JWT algorithm confusion in Hono JWK Auth Middleware when JWK lacks "alg" (untrusted header.alg fallback)

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Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.11.4, there is a flaw in Hono’s JWK/JWKS JWT verification middleware allowed the algorithm specified in the JWT header to influence signature verification when the selected JWK did not explicitly define an algorithm. This could enable JWT algorithm confusion and, in certain configurations, allow forged tokens to be accepted. The JWK/JWKS JWT verification middleware has been updated to require an explicit allowlist of asymmetric algorithms when verifying tokens. The middleware no longer derives the verification algorithm from untrusted JWT header values. This vulnerability is fixed in 4.11.4.

Source published

13 Jan 2026 19:49:52 UTC

Source modified

15 Jan 2026 15:29:39 UTC

Saved here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.11.4, there is a flaw in Hono’s JWK/JWKS JWT verification middleware allowed the algorithm specified in the JWT header to influence signature verification when the selected JWK did not explicitly define an algorithm. This could enable JWT algorithm confusion and, in certain configurations, allow forged tokens to be accepted. The JWK/JWKS JWT verification middleware has been updated to require an explicit allowlist of asymmetric algorithms when verifying tokens. The middleware no longer derives the verification algorithm from untrusted JWT header values. This vulnerability is fixed in 4.11.4.

Source published

Jan 13, 2026, 7:49:52 PM (timezone not stated)

Source modified

Jan 15, 2026, 3:29:39 PM (timezone not stated)

Saved here

15 Nov 2026 07:35:36 UTC

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ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.11.4, there is a flaw in Hono’s JWK/JWKS JWT verification middleware allowed the algorithm specified in the JWT header to influence signature verification when the selected JWK did not explicitly define an algorithm. This could enable JWT algorithm confusion and, in certain configurations, allow forged tokens to be accepted. The JWK/JWKS JWT verification middleware has been updated to require an explicit allowlist of asymmetric algorithms when verifying tokens. The middleware no longer derives the verification algorithm from untrusted JWT header values. This vulnerability is fixed in 4.11.4.

Source published

13 Jan 2026 20:16:11 UTC

Source modified

17 Jun 2026 10:20:29 UTC

Saved here

25 Oct 2026 05:17:36 UTC

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CVSS 3.1 · [email protected] · Secondary · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

CVSS 3.1 · [email protected] · Primary · 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

## Summary A flaw in Hono’s JWK/JWKS JWT verification middleware allowed the algorithm specified in the JWT header to influence signature verification when the selected JWK did not explicitly define 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. If the JWK did not specify an algorithm, the middleware fell back to using the `alg` value provided in the unverified JWT header. Because the `alg` field in a JWK is optional and commonly omitted in real-world JWKS configurations, this behavior could allow an attacker to influence which algorithm is used for verification. In some environments, this may result in authentication or authorization bypass through crafted JWTs. The practical impact depends on application configuration, including which algorithms are accepted and how JWTs are used to make authorization decisions. ## Impact In affected configurations, an attacker may be able to forge JWTs with attacker-controlled claims, potentially leading to 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. ## Resolution Update to the latest patched release. **Breaking change:** The JWK/JWKS JWT verification middleware has been updated to require an explicit allowlist of asymmetric algorithms when verifying tokens. The middleware no longer derives the verification algorithm from untrusted JWT header values. Instead, callers must explicitly specify which asymmetric algorithms are permitted, and only tokens signed with those algorithms will be accepted. This prevents JWT algorithm confusion by ensuring that algorithm selection is fully controlled by application configuration. As part of this fix, the `alg` option is now required when using the JWK/JWKS middleware, and symmetric (HS*) algorithms are no longer accepted in this context. ### Before (vulnerable configuration) ```ts import { jwk } from 'hono/jwk' app.use( '/auth/*', jwk({ jwks_uri: 'https://example.com/.well-known/jwks.json', // alg was optional }) ) ``` ### After (patched configuration) ```ts import { jwk } from 'hono/jwk' app.use( '/auth/*', jwk({ jwks_uri: 'https://example.com/.well-known/jwks.json', alg: ['RS256'], // required: explicit asymmetric algorithm allowlist }) ) ```

Source published

13 Jan 2026 21:52:03 UTC

Source modified

10 Sep 2026 03:50:58 UTC

Saved here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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

Publisher rating · HIGH