CVE-2026-111763

A2A Java SDK is the official Java SDK for the Agent2Agent (A2A) Protocol. Prior to 1.3.0.Final, a malicious or compromised remote agent can cause the default JDK-based client to forward a configured API key to a third-party server. When a client application configures credentials through CredentialService, AuthInterceptor.intercept in client/transport/spi/src/main/java/org/a2aproject/sdk/client/transport/spi/interceptors/auth/AuthInterceptor.java uses APIKeySecurityScheme.name() from the remote AgentCard directly as the HTTP header name containing the credential, without checking that the declared location is HEADER. JdkA2AHttpClient uses followRedirects(NORMAL), so an agent-controlled cross-origin redirect forwards the custom credential-bearing header even when the standard Authorization header is stripped. The attack requires configured API key credentials for the agent's security scheme and control of the agent's AgentCard and redirect response, but no additional client privileges. This issue is fixed in version 1.3.0.Final.

A2A Java SDK is the official Java SDK for the Agent2Agent (A2A) Protocol. Prior to 1.3.0.Final, a malicious or compromised remote agent can cause the default JDK-based client to forward a configured API key to a third-party server. When a client application configures credentials through CredentialService, AuthInterceptor.intercept in client/transport/spi/src/main/java/org/a2aproject/sdk/client/transport/spi/interceptors/auth/AuthInterceptor.java uses APIKeySecurityScheme.name() from the remote AgentCard directly as the HTTP header name containing the credential, without checking that the declared location is HEADER. JdkA2AHttpClient uses followRedirects(NORMAL), so an agent-controlled cross-origin redirect forwards the custom credential-bearing header even when the standard Authorization header is stripped. The attack requires configured API key credentials for the agent's security scheme and control of the agent's AgentCard and redirect response, but no additional client privileges. This issue is fixed in version 1.3.0.Final.

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.

A2A Java SDK is the official Java SDK for the Agent2Agent (A2A) Protocol. Prior to 1.3.0.Final, a malicious or compromised remote agent can cause the default JDK-based client to forward a configured API key to a third-party server. When a client application configures credentials through CredentialService, AuthInterceptor.intercept in client/transport/spi/src/main/java/org/a2aproject/sdk/client/transport/spi/interceptors/auth/AuthInterceptor.java uses APIKeySecurityScheme.name() from the remote AgentCard directly as the HTTP header name containing the credential, without checking that the declared location is HEADER. JdkA2AHttpClient uses followRedirects(NORMAL), so an agent-controlled cross-origin redirect forwards the custom credential-bearing header even when the standard Authorization header is stripped. The attack requires configured API key credentials for the agent's security scheme and control of the agent's AgentCard and redirect response, but no additional client privileges. This issue is fixed in version 1.3.0.Final.

A2A Java SDK is the official Java SDK for the Agent2Agent (A2A) Protocol. Prior to 1.3.0.Final, a malicious or compromised remote agent can cause the default JDK-based client to forward a configured API key to a third-party server. When a client application configures credentials through CredentialService, AuthInterceptor.intercept in client/transport/spi/src/main/java/org/a2aproject/sdk/client/transport/spi/interceptors/auth/AuthInterceptor.java uses APIKeySecurityScheme.name() from the remote AgentCard directly as the HTTP header name containing the credential, without checking that the declared location is HEADER. JdkA2AHttpClient uses followRedirects(NORMAL), so an agent-controlled cross-origin redirect forwards the custom credential-bearing header even when the standard Authorization header is stripped. The attack requires configured API key credentials for the agent's security scheme and control of the agent's AgentCard and redirect response, but no additional client privileges. This issue is fixed in version 1.3.0.Final.

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.