Summary
Sinatra is vulnerable to ReDoS through ETag header value generation
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)
Impact
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-61921? CVE-2025-61921 is a low-severity uncontrolled resource consumption vulnerability in sinatra (rubygems), affecting versions < 4.2.0. It is fixed in 4.2.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- Which versions of sinatra are affected by CVE-2025-61921? sinatra (rubygems) versions < 4.2.0 is affected.
- Is there a fix for CVE-2025-61921? Yes. CVE-2025-61921 is fixed in 4.2.0. Upgrade to this version or later.
- Is CVE-2025-61921 exploitable, and should I be worried? Whether CVE-2025-61921 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-61921 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-61921? Upgrade
sinatrato 4.2.0 or later.