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How CVE-2026-69207 Works
The built-in CORS middleware (hono/cors) in the Hono web framework contains a Regular Expression Denial of Service (ReDoS) vulnerability. This flaw exists in the way the middleware parses the `Access-Control-Request-Headers` header during a CORS preflight `OPTIONS` request.
Under the default configuration, when `allowHeaders` is not explicitly set, the middleware reflects and parses the value of the `Access-Control-Request-Headers` header. The parsing is performed using a regular expression that is tolerant of whitespace. However, this regex suffers from catastrophic backtracking. When the header value contains a long sequence of whitespace without any delimiter, the regex engine’s backtracking causes the processing time to grow quadratically (O(n²)) with the length of the input.
An attacker can exploit this by sending a single preflight request with a crafted `Access-Control-Request-Headers` header containing an extended run of whitespace characters. Such a request can consume several seconds of CPU time to process. Because the header size is only limited by the deployment’s maximum HTTP header size, the impact can be significant.
On runtimes that share a single execution thread across all requests, like many JavaScript environments, this CPU spike will stall the processing of all concurrent requests, leading to a denial-of-service condition. This attack requires no authentication, special origin, or user interaction. It affects any application using `cors()` with the default or an empty `allowHeaders` setting. Applications that configure a non-empty `allowHeaders` do not trigger the vulnerable code path.
The vulnerability is identified as CVE-2026-69207 and is also tracked under the GitHub Security Advisory GHSA-8j4g-w8fx-2239.
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Platform: Hono
Version: < 4.12.34
Vulnerability: ReDoS
Severity: Critical
date: 2026-08-03
Prediction: 2026-08-03
What Undercode Say: Analytics
The vulnerability resides in the regex pattern used to parse the `Access-Control-Request-Headers` value. The following is a conceptual representation of the vulnerable logic:
// Conceptual vulnerable parsing logic const headerValue = "x-custom-header "; // Attacker-controlled const regex = /^[a-zA-Z0-9_-]+(?:\s,\s[a-zA-Z0-9_-]+)$/; // Simplified example const match = regex.test(headerValue); // Catastrophic backtracking on long whitespace
A single request can be crafted as follows:
curl -X OPTIONS https://target.example.com/api \
-H "Origin: https://attacker.com" \
-H "Access-Control-Request-Method: GET" \
-H "Access-Control-Request-Headers: $(python3 -c "print('x' + ' ' 100000)")"
The middleware’s regex, when faced with a long sequence of spaces, will enter a state of excessive backtracking, consuming CPU cycles and blocking the event loop.
How Exploit
An unauthenticated attacker can exploit this vulnerability by sending a high volume of malicious preflight requests. Each request includes an `Access-Control-Request-Headers` header with a long whitespace run. The server will spend an inordinate amount of time processing each request, leading to resource exhaustion. On single-threaded runtimes, this effectively halts all other request processing, resulting in a denial of service.
Protection
The vulnerability is patched in Hono version 4.12.34. Users are strongly advised to upgrade to this version or later. As a workaround, applications can set a non-empty `allowHeaders` configuration in the CORS middleware, which bypasses the vulnerable code path. Additionally, deploying a Web Application Firewall (WAF) or reverse proxy that can limit the size of the `Access-Control-Request-Headers` header may provide a layer of defense.
Impact
- Denial of Service: The primary impact is a denial of service. An attacker can render the application unresponsive by exhausting CPU resources.
- No Data Exposure: This vulnerability does not lead to data exposure or modification.
- Wide Applicability: Any Hono application using the default CORS configuration is affected, making this a broadly applicable threat.
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Sources:
Reported By: github.com
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