CVE-2026-71848: Hono languageDetector algorithmic complexity DoS

CVE-2026-71848 · Severity: medium · CVSS 5.3 · Published 2026-08-07

Hono's language detection middleware is vulnerable to a denial-of-service attack when processing specially crafted language tags with many hyphenated subtags. An attacker can send repeated requests with these malicious language tags to consume excessive CPU resources, blocking the application from handling legitimate user requests. This vulnerability affects the default configuration where language detection processes untrusted input from query parameters, cookies, and HTTP headers.

Technologies: hono (npm). Vendors: npm.

Quick answers

Is CVE-2026-71848 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-71848 being exploited in the wild.
How severe is CVE-2026-71848?
Medium severity, with a CVSS score of 5.3, published 7 August 2026.
Which products and versions are affected?
hono (npm) >= 4.12.0, < 4.12.34, SEMVER:introduced=4.12.0,fixed=4.12.34.
Is there a fix for CVE-2026-71848?
Yes. hono (npm): upgrade to 4.12.34 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-71848 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-71848?
Check whether you run hono (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • hono (npm): upgrade to 4.12.34 or later
    npm install hono@4.12.34

Technical details

The normalizeLanguage() function in Hono's languageDetector middleware implements progressive language-tag truncation by repeatedly calling parts.slice(0, i).join('-') for every possible prefix, resulting in quadratic string processing complexity relative to the number of hyphen-separated subtags in the input. Language values are sourced from user-controllable locations including query parameters, cookies, Accept-Language headers, or URL paths depending on detector configuration; the default configuration enables query-string, cookie, and header detection without authentication requirements. An attacker can craft requests with language tags containing a large number of subtags to trigger synchronous, blocking CPU-intensive processing that stalls the JavaScript event loop. While runtime request-size limits provide some mitigation, they do not eliminate the issue as inputs within typical limits can still cause noticeable event-loop blocking. The vulnerability was fixed in version 4.12.34 by optimizing the lookup to avoid reconstructing every shorter prefix and instead inspecting configured supported languages to select the longest matching value at a hyphen boundary.

Affected products

  • Honojs Hono 4.12.0 through 4.12.33

Identifiers

  • CVE-2026-71848
  • GHSA-54fx-42gc-7vw4

Timeline

  • 2026-08-07: disclosed: Security advisory GHSA-54fx-42gc-7vw4 published
  • 2026-08-03: patched: Fix released in version 4.12.34

References

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