CVE-2019-12519 in Web Proxy
Summary
by MITRE
An issue was discovered in Squid through 4.7. When handling the tag esi:when when ESI is enabled, Squid calls ESIExpression::Evaluate. This function uses a fixed stack buffer to hold the expression while it's being evaluated. When processing the expression, it could either evaluate the top of the stack, or add a new member to the stack. When adding a new member, there is no check to ensure that the stack won't overflow.
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Analysis
by VulDB Data Team • 05/26/2024
The vulnerability identified as CVE-2019-12519 represents a critical buffer overflow condition within the Squid proxy server software version 4.7 and earlier. This issue manifests specifically when the ESI (Edge Side Includes) functionality is enabled and the system processes esi:when directives. The fundamental flaw lies in the ESIExpression::Evaluate function implementation which employs a fixed-size stack buffer to manage expression evaluation operations. This design choice creates a scenario where the stack buffer can be exceeded during normal processing operations, leading to potential memory corruption and system instability.
The technical execution of this vulnerability occurs through the manipulation of ESI expressions containing esi:when tags, which trigger the problematic evaluation routine. During expression processing, the system must either evaluate existing stack contents or append new elements to the stack structure. The absence of bounds checking when adding new members to the stack creates an exploitable condition where an attacker can craft malicious ESI expressions that deliberately overflow the fixed buffer. This buffer overflow vulnerability directly maps to CWE-129, which describes improper validation of array index values, and specifically aligns with CWE-787, which addresses out-of-bounds write conditions. The vulnerability demonstrates a classic stack buffer overflow pattern where insufficient bounds checking allows for memory corruption that could potentially lead to arbitrary code execution.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially enable remote code execution attacks against systems running vulnerable Squid versions. When exploited, the buffer overflow could allow attackers to overwrite adjacent memory locations, potentially corrupting program execution flow or injecting malicious code into the target system. This vulnerability affects organizations that utilize Squid as their primary web proxy server and have ESI functionality enabled, which is common in content delivery networks and web application environments where dynamic content inclusion is required. The attack surface is particularly concerning given that Squid is widely deployed in enterprise and cloud environments, making the potential impact substantial across various network infrastructures.
Mitigation strategies for CVE-2019-12519 should prioritize immediate patching of affected Squid installations to version 4.8 or later, which contains the necessary fixes for the stack buffer overflow condition. Organizations should also consider disabling ESI functionality entirely if it is not required for their operations, as this eliminates the attack vector entirely. Network segmentation and monitoring should be implemented to detect anomalous ESI expression processing patterns that might indicate attempted exploitation. Security teams should also review their proxy server configurations to ensure that only necessary ESI features are enabled and that proper input validation is enforced on all user-supplied content. The ATT&CK framework categorizes this vulnerability under T1203, which covers Exploitation for Client Execution, and T1059, which addresses Command and Scripting Interpreter, highlighting the potential for both remote code execution and command injection attacks through this buffer overflow condition.