CVE-2026-40969 in Spring gRPC
Summary
by MITRE • 04/28/2026
The raw message of every server-side AuthenticationException is returned to the unauthenticated remote caller in the gRPC status description. This allows an attacker to obtain information about the authentication failure, which may be useful for further attacks.
Affected versions: Spring gRPC: 1.0.0 - 1.0.2 (fixed in 1.0.3). Older, unsupported versions are also affected.
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Analysis
by VulDB Data Team • 05/17/2026
The vulnerability described represents a critical information disclosure issue within Spring gRPC implementations that exposes authentication failure details to unauthenticated remote attackers. This flaw exists in the gRPC status description handling mechanism where server-side AuthenticationException messages are directly transmitted to clients without proper sanitization or abstraction. The exposure of authentication failure details creates a significant security risk that can be exploited by malicious actors to gather intelligence about the authentication system and potentially identify valid user accounts or authentication mechanisms.
This vulnerability directly maps to CWE-209, Information Exposure Through an Error Message, which specifically addresses the issue of sensitive information being disclosed through error handling mechanisms. The flaw operates at the application layer where gRPC status codes are populated with raw exception messages instead of generic authentication failure responses. Attackers can leverage this information to perform account enumeration attacks, identify valid usernames through different error messages, or understand the underlying authentication architecture. The vulnerability affects the fundamental security principle of least privilege by exposing system internals to unauthorized parties.
The operational impact of this vulnerability extends beyond simple information disclosure to enable more sophisticated attack vectors including credential stuffing, brute force attacks, and account takeover attempts. When authentication systems reveal specific failure reasons such as "invalid username" versus "invalid password", attackers can systematically determine valid account names and focus their efforts on password cracking. This weakness creates a reconnaissance opportunity that significantly reduces the effort required for subsequent attacks, as the attacker gains insight into the authentication system's behavior patterns and validation logic.
The security implications of this vulnerability align with several ATT&CK techniques including T1586 for credential access and T1078 for valid accounts. The exposure of authentication failure details can be exploited to establish persistent access through account compromise or to facilitate privilege escalation attacks. Organizations using affected Spring gRPC versions should immediately implement mitigations including the deployment of version 1.0.3 or higher which addresses this issue through proper error message abstraction. Additional protective measures include implementing rate limiting, account lockout mechanisms, and comprehensive monitoring of authentication failure patterns to detect potential exploitation attempts.
The fix implemented in Spring gRPC version 1.0.3 demonstrates the importance of proper error handling design and the principle of security by design in distributed systems. The solution involves abstracting authentication exceptions to provide generic status descriptions while maintaining internal logging for legitimate administrative purposes. This approach ensures that external callers receive minimal information about authentication failures while preserving the ability for system administrators to troubleshoot authentication issues through appropriate logging channels. Organizations should also consider implementing additional security controls such as authentication throttling, IP address monitoring, and comprehensive audit logging to further mitigate the risk of exploitation.