{"id":"CVE-2026-59930","aliases":["AZL-92223","DEBIAN-CVE-2026-59930","EUVD-2026-42330","GHSA-2hm2-hc3v-44h9","PYSEC-2026-2218","SUSE-SU-2026:22655-1","UBUNTU-CVE-2026-59930","openSUSE-SU-2026:21339-1"],"status":"published","title":"Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id=\"toc_N\"` content","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","severity":{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00192,"percentile":0.08895,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1284","CWE-345"],"published":"2026-07-08T16:13:21Z","modified":"2026-11-05T05:40:51Z","fixes":[{"ecosystem":"PyPI","package":"mistune","fixed_in":["3.3.0"],"sources":["ghsa","osv"]},{"ecosystem":"Azure Linux:3","package":"python-mistune","fixed_in":["3.3.0-1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.3.0"}]}]},{"source":"osv","ecosystem":"Azure Linux:3","package":"python-mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.3.0-1"}]}]},{"source":"osv","ecosystem":"Debian:12","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.0.4-1","2.0.4-2","3.0.2-1","3.0.2-2","3.1.3-1","3.1.4-1","3.3.4-1","3.4.0-1"]},{"source":"osv","ecosystem":"Debian:13","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.1.3-1","3.1.4-1","3.3.4-1","3.4.0-1"]},{"source":"osv","ecosystem":"Debian:14","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.1.3-1","3.1.4-1","3.3.4-1","3.4.0-1"]},{"source":"osv","ecosystem":"PyPI","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.3.0"}]}],"versions":["0.1.0","0.2.0","0.3.0","0.3.1","0.4","0.4.1","0.5","0.5.1","0.6","0.7","0.7.1","0.7.2","0.7.3","0.7.4","0.8","0.8.1","0.8.2","0.8.3","0.8.4","2.0.0","2.0.0a1","2.0.0a2","2.0.0a3","2.0.0a4","2.0.0a5","2.0.0a6","2.0.0rc1","2.0.1","2.0.2","2.0.3","2.0.4","2.0.5","2.1.0","3.0.0","3.0.0a1","3.0.0a2","3.0.0a3","3.0.0rc1","3.0.0rc2","3.0.0rc3","3.0.0rc4","3.0.0rc5","3.0.1","3.0.2","3.1.0","3.1.1","3.1.2","3.1.3","3.1.4","3.2.0","3.2.1"]},{"source":"osv","ecosystem":"PyPI","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.3.0"}]}],"versions":["0.1.0","0.2.0","0.3.0","0.3.1","0.4","0.4.1","0.5","0.5.1","0.6","0.7","0.7.1","0.7.2","0.7.3","0.7.4","0.8","0.8.1","0.8.2","0.8.3","0.8.4","2.0.0","2.0.0a1","2.0.0a2","2.0.0a3","2.0.0a4","2.0.0a5","2.0.0a6","2.0.0rc1","2.0.1","2.0.2","2.0.3","2.0.4","2.0.5","2.1.0","3.0.0","3.0.0a1","3.0.0a2","3.0.0a3","3.0.0rc1","3.0.0rc2","3.0.0rc3","3.0.0rc4","3.0.0rc5","3.0.1","3.0.2","3.1.0","3.1.1","3.1.2","3.1.3","3.1.4","3.2.0","3.2.1"]},{"source":"osv","ecosystem":"SUSE:Linux Enterprise Server 16.0","package":"python-mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.1.3-160000.5.1"}]}],"bundled":10},{"source":"osv","ecosystem":"SUSE:Linux Enterprise Server for SAP applications 16.0","package":"python-mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.1.3-160000.5.1"}]}],"bundled":10},{"source":"osv","ecosystem":"Ubuntu:16.04:LTS","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.6-2","0.7.1-1"]},{"source":"osv","ecosystem":"Ubuntu:18.04:LTS","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.7.4-1","0.8-1","0.8.1-1","0.8.3-2"]},{"source":"osv","ecosystem":"Ubuntu:20.04:LTS","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.8.4-1","0.8.4-2"]},{"source":"osv","ecosystem":"Ubuntu:22.04:LTS","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.8.4-4","0.8.4-5","2.0.0-1+really0.8.4-1"]},{"source":"osv","ecosystem":"Ubuntu:24.04:LTS","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.0.4-2","3.0.2-1"]},{"source":"osv","ecosystem":"Ubuntu:25.10","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.0.2-2","3.1.3-1"]},{"source":"osv","ecosystem":"Ubuntu:26.10","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.1.4-1"]},{"source":"osv","ecosystem":"Ubuntu:26.04:LTS","package":"mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.1.3-1","3.1.4-1"]},{"source":"osv","ecosystem":"openSUSE:Leap 16.0","package":"python-mistune","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.1.3-160000.5.1"}]}],"bundled":10}],"products":[{"vendor":"lepture","product":"mistune"},{"vendor":"mistune_project","product":"mistune"}],"references":[{"url":"https://github.com/lepture/mistune/security/advisories/GHSA-2hm2-hc3v-44h9","tags":["x_refsource_CONFIRM","exploit","Exploit","Third Party Advisory","advisory","web","evidence","report"]},{"url":"https://github.com/lepture/mistune/commit/c4093c4742ed0d10d9332fb8edb455869b7b581b","tags":["x_refsource_MISC","Patch","fix","web"]},{"url":"https://github.com/lepture/mistune/releases/tag/v3.3.0","tags":["x_refsource_MISC","Release Notes","web","advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59930","tags":["web","advisory"]},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2218.yaml","tags":["web"]},{"url":"https://github.com/advisories/GHSA-2hm2-hc3v-44h9"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/59xxx/CVE-2026-59930.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-59930","tags":["advisory"]},{"url":"https://github.com/lepture/mistune","tags":["package"]},{"url":"https://www.suse.com/support/update/announcement/2026/suse-su-202622655-1/","tags":["advisory"]},{"url":"https://bugzilla.suse.com/1271082","tags":["report"]},{"url":"https://bugzilla.suse.com/1271117","tags":["report"]},{"url":"https://bugzilla.suse.com/1271119","tags":["report"]},{"url":"https://bugzilla.suse.com/1271121","tags":["report"]},{"url":"https://bugzilla.suse.com/1271125","tags":["report"]},{"url":"https://bugzilla.suse.com/1271127","tags":["report"]},{"url":"https://bugzilla.suse.com/1271128","tags":["report"]},{"url":"https://bugzilla.suse.com/1271131","tags":["report"]},{"url":"https://bugzilla.suse.com/1271132","tags":["report"]},{"url":"https://www.suse.com/security/cve/CVE-2026-44896","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59922","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59923","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59924","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59925","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59926","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59927","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59928","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59929","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-59930","tags":["web"]},{"url":"https://ubuntu.com/security/CVE-2026-59930","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-59930","tags":["report"]},{"url":"","tags":["advisory"]}],"sources":{"cve":[{"id":"CVE-2026-59930","title":"Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id=\"toc_N\"` content","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T16:13:21.999Z","modified":"2026-07-09T13:54:33.627Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-345","CWE-1284"],"updated_at":"2026-11-02T14:19:58.944663Z"}],"euvd":[{"id":"EUVD-2026-42330","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T16:13:21Z","modified":"2026-07-09T13:54:33Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T18:20:04.523855Z"}],"ghsa":[{"id":"GHSA-2hm2-hc3v-44h9","title":"Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id=\"toc_N\"` content","description":"## Summary\n\n**Type:** Predictable identifier generation. The `toc` plugin and `TableOfContents` directive both default to generating heading IDs of the form `toc_1`, `toc_2`, `toc_3`, ... with no input-derived component. An attacker who can place a heading anywhere in the document can predict which `toc_N` ID it will receive, and can inject HTML elsewhere (in a non-heading context) that uses the same `id=\"toc_N\"` to either (a) shadow the legitimate heading anchor, breaking same-page navigation, or (b) collide with CSS or JavaScript that targets `#toc_N` selectors, redirecting click handlers and styling to attacker-chosen content.\n**File:** `src/mistune/toc.py` line 36-37 (`heading_id = lambda token, index: \"toc_\" + str(index + 1)`); `src/mistune/directives/toc.py` line 33 (same default).\n**Root cause:** the default `heading_id` callback ignores the heading's text content and uses only the headings's order in the document. Two documents rendered together (or one document with attacker-influenced headings spliced into trusted content) produce overlapping `toc_N` IDs. Because `id` attribute uniqueness is required by HTML, browsers behaviour on duplicate IDs is undefined; `document.getElementById('toc_1')` returns the first match, `getElementsByTagName + querySelector` semantics differ across paths, and CSS rules targeting `#toc_1` apply to whichever element matches first in tree order.\n\n## Affected Code\n\n**File:** `src/mistune/toc.py`, lines 33-39.\n\n```python\ndef add_toc_hook(md, min_level=1, max_level=3, heading_id=None):\n    if heading_id is None:\n        def heading_id(token, index):\n            return \"toc_\" + str(index + 1)               # \u003c-- BUG: index-only ID, no slug derived from heading text\n```\n\n**File:** `src/mistune/directives/toc.py`, lines 32-33.\n\n```python\nclass TableOfContents(DirectivePlugin):\n    def __init__(self, min_level=1, max_level=3):\n        # ...\n\n    def generate_heading_id(self, token, index):\n        return \"toc_\" + str(index + 1)                   # \u003c-- BUG: same predictable scheme\n```\n\n**Why it's wrong:** the standard markdown-engine convention (used by GitHub-flavoured Markdown, Sphinx, MkDocs, pandoc, every modern markdown renderer in production) is to slugify the heading TEXT for the ID — `\u003ch1 id=\"introduction\"\u003eIntroduction\u003c/h1\u003e` — with a numeric suffix appended only when slug collisions occur. mistune's default punts the slugification entirely and produces purely positional IDs that an attacker can predict in O(1).\n\nThe downstream impacts:\n- Same-page links with `[click](#toc_1)` go to whichever element with `id=\"toc_1\"` appears first in tree order. If the attacker can land any HTML element with `id=\"toc_1\"` before the real heading (via inline_html with `escape=False`, via the include-directive HTML branch, via attacker-supplied content earlier in the document), navigation is hijacked.\n- CSS rules targeting `#toc_1` apply to the wrong element.\n- JavaScript bound to `document.getElementById('toc_1')` operates on the wrong element.\n- The TOC's own `\u003ca href=\"#toc_1\"\u003e` link in the rendered TOC list points to whichever element wins the duplicate-ID race.\n\n## Exploit Chain\n\n1. Application uses mistune with `add_toc_hook(md)` or `TableOfContents` directive enabled (the documented setup for sites with TOC support).\n2. Application renders an attacker-supplied document, or splices attacker content into a trusted document. With `escape=False` (or via the include-directive `.html` branch covered by my prior advisory), the attacker can place `\u003ca id=\"toc_1\"\u003e...\u003c/a\u003e` anywhere in the document.\n3. mistune assigns `id=\"toc_1\"` to the first heading. Now there are two elements with `id=\"toc_1\"` in the page.\n4. The rendered TOC contains `\u003ca href=\"#toc_1\"\u003eFirst heading\u003c/a\u003e`. Clicking it navigates to whichever element with `id=\"toc_1\"` appears first in tree order. If the attacker placed their `\u003ca id=\"toc_1\"\u003e` BEFORE the heading, navigation is hijacked.\n5. Same-page CSS / JS / aria-described references to `#toc_1` similarly redirect.\n\n## Security Impact\n\n**Severity:** sec-low. Not a direct XSS or RCE; the issue is identifier confusion that enables UI-redirection / navigation-hijack attacks. The realistic attacker capability is \"make an internal anchor link go to attacker content instead of the real heading\", or \"make a CSS selector apply to attacker content\", or \"break aria/screen-reader associations\".\n**Attacker capability:** with the ability to plant any HTML element with `id=\"toc_N\"` in the document, hijack `\u003ca href=\"#toc_N\"\u003e` navigation and any CSS/JS targeting that ID. With `escape=False`, this is straightforward. With `escape=True`, the attacker needs another vector to land a raw `id` attribute (one of the include-directive branches, a sibling tooling pipeline that lets HTML through, etc.).\n**Preconditions:** application uses TOC + the default `heading_id` callback. If the application provides its own `heading_id` (e.g., one based on slugified heading text, with collision suffixes), this finding does not apply.\n**Differential:** PoC-verified against mistune@3.2.1:\n\n```python\nimport mistune\nfrom mistune.directives import RSTDirective, TableOfContents\nmd = mistune.create_markdown(plugins=[RSTDirective([TableOfContents()])])\n\nprint(md('''\n.. toc::\n\n# Heading 1\n\n# Heading 2\n'''))\n\n# Output (note: id=\"toc_1\" / id=\"toc_2\", purely positional):\n# \u003cdetails class=\"toc\" open\u003e\n#   \u003csummary\u003eTable of Contents\u003c/summary\u003e\n#   \u003cul\u003e\n#     \u003cli\u003e\u003ca href=\"#toc_1\"\u003eHeading 1\u003c/a\u003e\u003c/li\u003e\n#     \u003cli\u003e\u003ca href=\"#toc_2\"\u003eHeading 2\u003c/a\u003e\u003c/li\u003e\n#   \u003c/ul\u003e\n# \u003c/details\u003e\n# \u003ch1 id=\"toc_1\"\u003eHeading 1\u003c/h1\u003e\n# \u003ch1 id=\"toc_2\"\u003eHeading 2\u003c/h1\u003e\n```\n\nThe patched build (with the suggested fix below) produces text-derived slugs like `id=\"heading-1\"` and `id=\"heading-2\"`, which are tied to content rather than position.\n\n## Suggested Fix\n\nDefault to slugifying the heading text:\n\n```diff\n--- a/src/mistune/toc.py\n+++ b/src/mistune/toc.py\n@@ -33,9 +33,18 @@ def add_toc_hook(md, min_level=1, max_level=3, heading_id=None):\n     if heading_id is None:\n+        import re\n+        _slug_re = re.compile(r\"[^a-z0-9]+\")\n+        seen = {}\n         def heading_id(token, index):\n-            return \"toc_\" + str(index + 1)\n+            text = striptags(md.renderer(md.inline(token[\"text\"], {}), BlockState()))\n+            slug = _slug_re.sub(\"-\", text.lower()).strip(\"-\") or \"section\"\n+            n = seen.get(slug, 0)\n+            seen[slug] = n + 1\n+            return slug if n == 0 else f\"{slug}-{n}\"\n```\n\nSame change applies to `src/mistune/directives/toc.py:33`. Existing applications that have hardcoded `#toc_N` anchors will break; document the migration in the changelog and consider providing an opt-out flag for the legacy behaviour. Add a regression test that asserts heading IDs are slug-derived, not position-derived, and that collisions get a `-N` suffix.","status":"published","published":"2026-07-20T21:35:11Z","modified":"2026-07-20T21:35:12Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"cwes":["CWE-345","CWE-1284"],"updated_at":"2026-11-02T14:40:48.410776Z"}],"nvd":[{"id":"CVE-2026-59930","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T17:17:28.867Z","modified":"2026-10-25T17:10:00.197Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium","assessor":"secondary"}],"cwes":["CWE-345","CWE-1284"],"updated_at":"2026-11-02T14:12:28.4135Z"}],"osv":[{"id":"CVE-2026-59930","title":"Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id=\"toc_N\"` content","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T16:13:21.999Z","modified":"2026-10-28T02:30:42.357534214Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"cwes":["CWE-1284","CWE-345"],"updated_at":"2026-11-02T15:45:12.496548Z"},{"id":"DEBIAN-CVE-2026-59930","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T17:17:28.867Z","modified":"2026-10-26T12:02:38.479946514Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T15:45:57.932517Z"},{"id":"GHSA-2hm2-hc3v-44h9","title":"Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id=\"toc_N\"` content","description":"## Summary\n\n**Type:** Predictable identifier generation. The `toc` plugin and `TableOfContents` directive both default to generating heading IDs of the form `toc_1`, `toc_2`, `toc_3`, ... with no input-derived component. An attacker who can place a heading anywhere in the document can predict which `toc_N` ID it will receive, and can inject HTML elsewhere (in a non-heading context) that uses the same `id=\"toc_N\"` to either (a) shadow the legitimate heading anchor, breaking same-page navigation, or (b) collide with CSS or JavaScript that targets `#toc_N` selectors, redirecting click handlers and styling to attacker-chosen content.\n**File:** `src/mistune/toc.py` line 36-37 (`heading_id = lambda token, index: \"toc_\" + str(index + 1)`); `src/mistune/directives/toc.py` line 33 (same default).\n**Root cause:** the default `heading_id` callback ignores the heading's text content and uses only the headings's order in the document. Two documents rendered together (or one document with attacker-influenced headings spliced into trusted content) produce overlapping `toc_N` IDs. Because `id` attribute uniqueness is required by HTML, browsers behaviour on duplicate IDs is undefined; `document.getElementById('toc_1')` returns the first match, `getElementsByTagName + querySelector` semantics differ across paths, and CSS rules targeting `#toc_1` apply to whichever element matches first in tree order.\n\n## Affected Code\n\n**File:** `src/mistune/toc.py`, lines 33-39.\n\n```python\ndef add_toc_hook(md, min_level=1, max_level=3, heading_id=None):\n    if heading_id is None:\n        def heading_id(token, index):\n            return \"toc_\" + str(index + 1)               # \u003c-- BUG: index-only ID, no slug derived from heading text\n```\n\n**File:** `src/mistune/directives/toc.py`, lines 32-33.\n\n```python\nclass TableOfContents(DirectivePlugin):\n    def __init__(self, min_level=1, max_level=3):\n        # ...\n\n    def generate_heading_id(self, token, index):\n        return \"toc_\" + str(index + 1)                   # \u003c-- BUG: same predictable scheme\n```\n\n**Why it's wrong:** the standard markdown-engine convention (used by GitHub-flavoured Markdown, Sphinx, MkDocs, pandoc, every modern markdown renderer in production) is to slugify the heading TEXT for the ID — `\u003ch1 id=\"introduction\"\u003eIntroduction\u003c/h1\u003e` — with a numeric suffix appended only when slug collisions occur. mistune's default punts the slugification entirely and produces purely positional IDs that an attacker can predict in O(1).\n\nThe downstream impacts:\n- Same-page links with `[click](#toc_1)` go to whichever element with `id=\"toc_1\"` appears first in tree order. If the attacker can land any HTML element with `id=\"toc_1\"` before the real heading (via inline_html with `escape=False`, via the include-directive HTML branch, via attacker-supplied content earlier in the document), navigation is hijacked.\n- CSS rules targeting `#toc_1` apply to the wrong element.\n- JavaScript bound to `document.getElementById('toc_1')` operates on the wrong element.\n- The TOC's own `\u003ca href=\"#toc_1\"\u003e` link in the rendered TOC list points to whichever element wins the duplicate-ID race.\n\n## Exploit Chain\n\n1. Application uses mistune with `add_toc_hook(md)` or `TableOfContents` directive enabled (the documented setup for sites with TOC support).\n2. Application renders an attacker-supplied document, or splices attacker content into a trusted document. With `escape=False` (or via the include-directive `.html` branch covered by my prior advisory), the attacker can place `\u003ca id=\"toc_1\"\u003e...\u003c/a\u003e` anywhere in the document.\n3. mistune assigns `id=\"toc_1\"` to the first heading. Now there are two elements with `id=\"toc_1\"` in the page.\n4. The rendered TOC contains `\u003ca href=\"#toc_1\"\u003eFirst heading\u003c/a\u003e`. Clicking it navigates to whichever element with `id=\"toc_1\"` appears first in tree order. If the attacker placed their `\u003ca id=\"toc_1\"\u003e` BEFORE the heading, navigation is hijacked.\n5. Same-page CSS / JS / aria-described references to `#toc_1` similarly redirect.\n\n## Security Impact\n\n**Severity:** sec-low. Not a direct XSS or RCE; the issue is identifier confusion that enables UI-redirection / navigation-hijack attacks. The realistic attacker capability is \"make an internal anchor link go to attacker content instead of the real heading\", or \"make a CSS selector apply to attacker content\", or \"break aria/screen-reader associations\".\n**Attacker capability:** with the ability to plant any HTML element with `id=\"toc_N\"` in the document, hijack `\u003ca href=\"#toc_N\"\u003e` navigation and any CSS/JS targeting that ID. With `escape=False`, this is straightforward. With `escape=True`, the attacker needs another vector to land a raw `id` attribute (one of the include-directive branches, a sibling tooling pipeline that lets HTML through, etc.).\n**Preconditions:** application uses TOC + the default `heading_id` callback. If the application provides its own `heading_id` (e.g., one based on slugified heading text, with collision suffixes), this finding does not apply.\n**Differential:** PoC-verified against mistune@3.2.1:\n\n```python\nimport mistune\nfrom mistune.directives import RSTDirective, TableOfContents\nmd = mistune.create_markdown(plugins=[RSTDirective([TableOfContents()])])\n\nprint(md('''\n.. toc::\n\n# Heading 1\n\n# Heading 2\n'''))\n\n# Output (note: id=\"toc_1\" / id=\"toc_2\", purely positional):\n# \u003cdetails class=\"toc\" open\u003e\n#   \u003csummary\u003eTable of Contents\u003c/summary\u003e\n#   \u003cul\u003e\n#     \u003cli\u003e\u003ca href=\"#toc_1\"\u003eHeading 1\u003c/a\u003e\u003c/li\u003e\n#     \u003cli\u003e\u003ca href=\"#toc_2\"\u003eHeading 2\u003c/a\u003e\u003c/li\u003e\n#   \u003c/ul\u003e\n# \u003c/details\u003e\n# \u003ch1 id=\"toc_1\"\u003eHeading 1\u003c/h1\u003e\n# \u003ch1 id=\"toc_2\"\u003eHeading 2\u003c/h1\u003e\n```\n\nThe patched build (with the suggested fix below) produces text-derived slugs like `id=\"heading-1\"` and `id=\"heading-2\"`, which are tied to content rather than position.\n\n## Suggested Fix\n\nDefault to slugifying the heading text:\n\n```diff\n--- a/src/mistune/toc.py\n+++ b/src/mistune/toc.py\n@@ -33,9 +33,18 @@ def add_toc_hook(md, min_level=1, max_level=3, heading_id=None):\n     if heading_id is None:\n+        import re\n+        _slug_re = re.compile(r\"[^a-z0-9]+\")\n+        seen = {}\n         def heading_id(token, index):\n-            return \"toc_\" + str(index + 1)\n+            text = striptags(md.renderer(md.inline(token[\"text\"], {}), BlockState()))\n+            slug = _slug_re.sub(\"-\", text.lower()).strip(\"-\") or \"section\"\n+            n = seen.get(slug, 0)\n+            seen[slug] = n + 1\n+            return slug if n == 0 else f\"{slug}-{n}\"\n```\n\nSame change applies to `src/mistune/directives/toc.py:33`. Existing applications that have hardcoded `#toc_N` anchors will break; document the migration in the changelog and consider providing an opt-out flag for the legacy behaviour. Add a regression test that asserts heading IDs are slug-derived, not position-derived, and that collisions get a `-N` suffix.","status":"published","published":"2026-07-20T21:35:11Z","modified":"2026-07-20T21:46:43.352385519Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"cwes":["CWE-1284","CWE-345"],"updated_at":"2026-11-02T15:46:34.338541Z"},{"id":"PYSEC-2026-2218","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T17:17:28.867Z","modified":"2026-07-13T07:15:18.864143856Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T15:52:55.537808Z"},{"id":"UBUNTU-CVE-2026-59930","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T17:17:00Z","modified":"2026-10-17T12:59:14.371388936Z","cvss":[{"version":"3.1","score":4.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T16:56:41.911783Z"},{"id":"AZL-92223","title":"CVE-2026-59930 affecting package python-mistune for versions less than 3.3.0-1","description":"Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id=\"toc_N\" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.","status":"published","published":"2026-07-08T17:17:28Z","modified":"2026-11-05T05:40:51Z","updated_at":"2026-11-05T16:41:25.880689Z"},{"id":"openSUSE-SU-2026:21339-1","title":"Security update for python-mistune","description":"This update for python-mistune fixes the following issues\n\n- CVE-2026-59922: quadratic-time parsing on long runs of some markers in `formatting.py` can lead to DoS (bsc#1271117).\n- CVE-2026-59923: `HTMLRenderer.safe_url()` does not block percent-encoded javascript URIs and allows for XSS\n  (bsc#1271119).\n- CVE-2026-59924: improper processing of user-supplied include paths in `Include.parse()` can lead to path traversal\n  and arbitrary file reads (bsc#1271121).\n- CVE-2026-59925: quadratic-time parsing on long runs of some emphasis pairs in `inline_parser` can lead to DoS\n  (bsc#1271125).\n- CVE-2026-59926: improper escaping in `render_admonition()` can lead to atribute injection and XSS (bsc#1271127).\n- CVE-2026-59927: uncontrolled recursion when processing two markdown files that include each other can lead to a DoS\n   (bsc#1271128).\n- CVE-2026-59928: quadratic-time parsing on long lists of repeated reference-link definitions in `block_parser` can\n  lead to DoS (bsc#1271131).\n- CVE-2026-59929: HARMFUL_PROTOCOLS list misses legacy and chained schemes and allow arbitrary script execution in\n  user agents (bsc#1271132).\n- CVE-2026-59930: the `toc` plugin and `TableOfContents` directive generate heading IDs with predictable values and\n  allow for collisions with attacker-controlled `id=\"toc_N\"` content (bsc#1271082).\n","status":"published","published":"2026-07-14T10:41:18Z","modified":"2026-07-15T10:00:11.548963212Z","updated_at":"2026-11-02T16:58:41.885092Z"},{"id":"SUSE-SU-2026:22655-1","title":"Security update for python-mistune","description":"This update for python-mistune fixes the following issues\n\n- CVE-2026-59922: quadratic-time parsing on long runs of some markers in `formatting.py` can lead to DoS (bsc#1271117).\n- CVE-2026-59923: `HTMLRenderer.safe_url()` does not block percent-encoded javascript URIs and allows for XSS\n  (bsc#1271119).\n- CVE-2026-59924: improper processing of user-supplied include paths in `Include.parse()` can lead to path traversal\n  and arbitrary file reads (bsc#1271121).\n- CVE-2026-59925: quadratic-time parsing on long runs of some emphasis pairs in `inline_parser` can lead to DoS\n  (bsc#1271125).\n- CVE-2026-59926: improper escaping in `render_admonition()` can lead to atribute injection and XSS (bsc#1271127).\n- CVE-2026-59927: uncontrolled recursion when processing two markdown files that include each other can lead to a DoS\n   (bsc#1271128).\n- CVE-2026-59928: quadratic-time parsing on long lists of repeated reference-link definitions in `block_parser` can\n  lead to DoS (bsc#1271131).\n- CVE-2026-59929: HARMFUL_PROTOCOLS list misses legacy and chained schemes and allow arbitrary script execution in\n  user agents (bsc#1271132).\n- CVE-2026-59930: the `toc` plugin and `TableOfContents` directive generate heading IDs with predictable values and\n  allow for collisions with attacker-controlled `id=\"toc_N\"` content (bsc#1271082).\n","status":"published","published":"2026-07-14T10:37:38Z","modified":"2026-07-17T18:24:05.645518218Z","updated_at":"2026-11-02T15:54:03.080707Z"}]}}