CVE-2026-22817

A flaw in Hono’s JWK/JWKS JWT verification middleware allowed the JWT header’s alg value to influence signature verification when the selected JWK did not explicitly specify an algorithm. This could enable JWT algorithm confusion and, in certain configurations, allow forged tokens to be accepted.

Details

When verifying JWTs using JWKs or a JWKS endpoint, the middleware selected the verification algorithm based on the JWK’s alg field if present, but otherwise fell back to the alg value provided in the unverified JWT header.

Because the alg field in a JWK is optional and often omitted in real-world JWKS configurations, this behavior could allow an attacker to control the algorithm used for verification. In some environments, this may lead to authentication or authorization
bypass through crafted tokens.

The practical impact depends on application configuration, including which algorithms are accepted and how JWTs are used for authorization decisions.

Resolution

Update to the latest patched release.

Breaking change:

As part of this fix, the JWT middleware now requires the alg option to be explicitly specified. This prevents algorithm confusion by ensuring that the verification algorithm is not derived from untrusted JWT header values.

Applications upgrading must update their configuration accordingly.

Before (vulnerable configuration)

import { jwt } from 'hono/jwt'

app.use(
  '/auth/*',
  jwt({
    secret: 'it-is-very-secret',
    // alg was optional
  })
)

After (patched configuration)

import { jwt } from 'hono/jwt'

app.use(
  '/auth/*',
  jwt({
    secret: 'it-is-very-secret',
    alg: 'HS256', // required
  })
)

In affected configurations, an attacker may be able to forge JWTs with attacker-controlled claims, potentially resulting in authentication or authorization bypass.

Applications that do not use the JWK/JWKS middleware, do not rely on JWT-based authentication, or explicitly restrict allowed algorithms are not affected.

CVE-2026-22817 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.11.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-22817? CVE-2026-22817 is a high-severity security vulnerability in hono (npm), affecting versions < 4.11.4. It is fixed in 4.11.4.
2. How severe is CVE-2026-22817? CVE-2026-22817 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of hono are affected by CVE-2026-22817? hono (npm) versions < 4.11.4 is affected.
4. Is there a fix for CVE-2026-22817? Yes. CVE-2026-22817 is fixed in 4.11.4. Upgrade to this version or later.
5. Is CVE-2026-22817 exploitable, and should I be worried? Whether CVE-2026-22817 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-22817 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-22817? Upgrade hono to 4.11.4 or later.