CVE-2015-8877 in PHP
Summary
by MITRE
The gdImageScaleTwoPass function in gd_interpolation.c in the GD Graphics Library (aka libgd) before 2.2.0, as used in PHP before 5.6.12, uses inconsistent allocate and free approaches, which allows remote attackers to cause a denial of service (memory consumption) via a crafted call, as demonstrated by a call to the PHP imagescale function.
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Analysis
by VulDB Data Team • 08/21/2022
The gdImageScaleTwoPass function in the GD Graphics Library presents a critical memory management vulnerability that enables remote attackers to consume excessive system resources through crafted image processing operations. This vulnerability exists within the interpolation.c file of libgd versions prior to 2.2.0, affecting PHP installations before version 5.6.12 where the imagescale function is utilized. The flaw stems from inconsistent allocation and deallocation patterns that create memory consumption issues when processing specially crafted image scale operations. The vulnerability manifests when the function processes image scaling requests that trigger memory allocation followed by improper memory cleanup, leading to gradual memory exhaustion that can result in system instability or complete denial of service conditions. This type of vulnerability falls under the category of memory leak issues and can be classified as a CWE-401: Improper Release of Memory Before Removal from Pool, which is a well-documented weakness in memory management practices. The attack vector is particularly dangerous because it can be exploited remotely through web applications that process user-uploaded images or perform dynamic image scaling operations, making it a significant concern for web hosting environments and content management systems that rely on image processing capabilities.
The technical implementation of this vulnerability occurs when the gdImageScaleTwoPass function executes a two-pass scaling algorithm that requires temporary memory allocation for intermediate image data processing. During the first pass, the function allocates memory for scaled image buffers, but during the second pass, the memory management logic fails to properly account for all allocated memory blocks, resulting in memory fragmentation and eventual exhaustion. The inconsistency between allocation and deallocation approaches creates a scenario where allocated memory segments remain unreleased even after the scaling operation completes, allowing attackers to repeatedly trigger this function with crafted parameters to gradually consume all available memory on the target system. This memory consumption pattern aligns with ATT&CK technique T1499.001: Network Denial of Service, where adversaries leverage application vulnerabilities to exhaust system resources. The vulnerability is particularly concerning because it affects the core image processing libraries that are widely used across web applications, making it a prime target for exploitation in distributed attacks against vulnerable web servers.
The operational impact of this vulnerability extends beyond simple resource exhaustion to potentially compromise entire web server operations and application availability. When exploited successfully, attackers can cause sustained memory consumption that leads to application crashes, system slowdowns, or complete service unavailability, affecting multiple users and potentially causing cascading failures in dependent systems. The vulnerability affects PHP applications that utilize the imagescale function for dynamic image resizing, which is commonly implemented in e-commerce platforms, social media applications, and content management systems where user-generated content processing is prevalent. Organizations running vulnerable versions of libgd or PHP are at risk of experiencing denial of service conditions that can be difficult to detect and mitigate, as the memory consumption occurs gradually and may not immediately trigger alerts. The remediation process requires updating to patched versions of both libgd and PHP, which can be complex in environments with multiple applications and dependencies. Additionally, administrators should implement proper input validation and resource monitoring to detect and prevent exploitation attempts, as the vulnerability can be triggered through various image processing operations that are commonly performed in web applications, making it a persistent threat to system stability and availability.