RackApplication
CVE-2026-34829
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. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser only wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit. For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space. This results in a denial of service condition for Rack applications that accept multipart form data. This issue has been patched in versions 2.2.23, 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::Multipart::Parser only enforces upload size limits when a Content-Length header is present. When multipart/form-data requests arrive without Content-Length (such as via HTTP chunked transfer encoding), parsing continues unbounded. File uploads bypass the in-memory buffer limit and write directly to disk, allowing attackers to stream unlimited data and exhaust disk space for denial of service.
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
< 2.2.23>= 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 `ruby -e "require 'rack'; puts Rack::VERSION"` or check your Gemfile.lock for the rack gem versionAffected if The version is less than 2.2.23, or between 3.0.0 and 3.1.20 inclusive, or between 3.2.0 and 3.2.5 inclusive
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Confirm multipart form handling is in useSearch application code for `Rack::Multipart.parse`, `params[:file]`, or multipart file upload endpoints. Check middleware stack for Rack::Multipart or similar multipart handling middlewareAffected if The application parses multipart/form-data requests, particularly file uploads
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Check for chunked transfer encoding exposureReview server configuration and application code to determine if chunked Transfer-Encoding is accepted. Inspect incoming request headers for 'Transfer-Encoding: chunked' on multipart POST requestsAffected if The server accepts requests with Transfer-Encoding: chunked for multipart uploads without a Content-Length header
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Verify size constraint configurationInspect application configuration for any BoundedIO or max multipart size limits. Check if Rack::Multipart::Parser uses a :BoundedIO parameter or similar limit setting. Search for `max_part_size` or `buffer_size` configurationsAffected if No explicit size limit is enforced on chunked multipart uploads, or the BoundedIO constraint is not applied when Content-Length is absent
You are affected if your Rack version falls within the vulnerable ranges AND your application accepts chunked multipart/form-data uploads without enforcing explicit size limits.
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 · scoped2.2.233.1.213.2.6
Upgrade Rack to versions 2.2.23, 3.1.21, or 3.2.6 or later. Consider adding upstream request size limits at the web server or application layer as a defense-in-depth measure.
Upgrade to Rack 2.2.23, 3.1.21, or 3.2.6 (whichever aligns with the current major version line in use)
- Check the current Rack version in use by examining the Gemfile.lock or running `gem list rack`
- Identify which Rack major version line (2.x or 3.x) is currently in use
- Run `gem install rack -v VERSION` replacing VERSION with the appropriate fixed version: 2.2.23, 3.1.21, or 3.2.6
- Update the Gemfile to specify the fixed version: `gem 'rack', '~> X.X'` where X.X matches the version line chosen
- Run `bundle update rack` to update dependencies
- Restart the application to load the new Rack version
- Verify the upgrade was successful by checking the installed Rack version
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Rack
Scoped from the published advisory
- Consultation2.0 h
- Implementation2.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-34829 — 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-2026-34829 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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