Summary
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.
Impact
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.
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
})
)References
calloc134 Reporter
devanshbatham Reporter
Summary
A flaw in Hono’s JWK/JWKS JWT verification middleware allowed the JWT header’s
algvalue 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
algfield if present, but otherwise fell back to thealgvalue provided in the unverified JWT header.Because the
algfield 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 authorizationbypass through crafted tokens.
The practical impact depends on application configuration, including which algorithms are accepted and how JWTs are used for authorization decisions.
Impact
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.
Resolution
Update to the latest patched release.
Breaking change:
As part of this fix, the JWT middleware now requires the
algoption 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)
After (patched configuration)
References