CVE-2021-32708: 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.
Other sources
Flysystem is an open source file storage library for PHP. 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 are: 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. The unicode whitespace removal has been replaced with a rejection (exception). For 1.x users, upgrade to 1.1.4. For 2.x users, upgrade to 2.1.1.
— MITRE
TOCTOU Race Condition enabling remote code execution
Affected Software
Remediation
Event History
Frequently Asked Questions
What is CVE-2021-32708?
CVE-2021-32708 is a vulnerability in Flysystem, an open source file storage library for PHP, that enables a Time-of-Check Time-of-Use (TOCTOU) race condition, leading to potential remote code execution.
What is the severity of CVE-2021-32708?
The severity of CVE-2021-32708 is critical, with a CVSS v3.1 base score of 8.1.
Which software versions are affected by CVE-2021-32708?
Versions 1.0.0 up to but excluding 1.1.4, and versions 2.0.0 up to but excluding 2.1.1 of Flysystem are affected by CVE-2021-32708.
How can CVE-2021-32708 be exploited?
CVE-2021-32708 can be exploited by a malicious user under specific conditions, allowing them to execute code remotely.
Is there a fix for CVE-2021-32708?
Yes, a fix for CVE-2021-32708 is available. It is recommended to update Flysystem to a version that is not vulnerable.