CVE-2026-113713

virtualenv is a tool for creating isolated virtual python environments. From 21.14.8 until 21.14.10, the app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls. This issue is fixed in version 21.14.10.

virtualenv is a tool for creating isolated virtual python environments. From 21.14.8 until 21.14.10, the app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls. This issue is fixed in version 21.14.10.

Summary

The app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls.

Anthropic's OSS Scanner reported this as ANT-2026-ZOE2GEPC in virtualenv-20261015-005, received on 15 October 2026. This draft records source review and functional remediation checks. The reporter's cross-account code-execution claim has not been independently reproduced.

Affected area

Integrity of app-data references after temporary cache cleanup.

Attacker-controlled input

Symlink seeding must be enabled through VIRTUALENV_SYMLINK_APP_DATA or symlink_app_data in the configuration file, and app-data setup must fall back to a temporary directory. A missing or unwritable configured cache can cause that fallback. Ordinary copy seeding does not leave these references.

The security concern requires a shared temporary-directory parent where another account can acquire the deleted cache pathname, followed by use of the affected environment. That account need not already be able to modify the environment or the private cache while it exists. This differs from the security policy's excluded case of an account that already controls live app-data contents. On a platform with a temporary-directory parent private to the environment owner, that cross-account condition is absent. Windows functional checks do not establish cross-account exposure under Windows defaults.

Affected code

The relevant 21.14.8 source is in symlink seeder, temporary cache cleanup.

Mechanism

When app-data setup falls back to a temporary directory, the command-line --symlink-app-data action is disabled. A value supplied by VIRTUALENV_SYMLINK_APP_DATA or virtualenv.ini can still become the parser default, and the seeder constructor accepts that value without rechecking the cache lifetime. It links site-packages/pip into the temporary cache. Session cleanup deletes that cache, leaving a dangling link whose pathname can later be claimed by another account on a shared temporary filesystem.

Reproduction and evidence

The scanner ran its reproduction as two unprivileged users, with root used only to switch identities. The environment owner had an unwritable home, so app-data fell back to /tmp. The control used the command-line flag and received a real pip directory. With VIRTUALENV_SYMLINK_APP_DATA=1, the new environment instead contained a pip link into a deleted /tmp/tmpXXXXXXXX directory. A second uid recreated the target path and placed a fake pip/__init__.py there. When the owner ran venv/bin/pip --version, that module created a marker inside the owner's environment and printed this is not pip.

The cross-account path requires a shared temporary parent, a second account able to claim the deleted name, and later use of the environment. A private temporary parent removes that route. The report traces the incomplete temporary-cache guard to #1661, first released in 20.0.6. Public fix PR #3437 copies packages whenever app-data is temporary, whatever supplied the symlink option. The implementation review checked the remediation; it did not rerun the cross-account reproduction.

Attack sequence

sequenceDiagram
    participant U as Environment owner
    participant V as virtualenv seeder
    participant T as Temporary app-data in /tmp
    participant E as site-packages/pip link
    participant A as Other local…

virtualenv is a tool for creating isolated virtual python environments. From 21.14.8 until 21.14.10, the app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls. This issue is fixed in version 21.14.10.

virtualenv is a tool for creating isolated virtual python environments. From 21.14.8 until 21.14.10, the app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls. This issue is fixed in version 21.14.10.

virtualenv is a tool for creating isolated virtual python environments. From 21.14.8 until 21.14.10, the app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls. This issue is fixed in version 21.14.10.