RackApplication
CVE-2026-34230
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::Utils.select_best_encoding processes Accept-Encoding values with quadratic time complexity when the header contains many wildcard (*) entries. Because this method is used by Rack::Deflater to choose a response encoding, an unauthenticated attacker can send a single request with a crafted Accept-Encoding header and cause disproportionate CPU consumption on the compression middleware path. This results in a denial of service condition for applications using Rack::Deflater. 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::Utils.select_best_encoding has quadratic time complexity when processing Accept-Encoding headers with many wildcard (*) entries. Since this method is used by Rack::Deflater for response encoding selection, an unauthenticated attacker can send a single request with a crafted Accept-Encoding header to cause disproportionate CPU consumption, resulting in 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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Check installed Rack gem versionRun `gem list rack` or in Ruby code `puts Rack::VERSION` or `Gem.loaded_specs['rack'].version`Affected if 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 Rack::Deflater middleware is loadedCheck your Rack middleware stack - look for `use Rack::Deflater` in your config.ru or application configurationAffected if Rack::Deflater is in the middleware chain, as it invokes select_best_encoding for Accept-Encoding processing
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Verify Accept-Encoding header processing is activeCheck if your application processes the Accept-Encoding HTTP header from incoming requests (default behavior in most Rack-based frameworks like Rails, Sinatra, etc.)Affected if Your application accepts and processes Accept-Encoding headers from clients
You are affected if your Rack version is vulnerable AND Rack::Deflater or any component that uses select_best_encoding is active in your application stack.
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). As a temporary workaround, implement rate limiting or request throttling on the Accept-Encoding header.
Rack 2.2.23 (recommended for 2.x users) or Rack 3.2.6 (for 3.x users)
- Check your current Rack gem version by running `gem list rack` or inspecting your Gemfile.lock
- Update Rack to a fixed version: 2.2.23, 3.1.21, or 3.2.6 (or later) by running `bundle update rack` or adding the version to your Gemfile
- Run `bundle install` to install the updated gem
- Run your application's test suite to verify compatibility
- 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
- Implementation2.0 h
- Testing4.0 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 $2,144.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-34230 — 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-2026-34230 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
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