Security bugs on Windows servers: Flask 0.12.2 and Werkzeug 0.12.2 released

written by Markus Unterwaditzer on 2017-05-16 in Releases , Security

Flask 0.12.2 and Werkzeug 0.12.2 have been released. They contain the same security bugfix for the safe_join function in each package.

The problem only occurs if you are running your application on a Windows server.


David Lord initially found this bug (thanks!) and disclosed it to the other maintainers in a private email:

While going through PR #2059 about safe_join, I looked up Python's ntpath.join and discovered a vulnerability that safe_join on Windows doesn't cover. "os.path.join("c:", "foo") represents a path relative to the current directory on drive C: (c:foo)"
safe_join('\\root\\path', 'd:', 'test.txt') would break out of the trusted root directory and instead take the test file relative to the cwd on the d drive. This doesn't give completely arbitrary path access, since it's limited to the cwd, but it's still not good.

For the application developer this means that endpoints using safe_join could potentially be used to disclose arbitrary files in the server processes' current working directory on Windows.

What happens next

We strongly recommend upgrading to Flask 0.12.2 and Werkzeug 0.12.2, as this bug has been fixed there (Flask, Werkzeug).

A CVE has been requested on Tue, 16 May 2017 06:51:09 +0000, the CVE CVE-2017-9088 was assigned.