CVE-2026-28350: Lxml html clean Lxml-html-clean Has <base> Tag Injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xvp8-3mhv-424c",
  "modified" : "2026-03-05T22:49:24Z",
  "published" : "2026-03-02T19:35:52Z",
  "aliases" : [ "CVE-2026-28350" ],
  "summary" : "lxml-html-clean has <base> tag injection through default Cleaner configuration",
  "details" : "### Summary\nThe `<base>` tag passes through the default `Cleaner` configuration. While `page_structure=True` removes `html`, `head`, and `title` tags, there is no specific handling for `<base>`, allowing an attacker to inject it and hijack relative links on the page.\n\n### Details\nThe `<base>` tag is not currently in the `page_structure` kill set. Even though the specification says `<base>` must be inside `<head>`, browsers accept `<base>` tags outside of the head.\n\nIf an attacker injects a `<base>` tag, it changes the base URL for all relative URLs on the page (links, images, scripts) to a domain controlled by the attacker.\n\n### PoC\n```python\nfrom lxml_html_clean import clean_html\n\n# The base tag is preserved in the output\nresult = clean_html('<base href=\"http://evil.com/\"><a href=\"/account\">Account</a>')\nprint(result)\n# Output: <div><base href=\"http://evil.com/\">...<a href=\"/account\">Account</a></div>\n```\n\n### Impact\nThe injection of a `<base>` tag allows an attacker to hijack the resolution of **all** relative URLs on the page. This results in three critical attack vectors:\n\n1.  **Phishing & Redirection:** Attackers can redirect user navigation (e.g., `<a href=\"/login\">`) and form submissions (e.g., `<form action=\"/auth\">`) to an attacker-controlled domain, effectively stealing credentials or sensitive data without the user realizing they have left the legitimate site.\n2.  **Cross-Site Scripting (XSS):** If the victim application loads JavaScript files using relative paths (e.g., `<script src=\"assets/app.js\">`), the browser will attempt to fetch the script from the attacker's domain. This upgrades the vulnerability from HTML injection to full Stored XSS.\n3.  **Defacement:** Relative references to images (`<img>`) and stylesheets (`<link>`) will be loaded from the attacker's server, allowing for UI redressing or defacement.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "lxml-html-clean"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.4.4"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.4.3"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fedora-python/lxml_html_clean/security/advisories/GHSA-xvp8-3mhv-424c"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-28350"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fedora-python/lxml_html_clean/commit/9c5612ca33b941eec4178abf8a5294b103403f34"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fedora-python/lxml_html_clean"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-116" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-02T19:35:52Z",
    "nvd_published_at" : "2026-03-05T20:16:16Z"
  }
}