KaTeX's maxExpand bypassed by Unicode sub/superscripts
### Impact
KaTeX users who render untrusted mathematical expressions could encounter malicious input using `\def` or `\newcommand` that causes a near-infinite loop, despite setting `maxExpand` to avoid such loops. This can be used as an availability attack, where e.g. a client rendering another user's KaTeX input will be unable to use the site due to memory overflow, tying up the main thread, or stack overflow.
### Patches
Upgrade to KaTeX v0.16.10 to remove this vulnerability.
### Workarounds
Forbid inputs containing any of the characters `₊₋₌₍₎₀₁₂₃₄₅₆₇₈₉ₐₑₕᵢⱼₖₗₘₙₒₚᵣₛₜᵤᵥₓᵦᵧᵨᵩᵪ⁺⁻⁼⁽⁾⁰¹²³⁴⁵⁶⁷⁸⁹ᵃᵇᶜᵈᵉᵍʰⁱʲᵏˡᵐⁿᵒᵖʳˢᵗᵘʷˣʸᶻᵛᵝᵞᵟᵠᵡ` before passing them to KaTeX.
(There is no easy workaround for the auto-render extension.)
### Details
KaTeX supports an option named `maxExpand` which aims to prevent infinitely recursive macros from consuming all available memory and/or triggering a stack overflow error. Unfortunately, support for "Unicode (sub|super)script characters"
https://github.com/KaTeX/KaTeX/commit/d8fc35e6a97f8e561c723b93ad275cf5a7f3094a allows an attacker to bypass this limit. Each sub/superscript group instantiated a separate Parser with its own limit on macro executions, without inheriting the current count of macro executions from its parent. This has been corrected in KaTeX v0.16.10.
### For more information
If you have any questions or comments about this advisory:
* Open an issue or security advisory in the KaTeX repository
https://github.com/KaTeX/KaTeX/
* Email us at [katex-security@mit.edu](mailto:katex-security@mit.edu)