RackApplication
CVE-2026-34786
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::Static#applicable_rules evaluates several header_rules types against the raw URL-encoded PATH_INFO, while the underlying file-serving path is decoded before the file is served. As a result, a request for a URL-encoded variant of a static path can serve the same file without the headers that header_rules were intended to apply. In deployments that rely on Rack::Static to attach security-relevant response headers to static content, this can allow an attacker to bypass those headers by requesting an encoded form of the path. 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::Static's header_rules are evaluated against URL-encoded PATH_INFO, but files are served from a decoded path. This mismatch allows attackers to request URL-encoded variants of static file paths to bypass security headers that would normally be applied by header_rules.
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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Check installed Rack versionRun `gem list rack` or in Ruby: `puts Gem.loaded_specs['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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Identify if Rack::Static is in useSearch codebase for `Rack::Static` or `use Rack::Static` or `require 'rack/static'` in application configuration filesAffected if Rack::Static middleware is loaded and configured in the application
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Verify if header_rules or security headers are configured for static filesReview the Rack::Static configuration to check for `header_rules`, `headers`, or similar options being set for static file servingAffected if header_rules or custom security headers are defined for static file serving (this is required for the bypass to have impact)
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Confirm URL-encoded PATH_INFO handlingTest static file access by requesting URL-encoded variants such as `/static/%2e%2e/config` or similar encoded path segments and verify if security headers are still appliedAffected if URL-encoded requests to static files bypass the configured security headers while the decoded equivalent would include them
You are affected if your Rack version is vulnerable AND you use Rack::Static with header_rules or security headers, and an attacker can send URL-encoded requests to bypass those headers.
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, then verify that security headers are properly applied to static content served through Rack::Static.
Rack 2.2.23, 3.1.21, or 3.2.6 (depending on your major version branch)
- Identify your current Rack version by checking your Gemfile.lock or running `gem list rack`
- For Rack 2.x users: Upgrade to version 2.2.23 or later by running `gem install rack -v 2.2.23` or updating your Gemfile with `gem 'rack', '~> 2.2'`
- For Rack 3.0.x users: Upgrade to version 3.1.21 or later by running `gem install rack -v 3.1.21` or updating your Gemfile with `gem 'rack', '~> 3.1'`
- For Rack 3.2.x users: Upgrade to version 3.2.6 or later by running `gem install rack -v 3.2.6` or updating your Gemfile with `gem 'rack', '~> 3.2'`
- Run `bundle update rack` to update dependencies
- Verify the upgrade was successful by running `gem list rack` and confirming the installed version
- Test your application to ensure static file serving with header_rules still functions correctly
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-2026-34786 — 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-2026-34786 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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