CVE-2002-1783 in PHP
Summary
by MITRE
CRLF injection vulnerability in PHP 4.2.1 through 4.2.3, when allow_url_fopen is enabled, allows remote attackers to modify HTTP headers for outgoing requests by causing CRLF sequences to be injected into arguments that are passed to the (1) fopen or (2) file functions.
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Analysis
by VulDB Data Team • 06/28/2021
The CVE-2002-1783 vulnerability represents a critical cross-site scripting and header manipulation flaw affecting PHP versions 4.2.1 through 4.2.3 when the allow_url_fopen configuration directive is enabled. This vulnerability operates through a CRLF injection attack vector that exploits the improper handling of carriage return and line feed characters within HTTP header construction processes. The flaw specifically targets the fopen and file functions in PHP's URL handling mechanisms, creating a pathway for remote attackers to inject malicious CRLF sequences into outgoing HTTP requests.
The technical implementation of this vulnerability stems from insufficient input validation and sanitization within PHP's URL processing functions. When allow_url_fopen is enabled, PHP's internal functions process URL arguments without adequately filtering or escaping special characters that include the carriage return \r and line feed \n sequences. Attackers can craft malicious input strings containing these CRLF characters and pass them as arguments to the fopen or file functions, which then get incorporated into HTTP headers without proper sanitization. This injection allows attackers to manipulate the HTTP request headers, potentially enabling various malicious activities including HTTP response splitting, cache poisoning, and session hijacking attacks.
The operational impact of CVE-2002-1783 extends beyond simple header manipulation, as it provides attackers with significant control over HTTP communications from vulnerable PHP applications. The vulnerability can be exploited to inject malicious headers that redirect traffic, modify authentication tokens, or manipulate cookies, potentially leading to full session compromise. The attack surface is particularly concerning because it leverages the legitimate allow_url_fopen functionality that many web applications require for fetching remote content, making the vulnerability difficult to detect through routine security scanning. This flaw aligns with CWE-113, which specifically addresses improper neutralization of CRLF sequences in HTTP headers, and represents a classic example of how input validation failures can lead to serious security implications.
Mitigation strategies for this vulnerability focus on immediate configuration changes and code-level protections. The most effective immediate solution involves disabling the allow_url_fopen directive in php.ini configuration files, which prevents the vulnerable URL handling functions from operating entirely. Additionally, developers should implement comprehensive input validation and sanitization routines that filter out CRLF sequences from any user-supplied data passed to URL-related functions. The remediation process should include thorough code reviews to identify all instances where fopen or file functions receive external input, ensuring that proper escaping mechanisms are implemented. Organizations should also consider implementing web application firewalls that can detect and block CRLF injection attempts, while following ATT&CK technique T1071.004 for network protocol manipulation to prevent header injection attacks. The vulnerability demonstrates the critical importance of input sanitization and proper HTTP header handling in preventing serious security breaches, with implications that extend to modern web application security practices and the broader principles of secure coding standards.