CVE-2021-32781 in Envoy
Summary
by MITRE • 08/25/2021
Envoy is an open source L7 proxy and communication bus designed for large modern service oriented architectures. In affected versions after Envoy sends a locally generated response it must stop further processing of request or response data. However when local response is generated due the internal buffer overflow while request or response is processed by the filter chain the operation may not be stopped completely and result in accessing a freed memory block. A specifically constructed request delivered by an untrusted downstream or upstream peer in the presence of extensions that modify and increase the size of request or response bodies resulting in a Denial of Service when using extensions that modify and increase the size of request or response bodies, such as decompressor filter. Envoy versions 1.19.1, 1.18.4, 1.17.4, 1.16.5 contain fixes to address incomplete termination of request processing after locally generated response. As a workaround disable Envoy's decompressor, json-transcoder or grpc-web extensions or proprietary extensions that modify and increase the size of request or response bodies, if feasible.
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Analysis
by VulDB Data Team • 08/26/2021
CVE-2021-32781 represents a critical memory safety vulnerability in the Envoy proxy that falls under the CWE-416 category of Use After Free conditions. This vulnerability specifically affects Envoy's handling of locally generated responses within its filter chain processing mechanism, where the proxy fails to properly terminate request or response processing after generating a local response. The flaw manifests when internal buffer overflows occur during the processing of requests or responses that are modified by extensions, particularly those that increase the size of request or response bodies. The vulnerability is particularly dangerous because it can lead to accessing freed memory blocks, creating potential for arbitrary code execution or system instability.
The technical exploitation of this vulnerability requires a specifically crafted request delivered by an untrusted peer in environments where extensions modifying request or response bodies are active. The most commonly affected extensions include decompressor filters, json-transcoder filters, and grpc-web extensions that inherently modify data sizes during processing. When these extensions encounter buffer overflow conditions while processing requests, the normal termination sequence fails to complete properly, leaving memory pointers in an invalid state. This creates a scenario where subsequent operations on the freed memory can cause unpredictable behavior, including crashes or potential code execution. The vulnerability is particularly severe in service-oriented architectures where Envoy acts as a communication bus handling numerous requests and responses.
The operational impact of CVE-2021-32781 extends beyond simple denial of service to potentially compromise entire service mesh infrastructures. Organizations using Envoy in production environments face significant risks when extensions that modify data sizes are enabled, as these provide the attack vector for exploitation. The vulnerability affects multiple versions of Envoy including 1.19.1, 1.18.4, 1.17.4, and 1.16.5, indicating it has been present across several major releases and affects organizations with varying deployment scenarios. The attack surface is broad given that decompressor and similar extensions are commonly used in modern microservice architectures for handling compressed content and protocol translation. Security teams must consider the potential for cascading failures when this vulnerability is exploited, as it can affect the stability of entire communication pathways within service mesh deployments.
Mitigation strategies for CVE-2021-32781 focus on both immediate workarounds and long-term remediation. The recommended immediate action involves disabling affected extensions such as decompressor, json-transcoder, and grpc-web filters when feasible, which directly eliminates the attack vector by preventing the buffer overflow conditions that trigger the vulnerability. Organizations should also consider upgrading to patched versions of Envoy where available, as these releases contain proper termination logic for request processing after local responses are generated. The ATT&CK framework categorizes this vulnerability under T1499.004 for Network Denial of Service, and potentially T1059.007 for Command and Scripting Interpreter if exploitation leads to code execution. Additionally, implementing proper input validation and buffer management practices within the filter chain processing would help prevent similar issues in future deployments. Organizations should also conduct comprehensive security assessments of their Envoy deployments to identify all instances where the vulnerable extensions are active and ensure proper patching or mitigation strategies are implemented across their service mesh infrastructure.