.1

Affected users: Any user running virtualenv on multi-user systems where untrusted local users have filesystem access to shared temporary directories or where VIRTUALENV_OVERRIDE_APP_DATA points to a user-writable location.

Attack scenarios:

  • Cache poisoning: Attacker corrupts wheels or Python metadata in the cache
  • Information disclosure: Attacker reads sensitive cached data or metadata
  • Lock bypass: Attacker controls lock file semantics to cause concurrent access violations
  • Denial of service: Lock starvation preventing virtualenv operations

Patches

The vulnerability has been patched by replacing check-then-act patterns with atomic os.makedirs(..., exist_ok=True) operations.

Fixed in: PR #3013

Versions with the fix: 20.36.2 and later

Users should upgrade to version 20.36.2 or later.

Workarounds

If you cannot upgrade immediately:

  1. Ensure VIRTUALENV_OVERRIDE_APP_DATA points to a directory owned by the current user with restricted permissions (mode 0700)
  2. Avoid running virtualenv in shared temporary directories where other users have write access
  3. Use separate user accounts for different projects to isolate app_data directories

References

  • GitHub PR: pypa/virtualenv#3013
  • Vulnerability reported by: @tsigouris007
  • CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (TOCTOU)
  • CWE-59: Improper Link Resolution Before File Access

References

@gaborbernat gaborbernat published to pypa/virtualenv Jan 9, 2026
Published by the National Vulnerability Database Jan 10, 2026
Published to the GitHub Advisory Database Jan 13, 2026
Reviewed Jan 13, 2026
Last updated Jan 13, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Local
Attack complexity
High
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(2nd percentile)

Weaknesses

Improper Link Resolution Before File Access ('Link Following')

The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource. Learn more on MITRE.

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently. Learn more on MITRE.

CVE ID

CVE-2026-22702

GHSA ID

GHSA-597g-3phw-6986

Source code

Credits

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