CVE-2007-1454 in PHP
Summary
by MITRE
ext/filter in PHP 5.2.0, when FILTER_SANITIZE_STRING is used with the FILTER_FLAG_STRIP_LOW flag, does not properly strip HTML tags, which allows remote attackers to conduct cross-site scripting (XSS) attacks via HTML with a < character followed by certain whitespace characters, which passes one filter but is collapsed into a valid tag, as demonstrated using %0b.
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Analysis
by VulDB Data Team • 07/18/2019
The vulnerability described in CVE-2007-1454 represents a critical flaw in PHP's input sanitization mechanism that undermines the security of web applications relying on the ext/filter extension. This issue specifically affects PHP version 5.2.0 and demonstrates how seemingly innocuous filtering operations can create unexpected attack vectors through improper handling of whitespace characters and HTML tag parsing. The vulnerability stems from the interaction between the FILTER_SANITIZE_STRING filter and the FILTER_FLAG_STRIP_LOW flag, creating a scenario where certain malicious inputs bypass intended security measures.
The technical flaw occurs when PHP processes user input through the filter_var function with the specified sanitization flags. The FILTER_FLAG_STRIP_LOW flag is designed to remove low ASCII characters from input, but it fails to properly handle HTML tag structures that contain non-printable characters such as the vertical tab character %0b. When a malicious actor crafts input containing a < character followed by whitespace characters including %0b, the filter strips the low characters but leaves the HTML structure intact. This allows the malicious payload to pass through the first filter layer while still maintaining its executable nature in the browser context, as the browser interprets the collapsed HTML structure as a valid tag.
The operational impact of this vulnerability is significant for web applications that rely on PHP's built-in filtering capabilities for input validation and sanitization. Attackers can exploit this weakness to inject malicious scripts that bypass the intended security controls, potentially leading to session hijacking, credential theft, or arbitrary code execution within the victim's browser context. The vulnerability particularly affects applications that use the FILTER_SANITIZE_STRING filter as part of their input validation pipeline, especially those that do not implement additional security measures or manual validation checks. This creates a dangerous scenario where developers believe their applications are protected against XSS attacks through automated filtering mechanisms, while simultaneously remaining vulnerable to sophisticated attacks that leverage the specific interaction between whitespace handling and HTML tag parsing.
This vulnerability aligns with CWE-79: Improper Neutralization of Input During Web Page Generation, which addresses the failure to properly sanitize user input that could lead to cross-site scripting attacks. The flaw also maps to ATT&CK technique T1203: Exploitation for Client Execution, as it enables attackers to execute malicious code within the victim's browser environment through crafted input that bypasses security controls. Additionally, the issue demonstrates characteristics of T1566: Phishing with Social Engineering, as the vulnerability can be exploited to deliver malicious payloads that trick users into executing harmful scripts. The root cause of this vulnerability highlights the complexity of input sanitization and the importance of understanding how different filtering mechanisms interact with each other, particularly in the context of HTML parsing and character encoding. Organizations should implement comprehensive input validation strategies that go beyond simple filtering, including proper HTML escaping, Content Security Policy implementation, and regular security testing to identify similar edge cases that could compromise application security.
The remediation approach for this vulnerability requires immediate patching of affected PHP installations to versions that properly handle the interaction between HTML tag parsing and whitespace character stripping. Developers should also review their input validation logic to ensure that multiple filtering steps are properly coordinated and that no single filter can be bypassed through clever input construction. Additionally, implementing proper HTML escaping mechanisms, validating input against known good patterns, and employing Content Security Policy headers can provide additional layers of defense against similar vulnerabilities. Organizations should conduct thorough security assessments of their applications to identify other potential edge cases in input sanitization and ensure that their security measures are robust against sophisticated attack techniques that exploit subtle interactions between different security controls.