CVE-2026-31958
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. In versions of Tornado prior to 6.5.5, the only limit on the number of parts in multipart/form-data is the max_body_size setting (default 100MB). Since parsing occurs synchronously on the main thread, this creates the possibility of denial-of-service due to the cost of parsing very large multipart bodies with many parts. This vulnerability is fixed in 6.5.5.
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 confidenceTornado web framework before 6.5.5 lacks a limit on the number of parts in multipart/form-data requests, with only the max_body_size (default 100MB) as a constraint. Since parsing occurs synchronously on the main thread, attackers can send requests with many small parts that consume significant CPU time, creating a denial-of-service condition.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 6.5.5CVSS 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
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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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Identify if Tornado is in useSearch for 'tornado' in your requirements.txt, pip list output, or Python import statements across your codebaseAffected if Tornado is not found in your environment (not applicable)
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Determine installed Tornado versionRun 'python -c "import tornado; print(tornado.version)"' or check via 'pip show tornado'Affected if Version is lower than 6.5.5 (e.g., 6.5.4, 6.4.x, 6.3.x, etc.)
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Verify multipart/form-data handling is enabledSearch your application code for RequestHandler classes that accept file uploads or use the @stream_body decorator, and check for 'multipart' in your request handling configurationAffected if Your application accepts multipart/form-data requests (this is the attack surface)
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Confirm default max_body_size is in useCheck your application configuration files and startup code for the 'max_body_size' setting in tornado.web.Application or tornado.httpserver.HTTPServer configurationAffected if Using default 100MB limit without additional part count restrictions (vulnerability is present)
You are affected if you are running any Tornado version below 6.5.5 AND your application accepts multipart/form-data requests, since the vulnerability allows attackers to send many small parts to exhaust CPU resources.
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 · scoped6.5.5
Upgrade to Tornado 6.5.5 or later to receive the patch that adds limits on multipart part count. Consider implementing request rate limiting or part count limits at the load balancer/proxy level as an additional defense-in-depth measure.
Tornado 6.5.5
- Check the current Tornado version by running `python -c "import tornado; print(tornado.version)"` or `pip show tornado`
- Upgrade Tornado to version 6.5.5 or later using pip: `pip install --upgrade tornado>=6.5.5`
- Verify the upgrade was successful by checking the new version matches or exceeds 6.5.5
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing4.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-31958 — 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-2026-31958 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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