KaTeX missing normalization of the protocol in URLs allows bypassing forbidden protocols

Impact

Code that uses KaTeX’s trust option, specifically that provides a function to block-list certain URL protocols, can be fooled by URLs in malicious inputs that use uppercase characters in the protocol. In particular, this can allow for malicious input to generate javascript: links in the output, even if the trust function tries to forbid this protocol via trust: (context) => context.protocol !== 'javascript'.

Patches

Upgrade to KaTeX v0.16.10 to remove this vulnerability.

Workarounds

• Allow-list instead of block protocols in your trust function.
• Manually lowercase context.protocol via context.protocol.toLowerCase() before attempting to check for certain protocols.
• Avoid use of or turn off the trust option.

Details

KaTeX did not normalize the protocol entry of the context object provided to a user-specified trust-function, so it could be a mix of lowercase and/or uppercase letters.

It is generally better to allow-list by protocol, in which case this would normally not be an issue. But in some cases, you might want to block-list, and the KaTeX documentation even provides such an example:

> Allow all commands but forbid specific protocol: trust: (context) => context.protocol !== 'file'

Currently KaTeX internally sees file: and File: URLs as different protocols, so context.protocol can be file or File, so the above check does not suffice. A simple workaround would be:

> trust: (context) => context.protocol.toLowerCase() !== 'file'

Most URL parsers normalize the scheme to lowercase. For example, RFC3986 says:

> Although schemes are case-insensitive, the canonical form is lowercase and documents that specify schemes must do so with lowercase letters. An implementation should accept uppercase letters as equivalent to lowercase in scheme names (e.g., allow “HTTP” as well as “http”) for the sake of robustness but should only produce lowercase scheme names for consistency.