CVE-2025-61921: Sinatra has ReDoS vulnerability in ETag header value generation

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2025-10-10 · updated 2026-10-19
Weakness: CWE-1333

Description

Sinatra is a domain-specific language for creating web applications in Ruby. In versions prior to 4.2.0, 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. 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. Version 4.2.0 fixes the issue.

Affected products

sinatra: before 4.2.0 — fixed in 4.2.0

How to fix

Upgrade to a fixed version: sinatra 4.2.0. Check the vendor advisory for mitigations.

References

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