MathJax TeX Macro Shadowing in GitHub Markdown

Topic Overview

This topic covers a vulnerability in GitHub’s rendering of mathematical notation with MathJax.

TeX macros can define custom commands that MathJax uses while rendering content.

A macro lookup-order issue allowed user-defined commands to shadow native mathematical operators.

An attacker could use this behavior to alter how content appeared across GitHub surfaces.

GitHub mitigated the issue by disabling the newcommand package.

Main Concepts

Concept 1

Definition: MathJax’s TeX input processor converts TeX notation into an internal representation, including MathML, for rendering.
Simple Explanation: TeX commands are parsed and turned into the mathematical content displayed on a page.
Example: A TeX command such as \sum is rendered as a summation operator.
Key Note: GitHub used MathJax to render mathematical expressions in places such as README files, Wiki pages, comments, and issues.

Concept 2

Definition: TeX macro commands such as \def, \newcommand, and \let allow users to define or modify commands.
Simple Explanation: A macro gives a command a custom meaning, which MathJax uses when it processes later notation.
Example: A user could define a custom macro such as \RR to display a bold “R”.
Key Note: MathJax’s newcommand package stored these definitions in a dedicated CommandMap named new-Command.

Concept 3

Definition: Macro shadowing occurs when a higher-priority user-defined macro takes precedence over a native command with the same name.
Simple Explanation: The native command is not removed; the parser finds the custom definition first and does not reach the native one.
Example: Redefining \sum could cause it to render as a bold “B” instead of the usual summation symbol.
Key Note: In the reported issue, the custom macro map was checked before the base maps, and macro definitions were added without checking for name conflicts.

Relationships, Processes, and Diagrams

Macro definition and lookup

\def or \newcommand
        ↓
addMacro writes definition to new-Command
        ↓
Parser checks macro maps in priority order
        ↓
new-Command matches first
        ↓
User-defined rendering is used; native command is not reached

The new-Command map was registered ahead of the base maps with priority -1. The write path added definitions to that map without checking whether the same command already existed in a base map.

Impact path

Injected macro in a GitHub Issue
        ↓
Native operator is shadowed during rendering
        ↓
Affected content appears with altered rendering on GitHub surfaces

Comparisons and Categorisations

Item	Feature	Use Case	Note
`\def`, `\newcommand`	Define or redefine macros	Change how commands or operators render	Reported as the highest-impact variants
`\let`	Assign one command’s meaning to another	Modify command behavior	Stored in the `new-Command` map
`\definecolor`	Define or alter text color	Change rendered text color	Described as a lower-impact variant
Base MathJax maps	Provide native commands such as `\sum`	Render standard mathematical notation	Shadowed when a higher-priority custom definition matched first

Examples and Applications

Example: A payload in a public repository’s GitHub Issue redefined the native \sum command so that it appeared as a bold “B” when related content was rendered.
Why? The parser checked the user-controlled new-Command map before the base map containing the native operator. The custom map therefore supplied the first match.
Result: The rendering was affected on github.com, gist.github.com, and GitHub mobile apps. The report described the result as altered content in the repository’s README rendering.

Key Notes and Exceptions

⭐ Key Notes

The native operator was occluded, not deleted: the base definition remained present but was not reached during lookup.
The root cause involved both map priority and the lack of a base-command conflict check when storing user macros.
The reported variants differed in impact, from changing displayed operators or content to changing text color.
The issue was reported to GitHub on May 20, 2022, and GitHub fixed it that same day. Additional variants were reported between May and August 2022.

⚠️ Exception: The described behavior concerns the vulnerability as reported. GitHub’s retest found that disabling the newcommand package prevented use of \def and related macros in this context.

Mitigation: Disable the newcommand package in MathJax’s TeX configuration:

window.MathJax = { tex: { packages: { '[-]': ['newcommand'] } } };

One-Minute Summary

GitHub used MathJax to render TeX notation in Markdown and other content.
The newcommand package allowed users to define macros.
User macros were stored in a high-priority new-Command map.
Definitions were added without checking for conflicts with native commands.
A matching custom macro could shadow a native operator such as \sum.
This changed rendered output across GitHub surfaces.
GitHub fixed the reported issue by disallowing the newcommand package.

OperatorOverwrite