PYSEC-2026-2009

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: https://github.com/pypa/virtualenv/pull/3013 - Vulnerability reported by: @tsigouris007 - CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (TOCTOU) - CWE-59: Improper Link Resolution Before File Access