# Mistune toc / TableOfContents directive: heading IDs use predictable \`toc_N\` numbering with no slugification, allowing collision with attacker-controlled \`id="toc_N"\` content

Advisory record · `GHSA-2hm2-hc3v-44h9`

- **Identifiers:** `GHSA-2hm2-hc3v-44h9` · `CVE-2026-59930` · `PYSEC-2026-2218`
- **State:** Confirmed
- **Severity:** CVSS 4.3 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-345](https://cwe.mitre.org/data/definitions/345.html) · [CWE-1284](https://cwe.mitre.org/data/definitions/1284.html)
- **Tags:** [CWE-345](https://agentprime.io/advisories/tag/cwe-345.md) · [CWE-1284](https://agentprime.io/advisories/tag/cwe-1284.md)
- **Published:** 20 Jul 2026
- **Updated:** 20 Jul 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 20 Jul 2026, 21:35 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `mistune` (PyPI): affected `<3.3.0`; fixed in `3.3.0`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> mistune versions before 3.3.0 give table-of-contents headings predictable IDs like toc_1 and toc_2. An attacker who can inject raw HTML with id attributes can redirect same-page links and CSS or JavaScript aimed at those IDs. Upgrade to mistune 3.3.0 or later.

Written 5 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 4.3 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0019, the 8th percentile, on 7 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:42 UTC: First seen by agentprime
- 31 Oct 2026, 14:42 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-2hm2-hc3v-44h9)

## Sources

- [GitHub Advisory Database: GHSA-2hm2-hc3v-44h9](https://github.com/advisories/GHSA-2hm2-hc3v-44h9)
- [OSV: GHSA-2hm2-hc3v-44h9](https://osv.dev/vulnerability/GHSA-2hm2-hc3v-44h9)
- <https://github.com/lepture/mistune/security/advisories/GHSA-2hm2-hc3v-44h9>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-59930>
- <https://github.com/lepture/mistune/commit/c4093c4742ed0d10d9332fb8edb455869b7b581b>
- <https://github.com/lepture/mistune>
- <https://github.com/lepture/mistune/releases/tag/v3.3.0>
- <https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2218.yaml>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-2hm2-hc3v-44h9>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-2hm2-hc3v-44h9/>. Everything for agents: <https://agentprime.io/llms.txt>.