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Low severityNVD Advisory· Published Oct 10, 2025· Updated Oct 10, 2025

Sinatra has ReDoS vulnerability in ETag header value generation

CVE-2025-61921

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.

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Sinatra 4.1.1 and earlier has a ReDoS vulnerability in the `If-Match` and `If-None-Match` header parser when the `etag` method is used, leading to denial of service.

Vulnerability

CVE-2025-61921 is a denial-of-service vulnerability in the Sinatra web framework for Ruby, affecting versions prior to 4.2.0. The bug resides in the parsing of If-Match and If-None-Match HTTP headers when the application uses the etag method to generate response ETags [2]. The root cause is a regular expression that exhibits catastrophic backtracking, a classic ReDoS (Regular Expression Denial of Service) pattern [4].

Exploitation

An attacker can exploit this vulnerability by sending a crafted If-Match or If-None-Match header to a Sinatra endpoint that utilizes the etag method [2]. No prior authentication is required; the attack is purely network-based, sending a specially crafted input can cause the header parser to take an exponentially long time, overwhelming the server's request-handling capacity [2].

Impact

Successful exploitation leads to a denial of service condition. The targeted Sinatra application becomes unresponsive to legitimate requests, impacting availability. The vulnerability is network-triggerable without authentication, making it a practical vector for disrupting web services [2].

Mitigation

The vulnerability is fixed in Sinatra version 4.2.0 [2]. The fix addresses the ReDoS in the regex used by etag_matches? [3]. All users running versions prior to 4.2.0 who use the etag method should upgrade immediately. There is no known workaround; upgrading is the only complete mitigation [2].

References
  1. NVD - CVE-2025-61921
  2. Fix regex in `etag_matches?` to prevent ReDoS by gecunps · Pull Request #2121 · sinatra/sinatra
  3. Feature #19104: Introduce the cache-based optimization for Regexp matching - Ruby

AI Insight generated on May 19, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
sinatraRubyGems
< 4.2.04.2.0

Affected products

2

Patches

2
3fe8c38dc405

Fix regex in `etag_matches?` to prevent ReDoS (#2121)

https://github.com/sinatra/sinatragecunpsOct 8, 2025via ghsa
commit
1 file changed · +1 −1
  • lib/sinatra/base.rb+1 −1 modified
    @@ -711,7 +711,7 @@ def time_for(value)
     def etag_matches?(list, new_resource = request.post?)
       return !new_resource if list == '*'
 
-      list.to_s.split(/\s*,\s*/).include? response['ETag']
+      list.to_s.split(',').map(&:strip).include?(response['ETag'])
     end
 
     def with_params(temp_params)
8ff496bd4877

Merge pull request #1823 from ooooooo-q/fix/redos

https://github.com/sinatra/sinatraJordan OwensOct 9, 2022via ghsa
commit
1 file changed · +1 −1
  • rack-protection/lib/rack/protection/ip_spoofing.rb+1 −1 modified
    @@ -16,7 +16,7 @@ class IPSpoofing < Base
       def accepts?(env)
         return true unless env.include? 'HTTP_X_FORWARDED_FOR'
 
-        ips = env['HTTP_X_FORWARDED_FOR'].split(/\s*,\s*/)
+        ips = env['HTTP_X_FORWARDED_FOR'].split(',').map(&:strip)
         return false if env.include?('HTTP_CLIENT_IP') && (!ips.include? env['HTTP_CLIENT_IP'])
         return false if env.include?('HTTP_X_REAL_IP') && (!ips.include? env['HTTP_X_REAL_IP'])

Vulnerability mechanics

Generated on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

10

News mentions

0

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