CVE-2018-20840 in Sign-In with Google API C++ Client
Summary
by MITRE
An unhandled exception vulnerability exists during Google Sign-In with Google API C++ Client before 2019-04-10. It potentially causes an outage of third-party services that were not designed to recover from exceptions. On the client, ID token handling can cause an unhandled exception because of misinterpretation of an integer as a string, resulting in denial-of-service and then other users can no longer login/sign-in to the affected third-party service. Once this third-party service uses Google Sign-In with google-api-cpp-client, a malicious user can trigger this client/auth/oauth2_authorization.cc vulnerability by requesting the client to receive the ID token from a Google authentication server.
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Analysis
by VulDB Data Team • 09/25/2023
The vulnerability described in CVE-2018-20840 represents a critical unhandled exception flaw within the Google API C++ Client library that specifically affects Google Sign-In implementations. This issue stems from a fundamental type interpretation error where an integer value is incorrectly processed as a string during the handling of ID tokens, creating a scenario where legitimate authentication flows can abruptly terminate due to runtime exceptions. The vulnerability impacts third-party services that integrate Google Sign-In functionality, particularly those utilizing the google-api-cpp-client library version prior to the 2019-04-10 release, making it a widespread concern for organizations relying on Google authentication services.
The technical execution of this vulnerability occurs within the client/auth/oauth2_authorization.cc component of the C++ client library, where the misinterpretation of integer values as strings during ID token processing creates an unhandled exception condition. This flaw specifically manifests when the client receives and processes authentication responses from Google's authentication servers, particularly during the token validation phase. The integer-to-string conversion error causes the application to crash or enter an undefined state, effectively rendering the authentication service unavailable. This type of vulnerability aligns with CWE-476 which describes NULL pointer dereference conditions, though the specific manifestation here involves improper type handling rather than null pointer access. The operational impact extends beyond simple service interruption as the vulnerability creates a cascading effect where users cannot successfully authenticate through the affected third-party services, leading to complete denial-of-service for authentication functionality.
From an operational perspective, this vulnerability creates a significant risk for third-party service providers who implement Google Sign-In as part of their authentication infrastructure. The malicious user can trigger this vulnerability by simply initiating an authentication request that results in the receipt of an ID token from Google's authentication server, making the attack surface relatively broad and accessible. Once exploited, the vulnerability results in sustained denial-of-service conditions that can persist until the affected service is manually restarted or the underlying code is patched. The impact extends beyond immediate service interruption to include potential data loss, user frustration, and reputational damage for affected organizations. The vulnerability's persistence in the authentication flow means that even legitimate users attempting to sign in will encounter failures, creating a systematic breakdown in service availability that can affect business operations and user engagement. This scenario aligns with ATT&CK technique T1499.004 which describes network denial of service attacks, though the specific vector here involves application-level exception handling rather than network-level disruption.
The mitigation strategy for this vulnerability requires immediate patching of the google-api-cpp-client library to version 2019-04-10 or later, which contains the necessary fixes for the integer-to-string conversion handling. Organizations should also implement robust exception handling mechanisms within their own applications that integrate Google Sign-In to prevent cascading failures when such exceptions occur. Additional defensive measures include implementing circuit breaker patterns for authentication services, adding proper logging and monitoring around authentication flows, and ensuring that authentication failures do not result in complete service outages. Security teams should also consider implementing automated alerting for unusual authentication patterns that might indicate exploitation attempts, and establish incident response procedures specifically for authentication service disruptions. The vulnerability highlights the importance of proper type safety in authentication libraries and demonstrates how seemingly minor implementation flaws can create significant operational risks in security-critical components of software systems.