CVE-2026-49825

Ein entfernter, anonymer Angreifer kann eine Schwachstelle in Red Hat Enterprise Linux ausnutzen, um Sicherheitsvorkehrungen zu umgehen.

Ein Angreifer kann mehrere Schwachstellen in Red Hat Ansible Automation Platform ausnutzen, um seine Privilegien zu erhöhen, beliebigen Code auszuführen, Sicherheitsmaßnahmen zu umgehen, Daten zu manipulieren oder offenzulegen und einen Denial-of-Service-Zustand herbeizuführen.

# `lxml_html_clean.Cleaner` does not strip `javascript:` URLs from namespaced URL attributes (`xlink:href`) **Reporter:** Guillem Lefait <guillem@datamq.com> · **Date:** 2026-05-10 **Affected:** `lxml` ≤ 6.1.0 and `lxml_html_clean` ≤ 0.4.4 (latest stable) **Confirmed against:** lxml 6.1.0 + lxml_html_clean 0.4.4 on Python 3.13.5, 3.14.4, and 3.15.0a8 (libxml2 2.14.6 / 2.9.14 — bug is in pure-Python sanitizer logic, independent of the libxml2 backend) **Root-cause class:** same as CVE-2021-28957 (`formaction` missing from `link_attrs`) ## Summary `Cleaner` filters URL schemes (`javascript:`, `vbscript:`, …) by walking links via `rewrite_links()`, which delegates to `iterlinks()`, which only yields attributes named in `lxml.html.defs.link_attrs`. That allow-list contains no prefixed names (`xlink:href`) and no `srcset`. As a result, when `Cleaner` is configured with `safe_attrs_only=False` — a documented option for callers that want lenient attribute handling but still expect URL-scheme scrubbing — `<a xlink:href="javascript:…">` survives sanitization untouched, and any browser that follows the SVG-anchor specification will execute the JavaScript when the rendered link is clicked. **CWE:** CWE-79 (XSS), with CWE-184 (Incomplete List of Disallowed Inputs) as the underlying defect class. ## Affected components | Package | Versions tested | File / line | |--------------------|------------------------|----------------------------------| | `lxml` | 4.9.x, 5.2.1, 6.1.0 | `src/lxml/html/defs.py:20` | | `lxml` | " | `src/lxml/html/__init__.py:485-528` | | `lxml_html_clean` | 0.4.0 – 0.4.4 | `lxml_html_clean/clean.py:348,576` | The legacy `lxml.html.clean` module — bundled in `lxml < 5.2.0` and still installable on newer versions via the `lxml[html_clean]` extra — shares the same bug. ## Root cause `defs.link_attrs` is a flat string set; the literal `xlink:href` is absent: ```python # lxml/html/defs.py link_attrs = frozenset([ 'action', 'archive', 'background', 'cite', 'classid', 'codebase', 'data', 'href', 'longdesc', 'profile', 'src', 'usemap', 'dynsrc', 'lowsrc', 'formaction', ]) ``` `HtmlMixin.iterlinks()` (`lxml/html/__init__.py:526-528`) only yields attributes whose key is in that set: ```python for attrib in link_attrs: if attrib in attribs: yield (el, attrib, attribs[attrib], 0) ``` `Cleaner.__call__` registers the URL-scheme filter via `rewrite_links` (`lxml_html_clean/clean.py:348`), which is a thin wrapper around `iterlinks()`. Because `xlink:href` is never yielded, `_remove_javascript_link` (`clean.py:576`) is never invoked for it. ## Minimal reproducer ```python from lxml import html from lxml_html_clean import Cleaner for payload in ( '<svg><a xlink:href="javascript:alert(1)">x</a></svg>', '<math><a xlink:href="javascript:alert(2)">y</a></math>', ): tree = html.fromstring(payload) Cleaner(safe_attrs_only=False)(tree) print(html.tostring(tree).decode()) print(' iterlinks:', list(html.fromstring(payload).iterlinks())) # <svg><a xlink:href="javascript:alert(1)">x</a></svg> ← unchanged # iterlinks: [] ← link rewriter blind # <math><a xlink:href="javascript:alert(2)">y</a></math> ← unchanged # iterlinks: [] ← link rewriter blind ``` Both SVG and MathML scopes are vulnerable — same allow-list gap, both render anchors that browsers treat as navigable. Other lab-confirmed surviving variants (same scope, different scheme encoding): mixed-case (`JaVaScRiPt:`), HTML-entity (`java&#x73;cript:`), embedded tab (`java\tscript:`). ## Impact A caller that uses `Cleaner` to neutralise untrusted HTML and chooses `safe_attrs_only=False` — typically because the application wants to allow custom data-/aria-/vendor attributes — will silently pass `javascript:` payloads carried on `xlink:href` through to victim renders. Stored XSS in any application that round-trips user-supplied HTML through this configuration. Reach is conditional on the `safe_attrs_only=False` toggle, but that is a documented public option; consumers reasonably expect URL-scheme scrubbing to be independent of attribute allow-listing. ## Suggested fix **Extend `link_attrs`** to include `xlink:href`. In HTML mode, `lxml.html` keeps prefixed attribute names verbatim — the parsed key is the literal string `xlink:href`, not a Clark-notation form — so the existing allow-list lookup is a plain string match. Same shape as the CVE-2021-28957 fix: ```diff # lxml/html/defs.py link_attrs = frozenset([ 'action', 'archive', 'background', 'cite', 'classid', 'codebase', 'data', 'href', 'longdesc', 'profile', 'src', 'usemap', 'dynsrc', 'lowsrc', 'formaction', + 'xlink:href', ]) ``` This single change closes the reported XSS for both SVG `<a xlink:href>` and MathML `<a xlink:href>`. `lxml_html_clean` is the canonical home of the `Cleaner` code (881 lines); `lxml.html.clean` is a 21-line backward-compat shim (`from lxml_html_clean import *`) that picks up the fix automatically once `link_attrs` is updated upstream. Since the upstream change requires lxml maintainer action, see the alternative below if a self-contained patch in `lxml_html_clean` is preferred. **Alternative (in-package fix, no lxml coordination needed):** add a namespaced-URL-attribute walk inside `Cleaner.__call__` so the URL-scheme filter doesn't depend on `link_attrs`. Sketch: ```python # lxml_html_clean/clean.py — supplements rewrite_links() in __call__ _NS_URL_ATTRS = ('xlink:href',) # extend as needed _BAD_SCHEME = re.compile(r'^\s*(javascript|vbscript|data):', re.I) for el in doc.iter(): for attr in _NS_URL_ATTRS: if attr in el.attrib and _BAD_SCHEME.match(el.attrib[attr]): del el.attrib[attr] ``` This decouples the cleaner from the upstream `link_attrs` set and matches the security-ownership boundary established when the cleaner was extracted in lxml 5.2.0. **Defense in depth (optional, regardless of which fix path is taken):** - Also handle `srcset`: the value is a `url 1x, url 2x, …` descriptor list, so split on commas and validate each candidate URL. Not directly executable in current browsers, but closes the same gap. - Also accept Clark-notation forms (`{http://www.w3.org/1999/xlink}href`) so XML-mode callers using `lxml.etree` get the same protection. HTML mode never produces this form, so not needed for the reported bug. ## Severity CVSS 3.1 base score: **8.2 / High** — `AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N` (stored XSS; victim must click the SVG anchor; scope-changed because script executes in the rendering origin). PR:N reflects the common case where untrusted HTML enters the sanitizer from anonymous sources (comments, support tickets); deployments that gate writes behind authentication can score with PR:L (→ 7.6). Severity is **CONDITIONAL** on the caller passing `safe_attrs_only=False`. With the class default (`True`), attribute allow-listing strips `xlink:href` before scheme scrubbing runs, and the bug does not fire — verified at HEAD: default-config `Cleaner()(<svg><a xlink:href="javascript:…">x</a></svg>)` → `<svg><a>x</a></svg>`. ## Prior art / novelty - **CVE-2021-28957 (lxml 4.6.3)** — same root cause, different attribute (`formaction`). Fix was a one-line extension of `link_attrs`. Direct precedent. - **CVE-2022-34473** (Mozilla Sanitizer API) — `xlink:href` URL bypass primitive in a different sanitizer. - **Bleach (Mozilla, Python)** explicitly handles the `xlink` namespace; `enshrined/svg-sanitize` (PHP) ships `cleanXlinkHrefs()`; DOMPurify scrubs `xlink:href` via `ALLOWED_URI_REGEXP`. - **`nh3`** (the alternative recommended in `lxml_html_clean`'s own README for security-sensitive use) is **not vulnerable** to this primitive — verified 2026-05-10 on `nh3==0.3.5`: with `<svg>`/`<math>`/`<a>` and `xlink:href` explicitly added to `tags`/`attributes`, both SVG and MathML payloads, all four scheme-encoding variants, are stripped (output e.g. `<svg><a rel="noopener noreferrer">x</a></svg>`). ## Coordination Filing as a private GHSA at `fedora-python/lxml_html_clean` — `lxml_html_clean` is the canonical maintainer of the `Cleaner` code (881 lines) and the security-responsible team since the lxml 5.2.0 split, where the cleaner was extracted out of lxml precisely so cleaner-security reports could land on the right team. The lxml side cannot be filed via GHSA (`https://github.com/lxml/lxml/security/advisories/new` returns 404 — private reporting is not enabled), so a parallel report has been emailed directly to the lxml maintainer for the upstream `defs.link_attrs` patch path. You're welcome to coordinate with them directly if you'd prefer the upstream fix over the in-package alternative above. Happy to provide a draft patch or PR on either path. No bounty expected.

# `lxml_html_clean.Cleaner` does not strip `javascript:` URLs from namespaced URL attributes (`xlink:href`) **Reporter:** Guillem Lefait <guillem@datamq.com> · **Date:** 2026-05-10 **Affected:** `lxml` ≤ 6.1.0 and `lxml_html_clean` ≤ 0.4.4 (latest stable) **Confirmed against:** lxml 6.1.0 + lxml_html_clean 0.4.4 on Python 3.13.5, 3.14.4, and 3.15.0a8 (libxml2 2.14.6 / 2.9.14 — bug is in pure-Python sanitizer logic, independent of the libxml2 backend) **Root-cause class:** same as CVE-2021-28957 (`formaction` missing from `link_attrs`) ## Summary `Cleaner` filters URL schemes (`javascript:`, `vbscript:`, …) by walking links via `rewrite_links()`, which delegates to `iterlinks()`, which only yields attributes named in `lxml.html.defs.link_attrs`. That allow-list contains no prefixed names (`xlink:href`) and no `srcset`. As a result, when `Cleaner` is configured with `safe_attrs_only=False` — a documented option for callers that want lenient attribute handling but still expect URL-scheme scrubbing — `<a xlink:href="javascript:…">` survives sanitization untouched, and any browser that follows the SVG-anchor specification will execute the JavaScript when the rendered link is clicked. **CWE:** CWE-79 (XSS), with CWE-184 (Incomplete List of Disallowed Inputs) as the underlying defect class. ## Affected components | Package | Versions tested | File / line | |--------------------|------------------------|----------------------------------| | `lxml` | 4.9.x, 5.2.1, 6.1.0 | `src/lxml/html/defs.py:20` | | `lxml` | " | `src/lxml/html/__init__.py:485-528` | | `lxml_html_clean` | 0.4.0 – 0.4.4 | `lxml_html_clean/clean.py:348,576` | The legacy `lxml.html.clean` module — bundled in `lxml < 5.2.0` and still installable on newer versions via the `lxml[html_clean]` extra — shares the same bug. ## Root cause `defs.link_attrs` is a flat string set; the literal `xlink:href` is absent: ```python # lxml/html/defs.py link_attrs = frozenset([ 'action', 'archive', 'background', 'cite', 'classid', 'codebase', 'data', 'href', 'longdesc', 'profile', 'src', 'usemap', 'dynsrc', 'lowsrc', 'formaction', ]) ``` `HtmlMixin.iterlinks()` (`lxml/html/__init__.py:526-528`) only yields attributes whose key is in that set: ```python for attrib in link_attrs: if attrib in attribs: yield (el, attrib, attribs[attrib], 0) ``` `Cleaner.__call__` registers the URL-scheme filter via `rewrite_links` (`lxml_html_clean/clean.py:348`), which is a thin wrapper around `iterlinks()`. Because `xlink:href` is never yielded, `_remove_javascript_link` (`clean.py:576`) is never invoked for it. ## Minimal reproducer ```python from lxml import html from lxml_html_clean import Cleaner for payload in ( '<svg><a xlink:href="javascript:alert(1)">x</a></svg>', '<math><a xlink:href="javascript:alert(2)">y</a></math>', ): tree = html.fromstring(payload) Cleaner(safe_attrs_only=False)(tree) print(html.tostring(tree).decode()) print(' iterlinks:', list(html.fromstring(payload).iterlinks())) # <svg><a xlink:href="javascript:alert(1)">x</a></svg> ← unchanged # iterlinks: [] ← link rewriter blind # <math><a xlink:href="javascript:alert(2)">y</a></math> ← unchanged # iterlinks: [] ← link rewriter blind ``` Both SVG and MathML scopes are vulnerable — same allow-list gap, both render anchors that browsers treat as navigable. Other lab-confirmed surviving variants (same scope, different scheme encoding): mixed-case (`JaVaScRiPt:`), HTML-entity (`java&#x73;cript:`), embedded tab (`java\tscript:`). ## Impact A caller that uses `Cleaner` to neutralise untrusted HTML and chooses `safe_attrs_only=False` — typically because the application wants to allow custom data-/aria-/vendor attributes — will silently pass `javascript:` payloads carried on `xlink:href` through to victim renders. Stored XSS in any application that round-trips user-supplied HTML through this configuration. Reach is conditional on the `safe_attrs_only=False` toggle, but that is a documented public option; consumers reasonably expect URL-scheme scrubbing to be independent of attribute allow-listing. ## Suggested fix **Extend `link_attrs`** to include `xlink:href`. In HTML mode, `lxml.html` keeps prefixed attribute names verbatim — the parsed key is the literal string `xlink:href`, not a Clark-notation form — so the existing allow-list lookup is a plain string match. Same shape as the CVE-2021-28957 fix: ```diff # lxml/html/defs.py link_attrs = frozenset([ 'action', 'archive', 'background', 'cite', 'classid', 'codebase', 'data', 'href', 'longdesc', 'profile', 'src', 'usemap', 'dynsrc', 'lowsrc', 'formaction', + 'xlink:href', ]) ``` This single change closes the reported XSS for both SVG `<a xlink:href>` and MathML `<a xlink:href>`. `lxml_html_clean` is the canonical home of the `Cleaner` code (881 lines); `lxml.html.clean` is a 21-line backward-compat shim (`from lxml_html_clean import *`) that picks up the fix automatically once `link_attrs` is updated upstream. Since the upstream change requires lxml maintainer action, see the alternative below if a self-contained patch in `lxml_html_clean` is preferred. **Alternative (in-package fix, no lxml coordination needed):** add a namespaced-URL-attribute walk inside `Cleaner.__call__` so the URL-scheme filter doesn't depend on `link_attrs`. Sketch: ```python # lxml_html_clean/clean.py — supplements rewrite_links() in __call__ _NS_URL_ATTRS = ('xlink:href',) # extend as needed _BAD_SCHEME = re.compile(r'^\s*(javascript|vbscript|data):', re.I) for el in doc.iter(): for attr in _NS_URL_ATTRS: if attr in el.attrib and _BAD_SCHEME.match(el.attrib[attr]): del el.attrib[attr] ``` This decouples the cleaner from the upstream `link_attrs` set and matches the security-ownership boundary established when the cleaner was extracted in lxml 5.2.0. **Defense in depth (optional, regardless of which fix path is taken):** - Also handle `srcset`: the value is a `url 1x, url 2x, …` descriptor list, so split on commas and validate each candidate URL. Not directly executable in current browsers, but closes the same gap. - Also accept Clark-notation forms (`{http://www.w3.org/1999/xlink}href`) so XML-mode callers using `lxml.etree` get the same protection. HTML mode never produces this form, so not needed for the reported bug. ## Severity CVSS 3.1 base score: **8.2 / High** — `AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N` (stored XSS; victim must click the SVG anchor; scope-changed because script executes in the rendering origin). PR:N reflects the common case where untrusted HTML enters the sanitizer from anonymous sources (comments, support tickets); deployments that gate writes behind authentication can score with PR:L (→ 7.6). Severity is **CONDITIONAL** on the caller passing `safe_attrs_only=False`. With the class default (`True`), attribute allow-listing strips `xlink:href` before scheme scrubbing runs, and the bug does not fire — verified at HEAD: default-config `Cleaner()(<svg><a xlink:href="javascript:…">x</a></svg>)` → `<svg><a>x</a></svg>`. ## Prior art / novelty - **CVE-2021-28957 (lxml 4.6.3)** — same root cause, different attribute (`formaction`). Fix was a one-line extension of `link_attrs`. Direct precedent. - **CVE-2022-34473** (Mozilla Sanitizer API) — `xlink:href` URL bypass primitive in a different sanitizer. - **Bleach (Mozilla, Python)** explicitly handles the `xlink` namespace; `enshrined/svg-sanitize` (PHP) ships `cleanXlinkHrefs()`; DOMPurify scrubs `xlink:href` via `ALLOWED_URI_REGEXP`. - **`nh3`** (the alternative recommended in `lxml_html_clean`'s own README for security-sensitive use) is **not vulnerable** to this primitive — verified 2026-05-10 on `nh3==0.3.5`: with `<svg>`/`<math>`/`<a>` and `xlink:href` explicitly added to `tags`/`attributes`, both SVG and MathML payloads, all four scheme-encoding variants, are stripped (output e.g. `<svg><a rel="noopener noreferrer">x</a></svg>`). ## Coordination Filing as a private GHSA at `fedora-python/lxml_html_clean` — `lxml_html_clean` is the canonical maintainer of the `Cleaner` code (881 lines) and the security-responsible team since the lxml 5.2.0 split, where the cleaner was extracted out of lxml precisely so cleaner-security reports could land on the right team. The lxml side cannot be filed via GHSA (`https://github.com/lxml/lxml/security/advisories/new` returns 404 — private reporting is not enabled), so a parallel report has been emailed directly to the lxml maintainer for the upstream `defs.link_attrs` patch path. You're welcome to coordinate with them directly if you'd prefer the upstream fix over the in-package alternative above. Happy to provide a draft patch or PR on either path. No bounty expected.