CVE-2020-8517 in Web Proxy
Summary
by MITRE
An issue was discovered in Squid before 4.10. Due to incorrect input validation, the NTLM authentication credentials parser in ext_lm_group_acl may write to memory outside the credentials buffer. On systems with memory access protections, this can result in the helper process being terminated unexpectedly. This leads to the Squid process also terminating and a denial of service for all clients using the proxy.
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Analysis
by VulDB Data Team • 03/28/2024
The vulnerability identified as CVE-2020-8517 represents a critical memory corruption flaw within the Squid proxy server software version 4.10 and earlier. This issue stems from inadequate input validation mechanisms within the NTLM authentication credentials parser, specifically affecting the ext_lm_group_acl helper process. The flaw manifests when processing authentication credentials, where the parser fails to properly bounds-check memory operations, leading to potential buffer overflow conditions. This vulnerability resides in the authentication handling subsystem of Squid, which is fundamental to proxy server operations and security management.
The technical implementation of this vulnerability demonstrates a classic buffer overflow scenario classified under CWE-121 as an insufficient bounds check during buffer copy operation. The flaw occurs within the NTLM authentication module where the ext_lm_group_acl helper process attempts to parse incoming authentication credentials without adequate validation of input length or format. When malformed or excessively long credential data is processed, the parser writes beyond the allocated buffer boundaries, potentially corrupting adjacent memory regions. This memory corruption directly impacts the helper process's execution environment, causing unexpected termination due to memory access violations that are typically enforced by modern operating system protections.
The operational impact of CVE-2020-8517 extends beyond simple process termination to encompass complete denial of service conditions for all proxy clients. When the helper process crashes, it affects the entire authentication infrastructure of the Squid proxy, rendering the service unavailable to legitimate users. This vulnerability can be exploited through various attack vectors including malformed NTLM authentication requests, potentially allowing malicious actors to repeatedly trigger the memory corruption condition and maintain persistent denial of service. The attack surface is particularly concerning as NTLM authentication is commonly used in enterprise environments where Squid proxies serve as critical network infrastructure components.
From a cybersecurity perspective, this vulnerability aligns with ATT&CK technique T1499.004 for network denial of service and represents a privilege escalation vector that can be leveraged for broader system compromise. The memory corruption aspect places this vulnerability in the category of software exploitation techniques that can be combined with other attack methods to achieve more severe outcomes. Organizations relying on Squid proxy services for authentication and access control face significant risk of service disruption, particularly in environments where proxy functionality is critical to business operations. The vulnerability's exploitation requires minimal privileges and can be executed remotely, making it particularly dangerous in networked environments.
Mitigation strategies for CVE-2020-8517 should prioritize immediate patching of affected Squid installations to version 4.10 or later where the vulnerability has been addressed through proper input validation and bounds checking. System administrators should implement network segmentation and access controls to limit exposure of vulnerable proxy servers to untrusted networks. Additional protective measures include monitoring for abnormal authentication request patterns and implementing intrusion detection systems to identify potential exploitation attempts. The fix implemented in version 4.10 incorporates enhanced input validation routines that properly check credential buffer boundaries before memory operations, preventing the unauthorized memory writes that previously led to process termination. Organizations should also consider disabling NTLM authentication where possible and implementing alternative authentication mechanisms that do not utilize the vulnerable helper process.