CVE-2023-5824 in Squid Web Proxy
Summary
by MITRE • 11/03/2023
Squid is vulnerable to Denial of Service attack against HTTP and HTTPS clients due to an Improper Handling of Structural Elements bug.
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Analysis
by VulDB Data Team • 09/13/2025
The vulnerability identified as CVE-2023-5824 affects the Squid proxy server, a widely deployed caching and forwarding HTTP proxy solution used extensively in enterprise environments and web infrastructure. This vulnerability represents a critical security flaw that can be exploited to disrupt service availability for legitimate users. The issue stems from improper handling of structural elements within the HTTP and HTTPS protocol implementations, creating a pathway for malicious actors to craft specific requests that can cause the proxy server to crash or become unresponsive. The vulnerability impacts both HTTP and HTTPS client connections, making it particularly dangerous as it can affect encrypted and unencrypted traffic flows simultaneously. Organizations relying on Squid for content filtering, caching, or web proxy services face significant operational risks when this vulnerability remains unpatched.
The technical root cause of this vulnerability lies in the insufficient validation and processing of structural elements within HTTP protocol messages. When Squid receives specially crafted HTTP requests containing malformed or unexpected structural components, the proxy server fails to properly handle these elements and instead enters an unstable state that leads to service disruption. This improper handling can occur during parsing of HTTP headers, request lines, or other structural components of the protocol. The vulnerability manifests as a denial of service condition where the proxy server either terminates its process or becomes unresponsive to legitimate client requests. According to CWE classification, this corresponds to CWE-704 improper handling of structural elements, which falls under the broader category of protocol implementation flaws that can lead to service disruption. The vulnerability is particularly concerning because it affects core protocol handling functions that are fundamental to the proxy server's operation.
The operational impact of CVE-2023-5824 extends beyond simple service interruption to potentially compromise entire network infrastructures that depend on Squid for web traffic management. In enterprise environments where Squid serves as a primary gateway for internet access, this vulnerability can lead to complete loss of web connectivity for users, resulting in productivity losses and business disruption. The attack vector requires minimal sophistication to exploit, making it particularly dangerous as it can be leveraged by both skilled attackers and automated tools. Network administrators may observe the service becoming unresponsive or crashing entirely, with no clear indication of the underlying cause until forensic analysis reveals the specific structural element manipulation. The vulnerability affects both HTTP and HTTPS protocols, meaning that encrypted traffic is not protected from this particular attack vector, which could be exploited to target sensitive environments where encrypted connections are expected to provide additional security. This issue can be particularly devastating in scenarios where Squid is deployed as a transparent proxy or in environments where it serves as a critical component of network security policies.
Organizations should immediately implement mitigations to address this vulnerability through patch management procedures and configuration hardening. The primary recommendation involves applying the latest security patches released by the Squid project, which contain fixes for the improper handling of structural elements in HTTP protocol parsing. Network administrators should also consider implementing additional protective measures such as rate limiting and connection monitoring to detect anomalous traffic patterns that may indicate exploitation attempts. The ATT&CK framework categorizes this type of vulnerability under T1499.004 - Endpoint Denial of Service, highlighting the importance of protecting critical infrastructure components from service disruption attacks. Organizations should also review their monitoring and alerting systems to ensure that server crashes or unresponsiveness are quickly detected and responded to. Configuration changes such as disabling unnecessary HTTP features and implementing stricter header validation can provide additional defense-in-depth measures. Regular security assessments of proxy server configurations should be conducted to identify and remediate similar vulnerabilities that may exist in other protocol implementations or custom configurations.