CVE-2025-67724: Tornado Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-pr2v-jx2c-wg9f",
  "modified" : "2026-07-20T18:55:11Z",
  "published" : "2026-07-20T18:55:11Z",
  "aliases" : [ "CVE-2025-67724" ],
  "summary" : "Tornado vulnerable to Header Injection and XSS via reason argument",
  "details" : "# Header injection and XSS via `reason` argument\n\n## Summary\n\nThe `reason` argument (used by both `RequestHandler.set_status` and `tornado.web.HTTPError` is designed to allow applications to pass custom \"reason\" phrases (the \"Not Found\" in `HTTP/1.1 404 Not Found`) to the HTTP status line (mainly for non-standard status codes). Vulnerabilities exist in Tornado versions prior to 6.5.3 if untrusted data is passed as the `reason` argument.\n\n## Details\n\nIn vulnerable versions, the supplied `reason` phrase is used unescaped in HTTP headers (where it could be used for header injection) or in HTML in the default error page (where it could be used for XSS).\n\n## Impact\n\n* **Type:** Reflected Cross-Site Scripting (CWE-79) in default error page, or header injection (CWE-644)\n* **Actors:** Remote attacker who can cause the application to raise `HTTPError`/`set_status` with an attacker-controlled `reason` (e.g., via query parameter used by developer).\n* **Effect:** Execution of arbitrary JavaScript in victims' browsers when they view the error page — possible session token theft, CSRF escalation, UI spoofing, or other client-side attacks depending on context.\n* **Scope:** Only applications that explicitly reflect untrusted input into `reason` are affected.\n\n## Mitigation\n\nAside from upgrading to Tornado 6.5.3 or newer, the vulnerability can be mitigated by not using untrusted data for the `reason` argument. In the intended use case the `reason` argument would generally be a string literal and not derived from user input. Also, the `reason` argument is rarely required (reason phrases are not used at all in HTTP/2) and can generally be omitted.\n\nFor a general-purpose error message in HTTPError, consider using the `log_message` argument instead of `reason`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.5.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-pr2v-jx2c-wg9f"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-67724"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/9c163aebeaad9e6e7d28bac1f33580eb00b0e421"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-265.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-20T18:55:11Z",
    "nvd_published_at" : "2025-12-12T06:15:41Z"
  }
}