{"id":"PYSEC-2026-3854","summary":"Uncontrolled recursion DoS in JustHTML() via deeply nested HTML","details":"### Summary\n\njusthtml through 1.9.1 allows denial of service via deeply nested HTML. During parsing, `JustHTML.__init__()` always reaches `TreeBuilder.finish()`, which unconditionally calls `_populate_selectedcontent()`. That function recursively traverses the DOM via `_find_elements()` / `_find_element()` without a depth bound, allowing attacker-controlled deeply nested input to trigger an unhandled `RecursionError` on CPython. Depending on the host application's exception handling, this can abort parsing, fail requests, or terminate a worker/process.\n\n### Details\n\n`TreeBuilder.finish()` ([`treebuilder.py#L476`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/treebuilder.py#L476)) unconditionally calls `_populate_selectedcontent(self.document)` at [line 494](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/treebuilder.py#L494). `_populate_selectedcontent()` ([`treebuilder.py#L1243`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/treebuilder.py#L1243)) calls `_find_elements()` ([`treebuilder.py#L1280`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/treebuilder.py#L1280)) to recursively search the DOM tree for `\u003cselect\u003e` elements:\n\n```python\ndef _find_elements(self, node: Any, name: str, result: list[Any]) -\u003e None:\n    \"\"\"Recursively find all elements with given name.\"\"\"\n    if node.name == name:\n        result.append(node)\n    if node.has_child_nodes():\n        for child in node.children:\n            self._find_elements(child, name, result)  # recursive call\n```\n\nWhen the DOM tree depth exceeds CPython's default recursion limit (1000), this raises an unhandled `RecursionError`. The full call path is:\n\n`JustHTML(html)` → `tokenizer.run()` → `tree_builder.finish()` → `_populate_selectedcontent(document)` → `_find_elements(root, \"select\", selects)` (recursive)\n\nDeeply nested DOM trees can be produced by nesting `\u003cdiv\u003e` tags ~1000 levels deep. On CPython with the default recursion limit, approximately 11 KB of `\u003cdiv\u003e` nesting is sufficient to trigger the error. The exact depth threshold is environment-dependent (CPython version, recursion limit setting, call stack depth at invocation).\n\nAdditional recursive functions are affected on already-parsed deep trees:\n- `Node.clone_node(deep=True)` ([`node.py#L523`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/node.py#L523)) — called during sanitization\n- `_node_to_html()` ([`serialize.py#L580`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/serialize.py#L580)) — used by `to_html(pretty=True)`\n- `_to_markdown_walk()` ([`node.py#L817`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/node.py#L817)) — used by `to_markdown()`\n\nNote: the library already uses iterative traversal in several comparable functions (e.g., `_node_to_html_compact` at [`serialize.py#L197`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/serialize.py#L197), `_to_text_collect` at [`node.py#L161`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/node.py#L161), `_is_blocky_element` at [`serialize.py#L405`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/serialize.py#L405), `apply_to_children` at [`transforms.py#L1642`](https://github.com/EmilStenstrom/justhtml/blob/a866b6077770d9ec4cb6b6f9bfe7c918f98455e4/src/justhtml/transforms.py#L1642)), demonstrating the correct pattern.\n\n### PoC\n\n```python\nfrom justhtml import JustHTML\n\nhtml = \"\u003cdiv\u003e\" * 1000 + \"x\" + \"\u003c/div\u003e\" * 1000\ndoc = JustHTML(html)  # raises RecursionError\n```\n\nTest environment: CPython 3.14.3, macOS ARM64 (Apple Silicon), justhtml 1.9.1, default recursion limit (1000)\n\n| Input | Size | Result |\n|-------|------|--------|\n| `\u003cdiv\u003e` × 500 | 5,501 bytes | OK |\n| `\u003cdiv\u003e` × 800 | 8,801 bytes | OK |\n| `\u003cdiv\u003e` × 1000 | 11,001 bytes | RecursionError |\n\nThe error occurs with both `sanitize=True` (default) and `sanitize=False`.\n\n### Impact\n\nAn attacker who can supply HTML for parsing can trigger an unhandled `RecursionError` during `JustHTML()` construction. The error is triggered during construction and is not avoided by `justhtml` configuration alone; mitigating it requires host-application exception handling or input constraints. Depending on the host application's exception handling, this can abort parsing, fail requests, or terminate a worker/process.\n\n### Suggested Fix\n\nConvert the recursive tree traversal functions to iterative implementations using an explicit stack. Example for `_find_elements`:\n\n```python\ndef _find_elements(self, node: Any, name: str, result: list[Any]) -\u003e None:\n    stack = [node]\n    while stack:\n        current = stack.pop()\n        if current.name == name:\n            result.append(current)\n        if current.has_child_nodes():\n            stack.extend(reversed(current.children))\n```\n\nThe same conversion should be applied to `_find_element`, `clone_node(deep=True)`, `_node_to_html()`, and `_to_markdown_walk()`.","aliases":["CVE-2026-9769","GHSA-v7cf-c9rm-wm3j"],"modified":"2026-09-10T12:15:07.010761627Z","published":"2026-09-10T09:44:49.394670Z","references":[{"type":"WEB","url":"https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-v7cf-c9rm-wm3j"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-9769"},{"type":"PACKAGE","url":"https://github.com/EmilStenstrom/justhtml"},{"type":"WEB","url":"https://github.com/EmilStenstrom/justhtml/releases/tag/v1.10.0"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/justhtml-before-denial-of-service-via-deeply-nested-html"},{"type":"PACKAGE","url":"https://pypi.org/project/justhtml"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-v7cf-c9rm-wm3j"}],"affected":[{"package":{"name":"justhtml","ecosystem":"PyPI","purl":"pkg:pypi/justhtml"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.10.0"}]}],"versions":["0.1.0","0.10.0","0.11.0","0.12.0","0.13.0","0.13.1","0.14.0","0.15.0","0.16.0","0.17.0","0.18.0","0.19.0","0.2.0","0.20.0","0.21.0","0.22.0","0.23.0","0.24.0","0.25.0","0.26.0","0.27.0","0.28.0","0.29.0","0.3.0","0.30.0","0.31.0","0.32.0","0.33.0","0.34.0","0.35.0","0.36.0","0.37.0","0.38.0","0.39.0","0.4.0","0.40.0","0.5.0","0.5.1","0.5.2","0.6.0","0.7.0","0.8.0","0.9.0","1.0.0","1.1.0","1.2.0","1.3.0","1.4.0","1.5.0","1.6.0","1.7.0","1.8.0","1.9.0","1.9.1"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/justhtml/PYSEC-2026-3854.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"}]}