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How the Mentioned CVE Works
This vulnerability is a JWT algorithm confusion attack targeting the `jwk` and `jwks` middleware in the Hono web framework. During JWT verification, the middleware fetches the JSON Web Key (JWK) corresponding to the token’s key ID (kid). The critical flaw occurred when determining which cryptographic algorithm to use for signature verification. If the retrieved JWK did not contain an explicit `alg` (algorithm) parameter—a common and optional field in real-world JWKS endpoints—the middleware would fall back to using the `alg` value from the unverified token’s header. An attacker could exploit this by forging a JWT, setting its header to use a symmetric algorithm like HS256, and signing it with a known or guessed secret. If the server’s public key (from the JWK) is mistakenly used as the symmetric verification key, the forged signature would be valid, allowing the attacker to bypass authentication and impersonate any user by injecting malicious claims.
DailyCVE Form
Platform: Hono
Version: Pre-patch versions
Vulnerability: JWT algorithm confusion
Severity: Critical
Date: 2026-01-14
Prediction: Already patched
What Undercode Say:
Analytics
Vulnerable code pattern detected in JWT verification logic. The system incorrectly trusted user-supplied `alg` header when JWK.alg was absent. This is a common misconfiguration leading to auth bypass.
Bash Commands and Codes
Check current Hono version npm list hono Update to the patched version npm update hono
// VULNERABLE CONFIGURATION (Before)
import { jwk } from 'hono/jwk';
app.use(
'/auth/',
jwk({
jwks_uri: 'https://example.com/.well-known/jwks.json',
// 'alg' parameter was optional here
})
);
// PATCHED CONFIGURATION (After)
import { jwk } from 'hono/jwk';
app.use(
'/auth/',
jwk({
jwks_uri: 'https://example.com/.well-known/jwks.json',
alg: ['RS256'], // 'alg' is now REQUIRED
})
);
How Exploit
An attacker crafts a malicious JWT with a header declaring alg: HS256. They sign this token using a symmetric secret, which could be a known value or even the public key from the JWKS endpoint encoded as a string. They then submit this token to an affected Hono endpoint. The server fetches the correct public JWK for the kid, but if that JWK lacks an `alg` field, the server uses the attacker’s supplied HS256. It then incorrectly uses the asymmetric public key as if it were a symmetric secret to verify the HS256 signature, which succeeds, accepting the forged token.
Protection from this CVE
Update the Hono framework to the latest patched version immediately. In the jwk/jwks middleware configuration, you must now explicitly provide the `alg` option with an allowlist of permitted asymmetric algorithms (e.g., ['RS256', 'ES384']). Ensure symmetric algorithms (HS) are never accepted in this context. Implement strict algorithm validation on the server side, never deriving it from the unverified token header. Regularly audit and rotate JWKS keys.
Impact
Successful exploitation leads to a full authentication and authorization bypass. Attackers can forge valid JWTs with any user identity or privileged claims (like admin: true). This grants unauthorized access to protected routes, data, and administrative functionalities, potentially resulting in complete application compromise, data breach, and privilege escalation. The impact is critical for applications using JWT for session management or access control.
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Sources:
Reported By: github.com
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