Executive brief
liboauth2 is a library used by applications to handle OAuth 2.0 and OpenID Connect authentication. A security flaw in how it handles AWS Application Load Balancer (ALB) tokens allows an attacker to trick the server into making unauthorized web requests to internal systems. This could lead to the exposure of internal configuration data or unauthorized access to private internal services.
Technical details
A Server-Side Request Forgery (SSRF) vulnerability exists in the oauth2_jose_jwks_aws_alb_resolve() function of liboauth2. The AWS ALB verifier extracts the 'signer' and 'kid' (Key ID) fields from an unverified JWT header. If the 'signer' matches a configured ARN, the 'kid' value is appended to the 'alb_base_url' without proper URL encoding or path sanitization. Because the HTTP GET request for the JWKS is issued before the JWT signature is verified, a remote attacker can provide a malicious 'kid' to force the server to perform a GET request to an arbitrary internal path. This issue is resolved in version 2.3.0 by implementing proper URL encoding for the 'kid' parameter.
Affected products
- OpenIDC liboauth2 < 2.3.0
Timeline
- 2026-07-02: advisory: NVD publication date
- 2026-07-02: disclosed
- 2026-07-02: patched: Fixed in version 2.3.0