RackApplication
CVE-2026-34827
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 · uneditedRack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.
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 confidenceRack's multipart parser has an algorithmic complexity vulnerability in handle_mime_head where repeated String#index searches combined with String#slice! prefix deletion causes super-linear (catastrophic backtracking-like) processing for escape-heavy quoted values in multipart/form-data headers. Attackers can trigger excessive CPU usage via crafted requests with many backslash-escaped parameter values.
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
RackApplication
>= 3.0.0, < 3.1.21>= 3.2.0, < 3.2.6CVSS 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 installed Rack versionRun `gem list rack` or check your Gemfile.lock for the rack gem versionAffected if Version is >= 3.0.0 and < 3.1.21, or >= 3.2.0 and < 3.2.6
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Confirm Rack handles multipart requestsInspect your application configuration (e.g., config.ru, Rails config) for use of Rack::Multipart or middleware that parses multipart form data such as Rack::Request params or file uploadsAffected if Your application uses Rack middleware that processes multipart/form-data requests (common in web apps accepting file uploads or form submissions)
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Verify exposed multipart parsing endpointReview your routes/controllers that accept POST requests with content-type multipart/form-data, typically for file uploads or form submissionsAffected if Any endpoint accepts multipart form data from untrusted sources without rate limiting or input restrictions
You are affected if your installed Rack version falls within the vulnerable range AND your application processes multipart/form-data requests from users or external clients.
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 · scoped3.1.213.2.6
Upgrade Rack to version 3.1.21 or 3.2.6 to patch the inefficient parsing algorithm. Consider implementing request size limits and timeout protections as defense-in-depth.
Rack 3.1.21 or 3.2.6 (or latest 3.2.x)
- Identify the current Rack version in use (check Gemfile.lock or run `bundle show rack`)
- Update the Gemfile to specify Rack version constraint: gem 'rack', '~> 3.1.21' or gem 'rack', '~> 3.2.6' (or '>= 3.2.6' to get the latest 3.2.x)
- Run `bundle update rack` to install the patched version
- Run application test suite to verify the upgrade does not break functionality
- Deploy the updated application
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Rack
Scoped from the published advisory
- Consultation1.0 h
- Implementation1.0 h
- Testing2.0 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 $1,376.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-34827 — 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-2026-34827 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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