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" 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
• Email us at katex-security@mit.edu