CVE-2026-71848: Hono: Algorithmic Complexity DoS in Language Middleware

Summary

The languageDetector middleware is vulnerable to algorithmic complexity denial of service when processing a crafted language tag containing a large number of hyphen-separated subtags.

Details

To implement progressive language-tag truncation, normalizeLanguage() repeatedly calls parts.slice(0, i).join('-') for every possible prefix. The total amount of string processing grows quadratically with the number of subtags.

Language values may come from a query parameter, cookie, Accept-Language header, or URL path, depending on the detector configuration. The default detector order enables query-string, cookie, and header detection, so applications using languageDetector() may expose this processing to unauthenticated requests.

Request-size limits reduce the maximum cost of a single request but do not eliminate the issue. Inputs accepted by common JavaScript runtimes can still cause noticeable synchronous event-loop blocking.

Impact

An attacker may repeatedly send requests containing long, hyphen-separated language tags, causing excessive CPU consumption and preventing unrelated requests from being processed.

The practical impact depends on the runtime's request-size limits, reverse-proxy configuration, and the detectors enabled by the application.

Resolution

The progressive lookup should avoid reconstructing every shorter prefix. The implementation can instead inspect the configured supported languages and select the longest value that matches the input at a hyphen boundary.