Remote service interruption via ETag header regex
Published Oct 10, 2025 · Updated Oct 10, 2025
ReDoS in Sinatra before 4.2.0 allows remote attackers to interrupt applications using etag through crafted conditional headers. The etag_matches? method splits If-Match and If-None-Match values with a backtracking regular expression, so whitespace-heavy input consumes excessive CPU. Exploitation requires an application to call etag while running on Ruby before 3.2; affected request handling can stall and deny service.
Summary
What happened
ReDoS in Sinatra before 4.2.0 allows remote attackers to interrupt applications using etag through crafted conditional headers. The etag_matches? method splits If-Match and If-None-Match values with a backtracking regular expression, so whitespace-heavy input consumes excessive CPU. Exploitation requires an application to call etag while running on Ruby before 3.2; affected request handling can stall and deny service.
The record
- CVE
- CVE-2025-61921
- Published
- Oct 10, 2025
- Updated
- Oct 10, 2025
- Vendor
- Sinatra
- Product
- Sinatra
- Classifications
- CWE-1333, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Oct 10, 2025CVE publishedPublication date reported by the CVE source.
- Oct 10, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
Compare your product and version with the public record. A matching version still requires validation against your environment.
Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 4.2.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Whitespace-heavy header ReDoS benchmarkproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
Check the software in your environment
Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.
Book a demo