CVE-2015-4024 in PHP
Summary
by MITRE
Algorithmic complexity vulnerability in the multipart_buffer_headers function in main/rfc1867.c in PHP before 5.4.41, 5.5.x before 5.5.25, and 5.6.x before 5.6.9 allows remote attackers to cause a denial of service (CPU consumption) via crafted form data that triggers an improper order-of-growth outcome.
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Analysis
by VulDB Data Team • 05/19/2022
The vulnerability identified as CVE-2015-4024 represents a critical algorithmic complexity issue within PHP's handling of multipart form data processing. This flaw exists in the multipart_buffer_headers function located in the main/rfc1867.c file of PHP's source code, affecting multiple versions including PHP 5.4.x before 5.4.41, 5.5.x before 5.5.25, and 5.6.x before 5.6.9. The vulnerability stems from an improper order-of-growth outcome when processing crafted form data, creating a significant security risk that can be exploited remotely by attackers to consume excessive CPU resources.
The technical implementation of this vulnerability involves the multipart_buffer_headers function's inefficient handling of malformed or specially crafted multipart form data. When PHP processes form data containing maliciously constructed boundaries or headers, the function exhibits quadratic time complexity behavior instead of the expected linear performance characteristics. This algorithmic degradation occurs because the function fails to properly validate the structure of multipart boundaries during header parsing, leading to nested loops that scale exponentially with input size. The flaw specifically manifests when the parser encounters form data with improperly ordered or overlapping boundary markers, causing the processor to repeatedly re-evaluate the same data segments.
From an operational perspective, this vulnerability creates a severe denial of service condition that can be exploited by remote attackers without authentication. The malicious actor can craft specific form data that, when processed by the vulnerable PHP application, causes the web server to consume excessive CPU cycles, potentially leading to complete service unavailability. The impact extends beyond simple resource exhaustion as the vulnerability can be leveraged in distributed denial of service attacks where multiple requests can be simultaneously processed to overwhelm server resources. Attackers can exploit this vulnerability through various attack vectors including file uploads, form submissions, and any web application that processes multipart form data, making it particularly dangerous in high-traffic environments where resource exhaustion can quickly cascade into system-wide outages.
The vulnerability maps directly to CWE-770, which addresses the allocation of resources without proper limits or refreshes, and aligns with ATT&CK technique T1499.004 for Network Denial of Service. Organizations affected by this vulnerability should implement immediate mitigations including upgrading to patched PHP versions, implementing input validation controls, and deploying rate limiting measures to prevent abuse. The recommended remediation involves updating to PHP 5.4.41, 5.5.25, or 5.6.9 respectively, which contain the necessary fixes to address the algorithmic complexity issue. Additionally, security teams should consider implementing web application firewalls, monitoring for unusual CPU usage patterns, and establishing automated alerting mechanisms to detect potential exploitation attempts. The vulnerability demonstrates the critical importance of algorithmic complexity analysis in security reviews, particularly for functions that process untrusted input data, as it highlights how seemingly benign parsing operations can become significant security threats when not properly constrained.