CVE-2015-8935 in PHP
Summary
by MITRE
The sapi_header_op function in main/SAPI.c in PHP before 5.4.38, 5.5.x before 5.5.22, and 5.6.x before 5.6.6 supports deprecated line folding without considering browser compatibility, which allows remote attackers to conduct cross-site scripting (XSS) attacks against Internet Explorer by leveraging (1) %0A%20 or (2) %0D%0A%20 mishandling in the header function.
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Analysis
by VulDB Data Team • 09/12/2022
The vulnerability identified as CVE-2015-8935 represents a critical cross-site scripting flaw in PHP's Simple API for PHP (SAPI) handling mechanism. This issue affects multiple PHP version lines including 5.4.x before 5.4.38, 5.5.x before 5.5.22, and 5.6.x before 5.6.6, making it a widespread concern for web application security. The vulnerability stems from the sapi_header_op function in main/SAPI.c which improperly handles deprecated line folding techniques, creating a pathway for malicious actors to exploit Internet Explorer's header parsing behavior. The flaw specifically targets how PHP processes HTTP headers containing line folding sequences that are no longer recommended but still supported for backward compatibility reasons.
The technical exploitation of this vulnerability relies on the improper handling of specific character sequences in HTTP headers that are processed by PHP's SAPI module. Attackers can leverage two primary methods to execute XSS attacks: the first involves using the sequence %0A%20 which represents a line feed followed by a space character, while the second uses %0D%0A%20 which represents carriage return, line feed, and space. These sequences exploit the way Internet Explorer processes headers with line folding, where the browser may interpret these sequences differently than other browsers, allowing attackers to inject malicious content that gets executed in the victim's browser context. The vulnerability specifically targets the header function's processing of these deprecated line folding patterns, creating a scenario where malicious headers can be injected and subsequently executed as JavaScript code within the target browser.
The operational impact of CVE-2015-8935 extends beyond simple XSS attacks, as it provides attackers with a vector to compromise Internet Explorer users specifically through header manipulation techniques that bypass normal security controls. This vulnerability aligns with CWE-79 which describes cross-site scripting flaws in web applications, and demonstrates how legacy code handling can introduce security gaps even in well-established software. The attack surface is particularly concerning for web applications running on PHP versions affected by this vulnerability, as any application that accepts user input through HTTP headers and subsequently processes these headers through PHP's header function could be compromised. The issue is further exacerbated by the fact that Internet Explorer's header parsing behavior differs from other browsers, creating a browser-specific attack vector that makes detection and prevention more complex. This vulnerability directly maps to ATT&CK technique T1203 which covers exploitation of web application vulnerabilities, specifically targeting the manipulation of HTTP headers to execute malicious code in victim browsers.
Organizations affected by this vulnerability should prioritize immediate patching of their PHP installations to versions that address this header processing issue. The recommended mitigation strategy involves upgrading to PHP 5.4.38, 5.5.22, or 5.6.6 respectively, depending on the version currently in use. Additionally, administrators should implement input validation measures that sanitize HTTP header values before processing them through PHP's header function, particularly focusing on removing or encoding potentially malicious line folding sequences. Network-level protections such as web application firewalls can also provide additional defense-in-depth measures by monitoring for suspicious header patterns. The vulnerability highlights the importance of maintaining secure coding practices around legacy compatibility features and demonstrates how seemingly minor implementation details in HTTP header processing can create significant security risks. Security teams should conduct thorough vulnerability assessments of their PHP-based applications to identify any custom code that might be using the affected header processing functions, ensuring comprehensive protection against this specific XSS attack vector targeting Internet Explorer users.