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