RackApplication
CVE-2026-34831
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::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire. Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters. This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value. 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 confidenceIn Rack::Files#fail, the Content-Length header is calculated using String#size (character count) instead of String#bytesize (byte count). When response bodies contain multibyte UTF-8 characters, this causes the declared Content-Length to be smaller than the actual bytes transmitted. An attacker can trigger this by requesting non-existent paths with percent-encoded UTF-8 characters, causing HTTP response desynchronization.
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
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/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 check your Gemfile.lock for the rack gem version, or run `ruby -e "require 'rack'; puts Rack::VERSION"`Affected if The version is < 2.2.23 OR >= 3.0.0 AND < 3.1.21 OR >= 3.2.0 AND < 3.2.6
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Confirm Rack::Files is in the stackCheck if your Rack-based application (Rails, Sinatra, etc.) uses Rack::Files middleware. In Rails, this is typically in the public file serving. Check your middleware stack with `rails middleware` or inspect `Rack::Files` presence in `Rails.configuration.middleware`Affected if Rack::Files middleware is present and handles file responses or error pages
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Verify application serves static files or custom error pagesCheck if the application serves static content from a public directory or generates custom 404/500 error pages that could contain multibyte UTF-8 charactersAffected if The application serves static files or generates error responses containing non-ASCII characters
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Test Content-Length behavior with UTF-8 pathsSend a request to a non-existent path with percent-encoded UTF-8 characters (e.g., `/nonexistent%E2%9C%93`) and inspect the Content-Length header in the 404 response. Compare the header value to the actual response body byte size using a tool like curl with `-w '%{size_download}'`Affected if The Content-Length header value is smaller than the actual bytes sent in the response body when UTF-8 characters are present
You are affected if your Rack version falls within the vulnerable ranges AND your application uses Rack::Files to serve responses containing multibyte UTF-8 characters, causing the Content-Length header to undercount actual response bytes.
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 version 2.2.23, 3.1.21, or 3.2.6 or later to patch the Content-Length calculation vulnerability.
Rack 2.2.23, 3.1.21, or 3.2.6 (depending on your current major version branch)
- Identify the current Rack gem version in use (e.g., run `gem list rack` or check Gemfile.lock)
- Determine which major version branch you are on (2.x, 3.0.x, or 3.2.x)
- Upgrade to the patched version for your branch: 2.2.23 if on 2.x, 3.1.21 if on 3.0.x or 3.1.x, or 3.2.6 if on 3.2.x
- Run `bundle update rack` or `gem install rack -v <version>` to apply the update
- Verify the application functions correctly and test that 404 responses with percent-encoded UTF-8 paths return correct Content-Length headers
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-2026-34831 — 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-2026-34831 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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- Version or environment caveats, and links to real fixes
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