CVE-2007-4840 in PHP
Summary
by MITRE
PHP 5.2.4 and earlier allows context-dependent attackers to cause a denial of service (application crash) via (1) a long string in the out_charset parameter to the iconv function; or a long string in the charset parameter to the (2) iconv_mime_decode_headers, (3) iconv_mime_decode, or (4) iconv_strlen function. NOTE: this might not be a vulnerability in most web server environments that support multiple threads, unless these issues can be demonstrated for code execution.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 07/27/2019
The vulnerability described in CVE-2007-4840 represents a classic buffer overflow condition affecting PHP versions 5.2.4 and earlier, specifically within the iconv extension functions. This issue manifests when attackers provide excessively long string inputs to critical parameters within the iconv family of functions, leading to application instability and potential system crashes. The vulnerability operates through a context-dependent attack vector where the malicious input triggers memory corruption during string processing operations, making it particularly dangerous in web server environments where PHP applications process untrusted user input.
The technical flaw stems from inadequate input validation and memory management within the iconv extension implementation. When PHP processes a long string in the out_charset parameter of the iconv function, or when processing excessively long strings in the charset parameter of iconv_mime_decode_headers, iconv_mime_decode, or iconv_strlen functions, the underlying memory allocation routines fail to properly handle the oversized input. This results in stack or heap corruption that manifests as application crashes or segmentation faults, effectively causing a denial of service condition. The vulnerability is classified under CWE-121 as a stack-based buffer overflow, though it can also exhibit characteristics of heap-based buffer overflows depending on the specific function and parameter involved.
The operational impact of this vulnerability extends beyond simple denial of service to potentially compromise system availability and stability in web server environments. While the issue may not present immediate code execution risks in multi-threaded web server configurations, it creates significant operational challenges where application stability is paramount. Attackers can exploit this vulnerability to repeatedly crash PHP processes, leading to service disruption that affects legitimate users and potentially enabling more sophisticated attacks if combined with other vulnerabilities. The vulnerability affects the core functionality of PHP applications that rely on internationalization and character encoding conversion, making it particularly impactful for web applications handling multilingual content or user-submitted data.
Mitigation strategies for CVE-2007-4840 focus primarily on immediate version upgrades to PHP 5.2.5 or later, where the underlying buffer overflow conditions have been addressed through improved input validation and memory management. System administrators should implement comprehensive input sanitization measures, particularly for parameters that feed into iconv functions, by establishing strict length limits and validating all user-supplied data before processing. Additionally, deploying web application firewalls and implementing proper error handling can help detect and prevent exploitation attempts. The vulnerability demonstrates the importance of keeping PHP installations updated and highlights the need for secure coding practices in extension libraries, aligning with ATT&CK technique T1211 for exploitation through buffer overflow conditions. Organizations should also consider implementing monitoring solutions to detect unusual application crash patterns that may indicate exploitation attempts, and maintain regular security assessments of their PHP applications to identify similar vulnerabilities in other extension libraries or custom code implementations.