Jinja vulnerable to HTML attribute injection when passing user input as keys to xmlattr filter

The xmlattr filter in affected versions of Jinja accepts keys containing non-attribute characters. XML/HTML attributes cannot contain spaces, /, >, or =, as each would then be interpreted as starting a separate attribute. If an application accepts keys (as opposed to only values) as user input, and renders these in pages that other users see as well, an attacker could use this to inject other attributes and perform XSS. The fix for the previous GHSA-h5c8-rqwp-cp95 CVE-2024-22195 only addressed spaces but not other characters.

Accepting keys as user input is now explicitly considered an unintended use case of the xmlattr filter, and code that does so without otherwise validating the input should be flagged as insecure, regardless of Jinja version. Accepting values as user input continues to be safe.