The vulnerability in pypa virtualenv (before 21.7.12) involves the download_wheel() function accepting pip and setuptools seed wheels without verifying their bytes against an authoritative digest (BUNDLE_SHA256). This lack of integrity check enables attackers controlling or intercepting the distribution source to substitute malicious wheels under the expected distribution, version, and filename. These attacker-controlled wheels are cached and subsequently used in newly created virtual environments. Verification is applied only to the default PyPI path and intentionally skipped for custom indexes configured via PIP_INDEX_URL, PIP_EXTRA_INDEX_URL, or PIP_INDEX. The vulnerability is resolved in version 21.7.12.

An attacker able to compromise the package index, a mirror, or intercept TLS connections can substitute malicious wheels that virtualenv will cache and use in new environments. This could lead to execution of attacker-controlled code within isolated Python environments created by virtualenv. The vulnerability affects the integrity of the software supply chain for Python environments relying on virtualenv prior to 21.7.12.

Upgrade virtualenv to version 21.7.12 or later, where the integrity check for downloaded wheels is implemented. Users relying on custom package indexes should be aware that verification is intentionally skipped for those indexes, so additional measures may be needed to ensure integrity in those cases. Patch status is confirmed fixed in 21.7.12.