CVE-2021-32778 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 envoy’s procedure for resetting a HTTP/2 stream has O(N^2) complexity, leading to high CPU utilization when a large number of streams are reset. Deployments are susceptible to Denial of Service when Envoy is configured with high limit on H/2 concurrent streams. An attacker wishing to exploit this vulnerability would require a client opening and closing a large number of H/2 streams. Envoy versions 1.19.1, 1.18.4, 1.17.4, 1.16.5 contain fixes to reduce time complexity of resetting HTTP/2 streams. As a workaround users may limit the number of simultaneous HTTP/2 dreams for upstream and downstream peers to a low number, i.e. 100.
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Analysis
by VulDB Data Team • 08/27/2021
The vulnerability described in CVE-2021-32778 represents a critical performance degradation issue within the Envoy proxy software that affects its handling of HTTP/2 stream reset operations. This flaw manifests as an O(N^2) time complexity during stream reset procedures, where N represents the number of streams being reset simultaneously. The issue specifically impacts Envoy's ability to efficiently manage concurrent HTTP/2 connections, creating a scenario where CPU utilization spikes dramatically when multiple streams are reset in rapid succession. This design flaw becomes particularly dangerous in production environments where Envoy is configured with high limits for concurrent HTTP/2 streams, as it creates an exploitable condition for denial of service attacks.
The technical implementation of this vulnerability stems from how Envoy processes stream reset operations in its HTTP/2 handling code. When a large number of streams are reset simultaneously, the proxy's algorithm exhibits quadratic time complexity, meaning that the processing time increases exponentially with the number of streams involved. This computational inefficiency occurs because the reset procedure involves nested loops or recursive operations that iterate through stream collections multiple times, resulting in substantial CPU overhead. The vulnerability affects versions of Envoy prior to 1.16.5, 1.17.4, 1.18.4, and 1.19.1, where the stream reset logic was not optimized to handle large volumes of concurrent stream resets efficiently. The underlying cause aligns with CWE-502, which addresses inefficient algorithms and data structure usage leading to performance degradation.
From an operational impact perspective, this vulnerability creates a significant risk for service-oriented architectures that rely heavily on HTTP/2 communication patterns. The attack vector requires an adversary to establish and rapidly close a large number of HTTP/2 streams, which can be achieved through a single client or multiple malicious connections. Once exploited, the vulnerability leads to sustained high CPU utilization that can exhaust system resources and prevent legitimate traffic from being processed. This creates a denial of service condition where the proxy becomes unresponsive or severely degraded, effectively disrupting service availability for all connected applications and clients. The impact extends beyond simple resource exhaustion to potentially affect the entire service mesh architecture that depends on Envoy for traffic management and proxying.
The mitigation strategies for CVE-2021-32778 involve both immediate workaround measures and permanent fixes through version upgrades. The recommended immediate workaround involves limiting the number of simultaneous HTTP/2 streams for both upstream and downstream connections to a low threshold, typically around 100 streams as suggested in the advisory. This approach effectively reduces the attack surface by limiting the number of streams that can be simultaneously reset, thereby preventing the quadratic complexity from manifesting. Organizations should also consider implementing rate limiting and connection pooling configurations to further constrain the potential impact. The permanent solution requires upgrading to affected versions of Envoy that contain the fixed stream reset algorithm, which implements optimized O(N) complexity for stream reset operations. This aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion, and demonstrates how proper algorithmic implementation can prevent such attacks. Security teams should also implement monitoring to detect unusual patterns of stream creation and destruction that might indicate exploitation attempts, particularly in environments where high concurrent stream limits are configured.