CVE-2026-31859: Craftcms/cms Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fvwq-45qv-xvhv",
  "modified" : "2026-03-11T20:44:34Z",
  "published" : "2026-03-11T00:26:13Z",
  "aliases" : [ "CVE-2026-31859" ],
  "summary" : "CraftCMS vulnerable to reflective XSS via incomplete return URL sanitization",
  "details" : "### Summary\n\nThe fix for CVE-2025-35939 in `craftcms/cms` introduced a `strip_tags()` call in `src/web/User.php` to sanitize return URLs before they are stored in the session. However, `strip_tags()` only removes HTML tags (angle brackets) -- it does not inspect or filter URL schemes. Payloads like `javascript:alert(document.cookie)` contain no HTML tags and pass through `strip_tags()` completely unmodified, enabling reflected XSS when the return URL is rendered in an `href` attribute.\n\n### Details\nThe patched code in is:\n\n```php\npublic function setReturnUrl($url): void\n{\n    parent::setReturnUrl(strip_tags($url));\n}\n```\n\n`strip_tags()` removes HTML tags (e.g., `<script>`, `<img>`) from a string, but it is **not** a URL sanitizer. When the sanitized return URL is subsequently rendered in an `href` attribute context (e.g., `<a href=\"{{ returnUrl }}\">`), the following dangerous payloads survive `strip_tags()` completely unmodified:\n\n1. **`javascript:` protocol URLs** -- `javascript:alert(document.cookie)` contains no HTML tags, so `strip_tags()` returns it verbatim. When placed in an `href`, clicking the link executes the JavaScript.\n\n2. **`data:` URIs** -- `data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==` uses Base64 encoding and contains no tags at all, bypassing `strip_tags()` entirely.\n\n3. **Protocol-relative URLs** -- `//evil.com/steal` contains no tags and is passed through unchanged. When rendered as an `href`, the browser resolves it relative to the current page’s protocol, redirecting the user to an attacker-controlled domain.\n\nThe core issue is that `strip_tags()` operates on HTML syntax (angle brackets) while the threat model here requires URL scheme validation. These are fundamentally different security concerns.\n\n### Impact\n\n**Reflected XSS via crafted return URL.** An attacker constructs a malicious link such as `https://target.example.com/craft/?returnUrl=javascript:alert(document.cookie)` and sends it to a victim. The attack flow is:\n\n1. Victim clicks the link, visiting the Craft CMS site.\n2. The application calls `setReturnUrl()` with the attacker-controlled value.\n3. `strip_tags()` processes the URL but finds no HTML tags -- it passes through unchanged.\n4. The URL is stored in the session and later rendered in an `href` attribute (e.g., a \"Return\" or \"Continue\" link).\n5. When the victim clicks that link, `javascript:alert(document.cookie)` executes in the context of the Craft CMS origin.\n\nThis enables:\n- **Session hijacking** via cookie theft (`document.cookie`)\n- **Data exfiltration** via `fetch()` to an attacker-controlled server\n- **Phishing** by redirecting to a lookalike domain (protocol-relative URL)\n- **CSRF** by performing actions on behalf of the authenticated user",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "craftcms/cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.15.3"
      }, {
        "fixed" : "4.17.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.17.2"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "craftcms/cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.7.5"
      }, {
        "fixed" : "5.9.7"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 5.9.6"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/craftcms/cms/security/advisories/GHSA-fvwq-45qv-xvhv"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-31859"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/craftcms/cms/commit/cc9921c14897ee2b592a431c2356af8a04ce4cfe"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/craftcms/cms"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-116", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-11T00:26:13Z",
    "nvd_published_at" : "2026-03-11T18:16:24Z"
  }
}