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

> Prototype pollution allows bypassing trust restrictions in KaTeX, enabling XSS via malicious math expressions in applications rendering unsanitized output.

- **Severity:** Low 2.1 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 1 Oct 2026
- **Updated:** 6 Oct 2026
- **Also known as:** CLSA-2026-1791284260, CLSA-2026-1792488994, DEBIAN-CVE-2026-103923, GHSA-238p-pmpm-9mq7, MINI-p78h-7q25-wggj, MINI-rhmf-wcw9-4rrr, UBUNTU-CVE-2026-103923, openSUSE-SU-2026:12075-1

## What is CVE-2026-103923?

CVE-2026-103923 is a low-severity cross-site scripting vulnerability in KaTeX, published on 1 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

KaTeX is a fast, easy-to-use JavaScript library for TeX math rendering on the web. From 0.11.0 until 0.18.2, KaTeX uses ordinary JavaScript property access for the renderer options object, the trust setting, default and processor setting metadata, and namespace lookup and group restoration, allowing inherited properties to be treated as explicitly supplied values. When Object.prototype is already polluted or an attacker controls the options object's prototype, attacker-controlled mathematical expressions can use an inherited trust value to enable trusted rendering and produce links capable of user-interaction cross-site scripting or loading attacker-selected external resources in a consuming application that inserts unsanitized KaTeX output into a page. KaTeX does not itself create the prototype pollution, and rendering an expression alone does not execute script. This issue is fixed in version 0.18.2.

Weakness: CWE-807 (Reliance on Untrusted Inputs in a Security Decision)

## Which versions are affected by CVE-2026-103923?

2 open source packages ship an affected version, including katex.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `katex` | npm | From 0.11.0 | 0.18.2 |
| `katex` | npm | From 0.11.0 | 0.18.2 |
| `python-jupyterlab-templates` | openSUSE:Tumbleweed | All versions | 0.5.3-3.1 |

## How do I fix CVE-2026-103923?

Upgrade katex to 0.18.2 or later.

## Is CVE-2026-103923 being exploited?

There is no known exploitation of CVE-2026-103923 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-103923 and fix it if it is affected.

What it is: CVE-2026-103923 is a low-severity cross-site scripting vulnerability in KaTeX, published on 1 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-103923 in the wild and no public exploit code.

Affected packages and fixed versions:
- katex (npm): affected from 0.11.0; fixed in 0.18.2
- katex (npm): affected from 0.11.0; fixed in 0.18.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-103923/ (Markdown: https://insights.sbomify.com/CVE-2026-103923/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-103923
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-103923
- OSV: https://osv.dev/vulnerability/CVE-2026-103923

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-103923/
Summaries are generated and may contain mistakes; the official description above is the record.