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The CVE-2025-45513 vulnerability is a ReDoS flaw in the Sinatra web framework. It originates from the component that parses the `If-Match` and `If-None-Match` HTTP headers. When an application uses Sinatra’s `etag` helper method to generate a response ETag, it triggers the parsing of these request headers. The vulnerability is present when using Ruby versions prior to 3.2. These older Ruby versions use a vulnerable regular expression engine for parsing the header values, which contains lists of entity tags. A malicious actor can craft a specific, complex string as the value of the `If-Match` or `If-None-Match` header. When this malicious string is processed by the vulnerable regex, it causes catastrophic backtracking. This forces the server’s CPU to spend an excessive amount of time evaluating the regular expression, leading to a denial of service as the request thread is blocked and unable to handle other requests.
Platform: Sinatra
Version: (Pre-patch, with Ruby < 3.2)
Vulnerability: ReDoS
Severity: Low
date: 2025-10-10
Prediction: Patch expected 2025-10-24
What Undercode Say:
Simulating a malicious request with a ReDoS payload in the If-Match header curl -H "If-Match: \"\", \"1\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"\"" http://vulnerable-sinatra-app.com/resource Using ab to demonstrate a simple DoS attempt ab -n 100 -c 10 -H "If-Match: \"bad_string\"" http://vulnerable-sinatra-app.com/
Example of a vulnerable Sinatra route using the 'etag' helper get '/data' do data = get_sensitive_data etag data.checksum This triggers header parsing, vulnerable to ReDoS on Ruby < 3.2 data.to_json end
How Exploit:
Craft malicious If-Match/If-None-Match headers with complex strings to trigger catastrophic backtracking in the Ruby regex parser, causing high CPU utilization and service unavailability.
Protection from this CVE
Upgrade to Ruby >= 3.2. Apply the Sinatra patch when released. Use a WAF to filter malicious header patterns.
Impact:
Application Denial of Service, resource exhaustion.
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Sources:
Reported By: github.com
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