# CVE-2025-61921: Sinatra is vulnerable to ReDoS through ETag header value generation

Source: https://junglewise.ai/threats/cve-2025-61921-sinatra-is-vulnerable-to-redos-through-etag-header-value  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-10-10  
License: CC BY 4.0 with a link back

CVE-2025-61921 · Severity: medium · CVSS 4 · Published 2025-10-10

Sinatra is vulnerable to ReDoS through ETag header value generation

Technologies: [sinatra (RubyGems)](https://junglewise.ai/threats/technologies/sinatra). Vendors: [RubyGems](https://junglewise.ai/threats/vendors/rubygems).

## Quick answers

**Is CVE-2025-61921 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2025-61921 being exploited in the wild.

**How severe is CVE-2025-61921?** Medium severity, with a CVSS score of 4, published 10 October 2025.

**Which products and versions are affected?** sinatra (RubyGems) ECOSYSTEM:introduced=0,fixed=4.2.0.

**Is there a fix for CVE-2025-61921?** Yes. sinatra (RubyGems): upgrade to 4.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2025-61921?** Check whether you run sinatra (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- sinatra (RubyGems): upgrade to 4.2.0 or later

  ```sh
  bundle update sinatra
  ```


## Affected products

- sinatra (RubyGems) ECOSYSTEM:introduced=0,fixed=4.2.0

## Identifiers

- CVE-2025-61921
- GHSA-mr3q-g2mv-mr4q

## Related threats

- [Sinatra vulnerable to Reliance on Untrusted Inputs in a Security Decision](https://junglewise.ai/threats/cve-2024-21510-sinatra-vulnerable-to-reliance-on-untrusted-inputs-in-a-security)
- [Sinatra vulnerable to Reflected File Download attack](https://junglewise.ai/threats/cve-2022-45442-sinatra-vulnerable-to-reflected-file-download-attack)
- [sinatra does not validate expanded path matches](https://junglewise.ai/threats/cve-2022-29970-sinatra-does-not-validate-expanded-path-matches)
- [Sinatra Cross-site Scripting vulnerability](https://junglewise.ai/threats/cve-2018-11627-sinatra-cross-site-scripting-vulnerability)
- [Sinatra Path Traversal vulnerability](https://junglewise.ai/threats/cve-2018-7212-sinatra-path-traversal-vulnerability)