CVE-2023-35944 in Envoy
Summary
by MITRE • 07/25/2023
Envoy is an open source edge and service proxy designed for cloud-native applications. Envoy allows mixed-case schemes in HTTP/2, however, some internal scheme checks are case-sensitive. Prior to versions 1.27.0, 1.26.4, 1.25.9, 1.24.10, and 1.23.12, this can lead to the rejection of requests with mixed-case schemes such as `htTp` or `htTps`, or the bypassing of some requests such as `https` in unencrypted connections. With a fix in versions 1.27.0, 1.26.4, 1.25.9, 1.24.10, and 1.23.12, Envoy will now lowercase scheme values by default, and change the internal scheme checks that were case-sensitive to be case-insensitive. There are no known workarounds for this issue.
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Analysis
by VulDB Data Team • 08/18/2023
The vulnerability identified as CVE-2023-35944 affects Envoy proxy, a widely-used open source edge and service proxy designed for cloud-native applications. This security flaw stems from inconsistent handling of HTTP/2 scheme validation where the proxy accepts mixed-case schemes in certain contexts while maintaining case-sensitive internal checks. The inconsistency creates a potential attack vector that could lead to either request rejection or unauthorized bypass of security controls. The vulnerability specifically impacts versions prior to 1.27.0, 1.26.4, 1.25.9, 1.24.10, and 1.23.12, where requests with mixed-case schemes like `htTp` or `htTps` could be improperly handled, potentially allowing malicious actors to exploit the scheme validation logic.
The technical flaw manifests in Envoy's internal scheme validation mechanism where the proxy's handling of scheme values demonstrates inconsistent behavior between external acceptance and internal processing. When mixed-case schemes are received, the system's internal checks remain case-sensitive while the external acceptance logic allows for mixed-case variations. This creates a scenario where legitimate requests with mixed-case schemes may be rejected, while potentially malicious requests could bypass security controls when the scheme is `https` but sent over unencrypted connections. The vulnerability essentially creates a bypass condition where the proxy's behavior becomes unpredictable based on scheme case variations, leading to potential security policy circumvention. This inconsistency directly relates to CWE-297, which addresses improper validation of certificate subject alternative names, as both issues involve inconsistent handling of case-sensitive validation logic.
The operational impact of CVE-2023-35944 extends beyond simple request processing failures to potentially compromise security controls within cloud-native environments. Organizations relying on Envoy for traffic management, security policy enforcement, and service mesh operations face risks of unauthorized access or policy bypass when the proxy fails to properly validate scheme values. The vulnerability could enable attackers to craft requests that bypass encryption requirements or authentication mechanisms, particularly when the scheme validation logic fails to properly enforce security policies. In service mesh deployments where Envoy acts as a critical component for traffic routing and security enforcement, this inconsistency could undermine the entire security posture. The vulnerability also impacts operational reliability as mixed-case scheme handling becomes unpredictable, potentially causing service disruptions or inconsistent behavior in traffic management.
The fix implemented in versions 1.27.0, 1.26.4, 1.25.9, 1.24.10, and 1.23.12 addresses the core issue by standardizing scheme value handling through default lowercase conversion and making internal scheme checks case-insensitive. This remediation aligns with security best practices for protocol implementation and ensures consistent behavior across all scheme validation scenarios. The solution follows ATT&CK technique T1071.004 which involves application layer protocol manipulation, ensuring that protocol handling becomes consistent and predictable. Organizations should prioritize upgrading to these fixed versions as there are no known workarounds for this issue. The remediation addresses the fundamental inconsistency in scheme handling that could have enabled various attack vectors including protocol manipulation, bypass of security controls, and potential elevation of privileges through malformed request handling. Security teams should monitor their Envoy deployments to ensure all instances are updated to prevent exploitation of this vulnerability that could compromise cloud-native application security.