CVE-2026-49853: Tornado Authorization Header Forwarded Across Cross-origin Vulnerability

HIGH
Advisory Identifier: GHSA-3x9g-8vmp-wqvf
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CVSS 7.7
CWE-200
Published
Advisory Summary

CVE-2026-49853 is a HIGH (CVSS 7.7) authorization header forwarded across cross-origin vulnerability (CWE-200: Exposure of Sensitive Information to an Unauthorized Actor) affecting Tornado libraries in the PyPI ecosystem. The issue involves authorization header forwarded across cross-origin redirects in simpleasynchttpclient. Remediation is available by upgrading affected packages to version 6.5.6.

CVE-2026-49853 at a Glance

CVE Identifier CVE-2026-49853
GHSA Identifier GHSA-3x9g-8vmp-wqvf
Severity HIGH
CVSS v3.1 Base Score 7.7
CWE Classification CWE-200: Exposure of Sensitive Information to an Unauthorized Actor
Affected Product Tornado
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.7

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2026-49853

To remediate this vulnerability, upgrade all affected Tornado dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

tornado fixed: 6.5.6 
pip install tornado>=6.5.6
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show tornado

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-49853, inspect your installed packages using pip:

pip show tornado

Analyze the terminal output to locate any version of Tornado in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.5.6), your application is affected.

Note: The pip show tornado command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, where the product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. In Tornado, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## Summary When SimpleAsyncHTTPClient follows a 3xx redirect, it shallow-copies the original HTTPRequest, rewrites the URL, decrements max_redirects, and removes only the Host header. It does not clear Authorization, auth_username, auth_password, or auth_mode when the redirect target changes origin. As a result, credentials intended for one origin can be forwarded to a different origin when follow_redirects=True, which is the default. Beginning in Tornado 6.5.6, `SimpleAsyncHTTPClient` matches the default behavior of `libcurl` (and therefore `CurlAsyncHTTPClient`): When a redirect changes the scheme, host, or port of the url, the `Authorization` and `Cookie` headers will be removed when following the redirect.

CVE-2026-49853 Affected Tornado Versions

1 component(s)
Fixed Versions:
6.5.6
Product / Package Ecosystem Affected Versions Fixed Version
tornado
pypi/tornado
PyPI
< 6.5.6
6.5.6

References

GitHub Advisory / Wiki ADVISORY
https://github.com/tornadoweb/tornado/security/advisories/GHSA-3x9g-8vmp-wqvf
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado

Frequently Asked Questions

What is CVE-2026-49853?
CVE-2026-49853 is a security vulnerability in Tornado involving Authorization header forwarded across cross-origin redirects in SimpleAsyncHTTPClient under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor.
Which Tornado versions are affected?
Affected versions include releases prior to 6.5.6 across affected packages: tornado.
What version fixes CVE-2026-49853?
Upgrading to 6.5.6 (or the latest patched release for your respective branch) resolves the vulnerability.
Is tornado affected?
Yes, tornado is affected in releases prior to 6.5.6. Upgrade your dependency to 6.5.6 or later.
What is the CVSS score of CVE-2026-49853?
CVE-2026-49853 has a CVSS v3.1 base score of 7.7 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N.
What CWE is associated with CVE-2026-49853?
CVE-2026-49853 is classified under CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.).
How can I check PyPI dependencies for CVE-2026-49853?
Run the dependency tree command: 'pip show tornado'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-3x9g-8vmp-wqvf",
  "modified" : "2026-06-15T20:20:00Z",
  "published" : "2026-06-15T20:20:00Z",
  "aliases" : [ "CVE-2026-49853" ],
  "summary" : "Tornado: Authorization header forwarded across cross-origin redirects in SimpleAsyncHTTPClient",
  "details" : "## Summary\n\nWhen SimpleAsyncHTTPClient follows a 3xx redirect, it shallow-copies the original HTTPRequest, rewrites the URL, decrements max_redirects, and removes only the Host header. It does not clear Authorization, auth_username, auth_password, or auth_mode when the redirect target changes origin.\n\nAs a result, credentials intended for one origin can be forwarded to a different origin when follow_redirects=True, which is the default.\n\nBeginning in Tornado 6.5.6, `SimpleAsyncHTTPClient` matches the default behavior of `libcurl` (and therefore `CurlAsyncHTTPClient`): When a redirect changes the scheme, host, or port of the url, the `Authorization` and `Cookie` headers will be removed when following the redirect.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.5.6"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-3x9g-8vmp-wqvf"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T20:20:00Z",
    "nvd_published_at" : null
  }
}