CVE-2025-4600 in Cloud Classic Application Load Balancer
Summary
by MITRE • 05/16/2025
A request smuggling vulnerability existed in the Google Cloud Classic Application Load Balancer due to improper handling of chunked-encoded HTTP requests. This allowed attackers to craft requests that could be misinterpreted by backend servers. The issue was fixed by disallowing stray data after a chunk, and is no longer exploitable. No action is required as Classic Application Load Balancer service after 2025-04-26 is not vulnerable.
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Analysis
by VulDB Data Team • 07/29/2025
The vulnerability identified as CVE-2025-4600 represents a critical request smuggling flaw within Google Cloud Classic Application Load Balancer infrastructure. This security weakness stems from inadequate processing of chunked-encoded HTTP requests, creating a scenario where malicious actors could manipulate request boundaries and potentially bypass security controls. The flaw specifically manifests when the load balancer fails to properly validate or reject malformed chunked encoding data that follows legitimate chunked request content, allowing for request smuggling attacks that could confuse backend servers about request boundaries and content interpretation.
The technical implementation of this vulnerability involves the improper handling of HTTP chunked transfer encoding, a standard method for transferring data in HTTP requests where data is sent in chunks rather than as a single unit. When the Classic Application Load Balancer processes chunked requests, it fails to strictly validate that no additional data exists after the final chunk, enabling attackers to inject malicious content that appears to be part of the chunked request but actually gets interpreted by backend servers as separate requests. This misinterpretation creates opportunities for request smuggling where an attacker can effectively send two different requests through a single HTTP connection, potentially allowing access to restricted resources or bypassing authentication mechanisms.
The operational impact of this vulnerability extends beyond simple data manipulation, as it fundamentally undermines the security assumptions of HTTP request processing within the load balancing layer. Attackers could exploit this weakness to perform various malicious activities including but not limited to bypassing security controls, accessing restricted resources, performing unauthorized operations against backend services, and potentially escalating privileges within the application stack. The vulnerability's exploitation capability is particularly concerning because it operates at the HTTP protocol level, making it difficult to detect through traditional network monitoring and potentially allowing for stealthy attacks that could go unnoticed for extended periods.
The remediation implemented by Google Cloud addresses the core issue by enforcing stricter validation of chunked encoding data, specifically disallowing any stray data that appears after the final chunk of a request. This solution aligns with established security practices for HTTP request handling and follows the principle of least privilege by ensuring that only properly formatted chunked requests are processed. The fix represents a defensive programming approach that prevents the accumulation of malformed data that could be misinterpreted by downstream systems, thereby eliminating the attack surface that previously existed. The implementation also demonstrates adherence to the CWE-444 principle of ensuring proper HTTP request parsing and validation, which is critical for maintaining the integrity of web application security controls.
Organizations utilizing Google Cloud Classic Application Load Balancer should be aware that the vulnerability was resolved through a service-side update that became effective as of April 26, 2025, making the service no longer exploitable for this specific weakness. This remediation approach follows industry best practices for handling HTTP protocol vulnerabilities, where the solution focuses on preventing the conditions that enable exploitation rather than attempting to patch individual attack vectors. The fix also aligns with ATT&CK framework techniques related to protocol manipulation and request smuggling, where the mitigation strategy involves hardening the protocol handling layer to prevent the conditions that enable such attacks. The resolution ensures that all subsequent requests processed by the Classic Application Load Balancer service will properly validate chunked encoding and reject any malformed data that could potentially be exploited by attackers.