Improper Authorization in Hono (JWT Audience Validation)

Hono’s JWT authentication middleware did not validate the aud (Audience) claim by default. As a result, applications using the middleware without an explicit audience check could accept tokens intended for other audiences, leading to potential cross-service access (token mix-up).

The issue is addressed by adding a new verification.aud configuration option to allow RFC 7519–compliant audience validation. This change is classified as a security hardening improvement, but the lack of validation can still be considered a vulnerability in deployments that rely on default JWT verification.

Recommended secure configuration

You can enable RFC 7519–compliant audience validation using the new verification.aud option:

ts import { Hono } from 'hono' import { jwt } from 'hono/jwt'

const app = new Hono()

app.use( '/api/', jwt({ secret: 'my-secret', verification: { // Require this API to only accept tokens with aud = 'service-a' aud: 'service-a', }, }) )

Below is the original description by the reporter. For security reasons, it does not include PoC reproduction steps, as the vulnerability can be clearly understood from the technical description.

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The original description by the reporter

Summary Hono’s JWT Auth Middleware does not provide a built-in aud (Audience) verification option, which can cause confused-deputy / token-mix-up issues: an API may accept a valid token that was issued for a different audience (e.g., another service) when multiple services share the same issuer/keys. This can lead to unintended cross-service access. Hono’s docs list verification options for iss/nbf/iat/exp only, with no aud support; RFC 7519 requires that when an aud claim is present, tokens MUST be rejected unless the processing party identifies itself in that claim.

Note: This problem likely exists in the JWK/JWKS-based middleware as well (e.g., jwk / verifyWithJwks)

Details - The middleware’s verifyOptions enumerate only iss, nbf, iat, and exp; there is no aud option. The same omission appears in the JWT Helper’s “Payload Validation” list. Developers relying on the middleware for complete standards-aligned validation therefore won’t check audience by default. - Standards requirement: RFC 7519 §4.1.3 states that each principal intended to process the JWT MUST identify itself with a value in the aud claim; if it does not, the JWT MUST be rejected (when aud is present). Lack of a first-class aud check increases the risk that tokens issued for Service B are accepted by Service A. - Real-world effect: In deployments with a single IdP/JWKS and shared keys across multiple services, a token minted for one audience can be mistakenly accepted by another audience unless developers implement a custom audience check. - For example, with Google Identity (OIDC), iss is always https://accounts.google.com (shared across apps), but aud differs per application because it is that app’s OAuth client ID; therefore, an attacker can host a separate service that supports “Sign in with Google,” obtain a valid ID token (JWT) for the victim user, and—if your API does not verify aud—use that token to access your API with the victim’s privileges.

Impact Type: Authentication/authorization weakness via token mix-up (confused-deputy).

Who is impacted: Any Hono user who: - shares an issuer/keys across multiple services (common with a single IdP/JWKS) - distinguishes tokens by intended recipient using aud.

What can happen: - Cross-service access: A token for Service B may be accepted by Service A. - Boundary erosion: ID tokens and access tokens, or separate API audiences, can be inadvertently intermixed. - This may causes unauthorized invocation of sensitive endpoints.

Recommended remediation: 1) Add verifyOptions.aud (string | string[] | RegExp) to the middleware and enforce RFC 7519 semantics: In verify method, if aud is present and does not match with specified audiences, reject. 2) Ensure equivalent aud handling exists in the JWK/JWKS flow (jwk middleware / verifyWithJwks) so users of external IdPs can enforce audience consistently.