Description

Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.12.34, the built-in CORS middleware, hono/cors, is vulnerable to a regular expression denial of service (ReDoS). During a preflight OPTIONS request, the middleware parses the attacker-controlled Access-Control-Request-Headers header using a whitespace-tolerant regular expression whose backtracking makes its running time quadratic in the input length. Because the header value is bounded only by the deployment's maximum HTTP header size, a single preflight carrying a long run of whitespace can consume seconds of CPU and block request processing. On runtimes that share one execution thread across requests, this stalls concurrent requests as well, and repeated requests can render the service unresponsive. This affects the default configuration, since the vulnerable path is reached whenever cors() is used with an unset or empty allowHeaders. Applications that set a non-empty allowHeaders are not affected. This issue is fixed in version 4.12.34.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-08-07T21:17:29.847Z",
  "pubdate": "2026-08-07T21:17:29.847Z",
  "executiveSummary": "A regular expression denial of service (ReDoS) vulnerability exists in the built-in CORS middleware of the Hono web application framework, specifically within hono/cors.\nThe vulnerability allows an unauthenticated remote attacker to cause excessive CPU consumption and block request processing by submitting specially crafted HTTP headers during a preflight OPTIONS request.\nThe affected component is the whitespace-tolerant regular expression used to parse the attacker-controlled Access-Control-Request-Headers header when the allowHeaders configuration is unset or empty.\nOn single-threaded JavaScript runtimes, a single malicious preflight request containing an extended sequence of whitespace can stall concurrent operations for seconds, while repeated requests can lead to a complete denial of service.\nApplications utilizing the default configuration of the cors() middleware without an explicitly defined, non-empty allowHeaders property are vulnerable to this denial of service condition.\nExploitation requires no special privileges or authentication beyond the ability to send standard HTTP preflight requests to the target web application.",
  "technicalDetails": "The root cause of the vulnerability resides in the internal parsing logic of the hono/cors middleware within the Hono web application framework prior to version 4.12.34.\nDuring the handling of HTTP preflight OPTIONS requests, the middleware inspects the attacker-controlled Access-Control-Request-Headers header.\nThe parsing mechanism employs a whitespace-tolerant regular expression that exhibits catastrophic backtracking behavior when processing specific input patterns.\nBecause the runtime evaluation time of the regular expression scales quadratically relative to the length of the input string, processing a sufficiently long run of whitespace can consume excessive CPU resources for multiple seconds.\nThe maximum length of the header value is bounded solely by the underlying web server or deployment environment's maximum HTTP header size limit.\nThe attack flow proceeds as follows: 1) An unauthenticated attacker crafts a malicious HTTP OPTIONS request directed at an endpoint utilizing the vulnerable hono/cors middleware. 2) The attacker populates the Access-Control-Request-Headers header with a long sequence of whitespace designed to trigger the catastrophic backtracking condition in the regular expression engine. 3) The Hono application receives the preflight request and invokes the CORS middleware to parse the malicious header. 4) The regular expression execution enters a high-complexity backtracking loop, monopolizing the CPU thread. 5) On JavaScript runtimes that share a single execution thread across concurrent requests, this computation stalls all simultaneous request processing. 6) Repeated transmission of these malicious payloads renders the web service entirely unresponsive, resulting in a denial of service.\nThe vulnerable component is explicitly the hono/cors middleware parsing logic.\nAffected software versions include all Hono framework versions prior to 4.12.34.\nApplications that explicitly configure a non-empty allowHeaders value bypass the vulnerable parsing path and are not affected by this issue."
}