CVE-2018-19131 in Web Proxy
Summary
by MITRE
Squid before 4.4 has XSS via a crafted X.509 certificate during HTTP(S) error page generation for certificate errors.
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Analysis
by VulDB Data Team • 06/05/2023
The vulnerability identified as CVE-2018-19131 represents a cross-site scripting flaw within the Squid proxy server software affecting versions prior to 4.4. This issue arises during the generation of HTTP(S) error pages when certificate errors occur, specifically through the handling of crafted X.509 certificates. The vulnerability demonstrates how proxy servers can become vectors for client-side attacks when processing and displaying certificate-related error information, creating a critical security gap in the SSL/TLS termination process that organizations rely upon for secure web traffic filtering and monitoring.
The technical flaw manifests in the improper sanitization of X.509 certificate data when Squid generates error pages for SSL/TLS certificate validation failures. When a client encounters a certificate error while accessing HTTPS resources through the proxy, Squid constructs an error page that displays certificate details to the user. However, the software fails to properly escape or sanitize the certificate information, allowing maliciously crafted certificate data to contain script payloads that execute in the context of the victim's browser. This occurs because the certificate fields, particularly those containing user-supplied or system-generated data, are directly embedded into HTML error pages without adequate input validation or output encoding, creating a classic XSS vulnerability.
The operational impact of this vulnerability extends beyond simple script injection, as it can enable attackers to perform a range of malicious activities against users of the affected proxy infrastructure. An attacker who can influence certificate data or manipulate the certificate validation process could inject malicious JavaScript into error pages, potentially leading to session hijacking, credential theft, or redirection to malicious sites. This vulnerability is particularly concerning in enterprise environments where Squid proxies are commonly deployed for content filtering, web traffic monitoring, and security enforcement, as it undermines the trust model that organizations place in their proxy infrastructure. The vulnerability affects both HTTP and HTTPS error handling, meaning that users accessing both secure and non-secure resources through the proxy could be exposed to the attack vector, potentially compromising the entire web browsing session.
Organizations should immediately upgrade to Squid version 4.4 or later to address this vulnerability, as no effective workarounds exist for the underlying issue. The mitigation strategy requires comprehensive patch management across all proxy server infrastructure, with particular attention to ensuring that all versions of Squid in use are updated to the patched releases. Security teams should also implement monitoring for suspicious certificate-related activities and consider additional network-level protections such as web application firewalls that can detect and block malicious payloads in real-time. This vulnerability aligns with CWE-79 Cross-site Scripting and follows patterns consistent with ATT&CK technique T1212 Exploitation for Credential Access, where the initial compromise occurs through a web-based attack vector that can be leveraged for broader system compromise. The attack surface is particularly wide given that proxy servers typically handle traffic from multiple users and applications, making this vulnerability potentially exploitable at scale within affected organizations.