CVE-2026-34785 — rack
HIGHCVE-2026-34785 is a high-severity (CVSS 7.5) CWE-187 vulnerability in rack. A fix is available for rack — see the affected versions and patch details below.
Rack: Local file inclusion in `Rack::Static` via URL Prefix Matching
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34785.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-34785 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
rack💎rack💎rackReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Summary
Rack::Static determines whether a request should be served as a static file using a simple string prefix check. When configured with URL prefixes such as "/css", it matches any request path that begins with that string, including unrelated paths such as "/css-config.env" or "/css-backup.sql".
As a result, files under the static root whose names merely share the configured prefix may be served unintentionally, leading to information disclosure.
Details
Rack::Static#route_file performs static-route matching using logic equivalent to:
@urls.any? { |url| path.index(url) == 0 }
This checks only whether the request path starts with the configured prefix string. It does not require a path segment boundary after the prefix.
For example, with:
use Rack::Static, urls: ["/css", "/js"], root: "public"
the following path is matched as intended:
/css/style.css
but these paths are also matched:
/css-config.env
/css-backup.sql
/csssecrets.yml
If such files exist under the configured static root, Rack forwards the request to the file server and serves them as static content.
This means a configuration intended to expose only directory trees such as /css/... and /js/... may also expose sibling files whose names begin with those same strings.
Impact
An attacker can request files under the configured static root whose names share a configured URL prefix and obtain their contents.
In affected deployments, this may expose configuration files, secrets, backups, environment files, or other unintended static content located under the same root directory.
Mitigation
- Update to a patched version of Rack that enforces a path boundary when matching configured static URL prefixes.
- Match only paths that are either exactly equal to the configured prefix or begin with
prefix + "/". - Avoid placing sensitive files under the
Rack::Staticroot directory. - Prefer static URL mappings that cannot overlap with sensitive filenames.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | rack | all versions | 2.2.23bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.0.0.beta1&&< 3.1.21 | 3.1.21bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.2.0&&< 3.2.6 | 3.2.6bundle update rack --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rack to 2.2.23 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34785 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34785 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34785. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2026-34785 in your dependencies?
O3 Security finds CVE-2026-34785 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.