CVE-2025-61921 — sinatra
Fix: sinatra/sinatra#1823CVE-2025-61921 is a CWE-1333 vulnerability in sinatra. A fix is available for sinatra — see the affected versions and patch details below.
Sinatra has ReDoS vulnerability in ETag header value generation
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-2025-61921.
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.
Real-World Exposure
sinatraReal-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
There is a denial of service vulnerability in the If-Match and If-None-Match header parsing component of Sinatra, if the etag method is used when constructing the response and you are using Ruby < 3.2.
Details
Carefully crafted input can cause If-Match and If-None-Match header parsing in Sinatra to take an unexpected amount of time, possibly resulting in a denial of service attack vector. This header is typically involved in generating the ETag header value. Any applications that use the etag method when generating a response are impacted if they are using Ruby below version 3.2.
Resources
- https://github.com/sinatra/sinatra/issues/2120 (report)
- https://github.com/sinatra/sinatra/pull/2121 (fix)
- https://github.com/sinatra/sinatra/pull/1823 (older ReDoS vulnerability)
- https://bugs.ruby-lang.org/issues/19104 (fix in Ruby >= 3.2)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | sinatra | all versions | 4.2.0bundle update sinatra --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for sinatra, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update sinatra to 4.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-61921 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-2025-61921 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-61921. 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-2025-61921 in your dependencies?
O3 Security finds CVE-2025-61921 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.