CVE-2026-49853
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedTornado is a Python web framework and asynchronous networking library. Prior to 6.5.6, SimpleAsyncHTTPClient shallow-copied redirected requests and removed only the Host header, leaving Authorization, auth_username, auth_password, and auth_mode in place when a redirect changed scheme, host, or port. This issue is fixed in version 6.5.6.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe SimpleAsyncHTTPClient in Tornado versions prior to 6.5.6 performs a shallow copy of redirected HTTP requests but only removes the Host header. Authentication credentials (Authorization header, auth_username, auth_password, and auth_mode) persist when a redirect changes the scheme, host, or port, potentially leaking credentials to an unintended server.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Changed
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check installed Tornado versionRun `python -c "import tornado; print(tornado.version)"` or `pip show tornado` to see the installed version numberAffected if Version is lower than 6.5.6 (e.g., 6.5.5, 6.4.x, 6.3.x, etc.)
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Verify SimpleAsyncHTTPClient is in useSearch your codebase for `SimpleAsyncHTTPClient` or inspect your HTTP client initialization to confirm this class is being used rather than other HTTP client implementationsAffected if Your code explicitly uses SimpleAsyncHTTPClient for HTTP requests
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Confirm authentication is configuredSearch your codebase for HTTP requests that set auth_username, auth_password, auth_mode parameters, or manually set an Authorization headerAffected if Your code makes authenticated HTTP requests using any of these authentication parameters
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Check if redirects are followedInspect your HTTP request configuration for follow_redirects=True (the default in Tornado HTTPClient) or verify your application logic triggers redirectsAffected if Your HTTP client is configured to follow redirects, which is the default behavior
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scopedUpgrade Tornado to version 6.5.6 or later to receive the fix that properly clears authentication credentials when following redirects that change scheme, host, or port.
6.5.6
- Check current Tornado version by running: pip show tornado
- Upgrade Tornado to version 6.5.6 or later using: pip install --upgrade tornado>=6.5.6
- Verify the upgrade was successful by running: pip show tornado and confirming the version is 6.5.6 or higher
- Test that your application functions correctly after the upgrade
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,936.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-49853 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-49853 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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