Hono: ReDoS in CORS middleware via Access-Control-Request-Headers

### Summary The built-in CORS middleware (`hono/cors`) parses the attacker-controlled `Access-Control-Request-Headers` request header during a preflight (`OPTIONS`) request using a regular expression whose running time is quadratic in the input length. A single request carrying a long run of whitespace can consume seconds of CPU, and repeated requests can render the service unresponsive. This parsing runs under the default configuration. ### Details On a CORS preflight, when `allowHeaders` is not configured - the default - the middleware reflects and parses the `Access-Control-Request-Headers` value. The parser used a whitespace-tolerant regular expression whose backtracking makes the work grow quadratically (O(n²)) with the length of the value when it contains a long whitespace sequence without a delimiter. Because the header value is bounded only by the deployment's maximum HTTP header size, a single preflight can block request processing for a noticeable amount of time; on runtimes that share one execution thread across requests, this stalls concurrent requests as well. No authentication, special origin, or user interaction is required. This issue arises for any application using `cors()` with the default (or an empty) `allowHeaders`. Applications that set a non-empty `allowHeaders` do not reach the affected path. ### Impact An unauthenticated attacker can send preflight requests that each consume disproportionate CPU relative to their size, degrading or denying service. This is a denial-of-service issue only; it does not expose or modify data.

Source published

3 Aug 2026 20:23:30 UTC

Source modified

10 Sep 2026 03:51:13 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Publisher rating · MODERATE