CVE-2020-11633 in Client Connector
Summary
by MITRE • 07/16/2021
The Zscaler Client Connector for Windows prior to 2.1.2.74 had a stack based buffer overflow when connecting to misconfigured TLS servers. An adversary would potentially have been able to execute arbitrary code with system privileges.
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Analysis
by VulDB Data Team • 07/19/2021
The vulnerability identified as CVE-2020-11633 represents a critical stack-based buffer overflow flaw within the Zscaler Client Connector for Windows platform. This security weakness affected versions prior to 2.1.2.74 and specifically manifested during connections to improperly configured tls servers. The vulnerability arises from insufficient input validation and memory management practices within the client connector's network communication handling mechanisms. The flaw exists in the client-side software that establishes secure connections to zscaler's cloud services, creating a potential attack surface when the software encounters malformed tls server responses or unexpected connection parameters. The buffer overflow condition occurs when the application attempts to write data beyond the allocated memory boundaries in the stack, potentially allowing an attacker to overwrite critical program execution data.
The technical exploitation of this vulnerability follows a classic stack buffer overflow attack pattern that aligns with common weakness enumerations such as cwe-121. When the zscaler client connector encounters a maliciously crafted tls server response, the application fails to properly validate the size of incoming data before copying it into fixed-size stack buffers. This validation failure allows an attacker to craft specific tls server responses that contain oversized data payloads, which when processed by the vulnerable client connector, overwrite adjacent stack memory locations. The overflow can potentially corrupt return addresses, function pointers, or other critical execution data structures, providing an attacker with the opportunity to redirect program execution flow. The attack requires the adversary to establish a connection to a misconfigured tls server that can be controlled or manipulated by the attacker, as the vulnerability specifically occurs during the tls handshake and connection establishment phases.
The operational impact of this vulnerability extends beyond simple code execution privileges, as the zscaler client connector operates with elevated system privileges to manage network traffic filtering and security policies. When successfully exploited, the buffer overflow could enable an attacker to execute arbitrary code with the same privileges as the zscaler client process, which typically runs with administrative rights on the target system. This privilege escalation capability allows the attacker to potentially gain complete system control, install persistent backdoors, modify system configurations, or exfiltrate sensitive data through the established secure network channel. The vulnerability affects organizations relying on zscaler's client connector for network security management, creating a significant risk for enterprises where the client connector is deployed across multiple endpoints. The attack vector leverages the trust relationship between the client software and the zscaler cloud services, making it particularly dangerous as users typically expect secure communication channels to be safe from such exploitation attempts.
Mitigation strategies for CVE-2020-11633 primarily focus on immediate software updates and configuration hardening measures. Organizations should prioritize updating the zscaler client connector to version 2.1.2.74 or later, which includes patches addressing the buffer overflow condition through proper input validation and memory boundary checks. System administrators should also implement network segmentation and monitoring to detect unusual tls connection patterns that might indicate exploitation attempts. The implementation of additional security controls such as network access control lists, tls certificate pinning, and regular security assessments of tls configurations can help reduce the attack surface. From a defensive perspective, this vulnerability aligns with several attack techniques documented in the attack pattern taxonomy, particularly those related to privilege escalation and code execution through client-side applications. Security teams should also consider implementing behavioral monitoring for the zscaler client process to detect anomalous memory access patterns or unexpected code execution that might indicate exploitation attempts. The vulnerability serves as a reminder of the critical importance of proper input validation in network security applications and the potential for client-side software to become attack vectors when proper memory management practices are not implemented.