CVE-2026-103923: KaTeX: Existing prototype pollution can bypass trust restrictions

KaTeX can act as a read-side prototype pollution gadget in applications where Object.prototype has already been polluted, or where an attacker can influence the prototype of the renderer options object. (KaTeX does not enable said prototype pollution. This advisory applies when combining KaTeX with other vulnerable software that allows for prototype pollution.)

Affected versions may treat inherited properties from Object.prototype as renderer options, internal setting metadata, or namespace entries. In particular, an inherited trust value will be treated as though the application explicitly enabled trusted rendering, rather than using the documented default of false.

With attacker-controlled mathematical expressions, this can produce links capable of user-interaction cross-site scripting or load attacker-selected external resources. Exploitation requires the consuming application to insert KaTeX output into a web page without a separate sanitizer. (KaTeX does not execute scripts merely by rendering an expression.)

Other inherited settings can alter rendering behavior or resource limits. Inherited setting metadata can affect how defaults and supplied options are processed, while inherited namespace properties can be mistaken for defined macros or other internal values.