CVE-2007-1900 in PHP
Summary
by MITRE
CRLF injection vulnerability in the FILTER_VALIDATE_EMAIL filter in ext/filter in PHP 5.2.0 and 5.2.1 allows context-dependent attackers to inject arbitrary e-mail headers via an e-mail address with a \n character, which causes a regular expression to ignore the subsequent part of the address string.
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Analysis
by VulDB Data Team • 06/12/2025
The CVE-2007-1900 vulnerability represents a critical cross-site scripting and header injection flaw within PHP's email validation mechanism. This vulnerability specifically affects PHP versions 5.2.0 and 5.2.1 where the FILTER_VALIDATE_EMAIL filter fails to properly sanitize email addresses containing carriage return line feed sequences. The flaw stems from improper handling of special characters within the validation process, creating a pathway for attackers to manipulate email headers through carefully crafted input strings.
The technical implementation of this vulnerability occurs when the FILTER_VALIDATE_EMAIL function processes email addresses containing CRLF sequences. When an attacker submits an email address containing these sequences, the regular expression validation logic becomes compromised and fails to properly validate the entire email address. This allows malicious input to bypass validation checks, enabling the injection of arbitrary email headers into the system. The vulnerability operates at the input sanitization layer, where the filter should ensure email format compliance but instead creates a condition where additional header fields can be appended to the validated address.
From an operational impact perspective, this vulnerability enables attackers to perform various malicious activities including email header injection, spam relay exploitation, and potentially more severe attacks depending on the application's email handling mechanisms. The context-dependent nature of this vulnerability means that its exploitation is highly dependent on how the validated email addresses are subsequently processed within the application. Attackers can leverage this weakness to inject additional header fields such as From, To, Subject, or even custom headers, which could be used for phishing attacks, spam distribution, or to manipulate email routing behavior.
The vulnerability aligns with CWE-113, which specifically addresses "Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')", and also relates to broader email security concerns within web applications. From an ATT&CK framework perspective, this vulnerability maps to T1190 - Exploit Public-Facing Application and T1078 - Valid Accounts, as it allows attackers to manipulate email systems and potentially escalate privileges through email-based attacks. The weakness demonstrates a classic input validation failure where the system does not properly sanitize user-supplied data before processing it in sensitive contexts.
Mitigation strategies for CVE-2007-1900 primarily involve upgrading to PHP versions 5.2.2 or later where this vulnerability has been addressed through improved input sanitization. Organizations should implement additional input validation layers beyond the built-in filters, particularly when email addresses are used in header construction or when the validated addresses are subsequently processed in email sending functions. Proper header sanitization techniques should be employed, including encoding special characters and implementing comprehensive input validation that explicitly checks for and removes CRLF sequences from email addresses before any header construction occurs. Additionally, applications should avoid using user-supplied email addresses directly in email header fields without additional sanitization and validation steps.