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Medium

Tornado vulnerable to Header Injection and XSS via reason argument

  • Disclosed: 12.12.2025
  • Updated: 11.10.2026 14:18

Header injection and XSS via reason argument

Summary

The 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.

Details

In 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).

Impact

  • Type: Reflected Cross-Site Scripting (CWE-79) in default error page, or header injection (CWE-644)
  • 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).
  • 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.
  • Scope: Only applications that explicitly reflect untrusted input into reason are affected.

Mitigation

Aside 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.

For a general-purpose error message in HTTPError, consider using the log_message argument instead of reason.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) tornado < 6.5.3 6.5.3

References

CVSS score 5.4/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
Weakness (CWE)
CWE-79
Platform
Python (PyPI)
Package
tornado
Disclosed
12.12.2025
Updated
11.10.2026 14:18

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2025-265 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-pr2v-jx2c-wg9f - CC BY 4.0

Vulnerability history: tornado

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (23) in the archive
Total entries
23
First → latest
05.2012 09.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 13
  • Medium 8
  • Low 2

Disclosures per year

earlier: 1

Severity:
  1. tornado: CurlAsyncHTTPClient enforces no response-size limit — decompression bomb drives unbounded memory accumulation to OOM
    High · 7.5 CVE-2026-103262 < 6.5.9 6.5.9
  2. Tornado: StaticFileHandler follows symlinks outside static root (path traversal)
    High CVE-2026-103263 < 6.5.9 6.5.9
  3. Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop
    High · 7.5 CVE-2026-82397 < 6.5.8 6.5.8
  4. Tornado: Authorization header forwarded across cross-origin redirects in SimpleAsyncHTTPClient
    High · 7.7 CVE-2026-49853 < 6.5.6 6.5.6
  5. tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)
    High · 7.5 CVE-2026-49855 < 6.5.6 6.5.6
  6. Tornado has cookie attribute injection via .RequestHandler.set_cookie
    High · 7.2 CVE-2026-35536 < 6.5.5 6.5.5
  7. Tornado is vulnerable to DoS due to too many multipart parts
    High · 7.5 CVE-2026-31958 < 6.5.5 6.5.5
  8. Tornado: Quadratic DoS via Crafted Multipart Parameters
    High · 7.5 CVE-2025-67726 < 6.5.3 6.5.3
  9. Tornado vulnerable to Header Injection and XSS via reason argument this entry
    Medium · 5.4 CVE-2025-67724 < 6.5.3 6.5.3
  10. Tornado: Quadratic DoS via Repeated Header Coalescing
    High · 7.5 CVE-2025-67725 < 6.5.3 6.5.3
  11. Tornado vulnerable to excessive logging caused by malformed multipart form data
    High · 7.5 CVE-2025-47287 < 6.5 6.5
  12. Tornado has an HTTP cookie parsing DoS vulnerability
    High · 7.5 CVE-2024-52804 < 6.4.2 6.4.2
  13. Tornado before 3.2.2 sends arbitrary responses that contain a fixed CSRF token and may be sent with HTTP compression, which makes it easier for remote attackers to conduct a BREACH attack and determin
    High · 6.5 CVE-2014-9720 < 3.2.2 3.2.2
  14. CRLF injection vulnerability in the tornado.web.RequestHandler.set_header function in Tornado before 2.2.1 allows remote attackers to inject arbitrary HTTP headers and conduct HTTP response splitting
    High · 7.5 CVE-2012-2374 < 2.2.1 2.2.1

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