Time-of-check Time-of-use (TOCTOU) Race Condition in league/flysystem

Impact

The whitespace normalisation using in 1.x and 2.x removes any unicode whitespace. Under certain specific conditions this could potentially allow a malicious user to execute code remotely.

The conditions:

• A user is allowed to supply the path or filename of an uploaded file.
• The supplied path or filename is not checked against unicode chars.
• The supplied pathname checked against an extension deny-list, not an allow-list.
• The supplied path or filename contains a unicode whitespace char in the extension.
• The uploaded file is stored in a directory that allows PHP code to be executed.

Given these conditions are met a user can upload and execute arbitrary code on the system under attack.

Patches

The unicode whitespace removal has been replaced with a rejection (exception).

The library has been patched in:

• 1.x: https://github.com/thephpleague/flysystem/commit/f3ad69181b8afed2c9edf7be5a2918144ff4ea32
• 2.x: https://github.com/thephpleague/flysystem/commit/a3c694de9f7e844b76f9d1b61296ebf6e8d89d74

Workarounds

For 1.x users, upgrade to 1.1.4. For 2.x users, upgrade to 2.1.1.