A2A Java SDK send client credentials under server-controlled HTTP header name, enabling leakage via redirect

Impact.* A malicious or compromised A2A agent can cause the client's API key credential to be forwarded to an arbitrary third-party server. The agent serves an AgentCard with an APIKeySecurityScheme specifying a custom header name, then responds with an HTTP redirect. The JDK HttpClient follows the redirect and forwards the custom header containing the credential, since only the standard Authorization header may be stripped on cross-origin redirects.

Where. client/transport/spi/src/main/java/org/a2aproject/sdk/client/transport/spi/interceptors/auth/AuthInterceptor.java:69 in intercept

What. The apiKeySecurityScheme.name() field from the remote agent's AgentCard (untrusted, server-controlled data) is used directly as the HTTP header name when placing client credentials. Combined with JdkA2AHttpClient's followRedirects(NORMAL) policy, credentials under custom header names survive cross-origin redirects (only Authorization is stripped on newer JDKs), allowing a malicious agent to leak client credentials to third-party servers. Additionally, AuthInterceptor does not check that location==HEADER before placing the credential as a header.

Exploit scenario. Client configures an API key credential for agent's security scheme myAuth. The malicious agent returns an AgentCard where securitySchemes includes myAuth as APIKeySecurityScheme with name='X-Leaked-Cred' and location=HEADER. AuthInterceptor places the credential in the X-Leaked-Cred header. The agent responds with HTTP 302 redirect to https://evil.com/collect. JdkA2AHttpClient follows the redirect, forwarding all custom headers including X-Leaked-Cred to evil.com. The attacker captures the client's credential.

Preconditions.

• Client application has configured API key credentials via CredentialService for the agent's security scheme
• The agent is malicious or compromised (controls its own AgentCard response)
• The default JdkA2AHttpClient is used (with followRedirects=NORMAL)
• The JDK version does not strip custom headers on cross-origin redirects (all current versions)

Fix. Validate the APIKeySecurityScheme.name() field against a whitelist of safe header names. Check that apiKeySecurityScheme.location() equals Location.HEADER before placing the credential as a header. Consider using followRedirects(NEVER) and implementing manual redirect handling that strips all authentication headers on cross-origin redirects.

Verification. 2/3 lens verifiers confirmed.