RackApplication
CVE-2026-26961
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 extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. 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 uses a greedy regex that selects the LAST boundary parameter from Content-Type headers instead of the FIRST. When multiple boundaries are present, upstream proxies/WAFs validate the first boundary while Rack parses the last, enabling attackers to smuggle malicious multipart content past security intermediaries.
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
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/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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Identify installed Rack versionRun `gem list rack` or inspect the Gemfile.lock to find the installed Rack gem versionAffected if The version is less than 2.2.23, or between 3.0.0 and 3.1.20, or between 3.2.0 and 3.2.5
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Confirm multipart form parsing is in useSearch the application codebase for multipart form handling: look for file upload forms, `Rack::Multipart` middleware inclusion, or requests with `enctype="multipart/form-data"`Affected if The application processes multipart form data which triggers the vulnerable `Rack::Multipart::Parser` code path
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Verify Content-Type header processingInspect how the application handles incoming Content-Type headers. Check if the app directly passes headers to Rack's multipart parser without preprocessingAffected if The application allows raw Content-Type headers to reach Rack's multipart parser without sanitizing duplicate boundary parameters
You are affected if your installed Rack version falls within the vulnerable ranges AND your application processes multipart form data using Rack's built-in multipart parser.
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 to patch the boundary parsing logic.
Upgrade to Rack 2.2.23, 3.1.21, or 3.2.6 (choose the appropriate version based on your major version branch)
- Check the currently installed Rack version by running `gem list rack` or checking your Gemfile
- Update the Rack gem to version 2.2.23, 3.1.21, or 3.2.6 depending on your major version branch
- For Bundler-based projects, update your Gemfile or Gemfile.lock with `bundle update rack` or specify the version directly (e.g., `gem 'rack', '~> 2.2.23'`)
- After updating, verify the correct version is installed with `gem list rack`
- Run your application's test suite to ensure compatibility with the patched version
- 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
- 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 $1,616.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-26961 — 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-2026-26961 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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