CVE-2025-61921: sinatra — Exploit & Fix

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.

Is your site exposed to CVE-2025-61921?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

7.5

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Weakness Type (CWE)

CWE-1333 CWE-1333

Affected Products

Vendor	Product	Versions
sinatrarb	sinatra	< 4.2.0

References

Advisories & Patches

https://github.com/sinatra/sinatra/pull/1823 https://github.com/sinatra/sinatra/pull/2121 https://github.com/sinatra/sinatra/security/advisories/GHSA-mr3q-g2mv-mr4q

Exploits

https://github.com/sinatra/sinatra/pull/1823

Other References

https://bugs.ruby-lang.org/issues/19104 https://github.com/sinatra/sinatra/issues/2120

Frequently Asked Questions

What is CVE-2025-61921? +

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. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2025-61921? +

CVE-2025-61921 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-61921? +

CVE-2025-61921 affects products from sinatrarb, specifically: sinatra. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-61921? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-45409

Internationalized Domain Names in Applications (IDNA) for Python provides support for Internationalized Domain Names in Applications (IDNA) and Unicode IDNA …

CVE-2025-48058

PowSyBl (Power System Blocks) is a framework to build power system oriented software. Prior to version 6.7.2, there is a …

CVE-2025-48059

PowSyBl (Power System Blocks) is a framework to build power system oriented software. In com.powsybl:powsybl-iidm-criteria versions 6.3.0 to before 6.7.2 …

CVE-2025-6998

ReDoS in strip_whitespaces() function in cps/string_helper.py in Calibre Web and Autocaliweb allows unauthenticated remote attackers to cause denial of service …

CVE-2025-54363

Microsoft Knack 0.12.0 allows Regular expression Denial of Service (ReDoS) in the knack.introspection module. extract_full_summary_from_signature employs an inefficient regular expression …

CVE-2025-54364

Microsoft Knack 0.12.0 allows Regular expression Denial of Service (ReDoS) in the knack.introspection module. option_descriptions employs an inefficient regular expression …

CVE-2025-61921