CVE-2024-47873 in PhpSpreadsheet
Summary
by MITRE • 11/18/2024
PhpSpreadsheet is a PHP library for reading and writing spreadsheet files. The XmlScanner class has a scan method which should prevent XXE attacks. However, prior to versions 1.9.4, 2.1.3, 2.3.2, and 3.4.0, the regexes used in the `scan` method and the findCharSet method can be bypassed by using UCS-4 and encoding guessing. An attacker can bypass the sanitizer and achieve an XML external entity attack. Versions 1.9.4, 2.1.3, 2.3.2, and 3.4.0 fix the issue.
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Analysis
by VulDB Data Team • 11/20/2024
The vulnerability identified as CVE-2024-47873 affects PhpSpreadsheet, a widely-used PHP library for handling spreadsheet file operations including reading and writing various formats such as xlsx, xls, and ods. This library serves as a critical component in numerous web applications and systems that process spreadsheet data, making it a significant target for security researchers and malicious actors. The vulnerability resides within the XmlScanner class, specifically in its scan method designed to prevent XML External Entity (XXE) attacks, which are well-known exploitation techniques that can lead to data exfiltration, server-side request forgery, and denial of service conditions.
The technical flaw stems from insufficient input validation mechanisms within the XmlScanner class that are meant to sanitize XML content and prevent XXE attacks. Prior to the patched versions 1.9.4, 2.1.3, 2.3.2, and 3.4.0, the regular expressions employed in both the scan method and findCharSet method can be circumvented through the strategic use of UCS-4 encoding and sophisticated encoding guessing techniques. This bypass allows attackers to inject malicious external entity declarations that would normally be filtered out by the regex sanitization process. The vulnerability specifically exploits weaknesses in character set detection and encoding handling, enabling attackers to craft payloads that appear legitimate to the sanitizer while containing malicious XXE constructs.
The operational impact of this vulnerability is substantial for organizations relying on PhpSpreadsheet for processing user-uploaded or externally sourced spreadsheet files. An attacker who successfully exploits this vulnerability could potentially read arbitrary files from the server filesystem, perform server-side request forgery attacks against internal systems, or execute denial of service attacks by consuming excessive server resources through recursive entity expansion. The attack vector typically involves uploading a specially crafted spreadsheet file that contains malicious XML entities, which the vulnerable library fails to properly sanitize. This could lead to unauthorized data access, system compromise, and potential lateral movement within network environments where such libraries are deployed.
The security implications extend beyond immediate exploitation as this vulnerability demonstrates a critical flaw in input validation and sanitization processes. According to CWE-611, this vulnerability relates to improper neutralization of character data, specifically in XML processing contexts. The ATT&CK framework categorizes this under T1190 - Exploit Public-Facing Application, where attackers target vulnerable web applications to gain unauthorized access. Organizations using PhpSpreadsheet should immediately implement mitigation strategies including updating to the patched versions, implementing additional input validation layers, and monitoring for suspicious file upload activities. The vulnerability highlights the importance of robust XML parsing and sanitization in web applications, particularly those handling user-supplied data, and underscores the need for comprehensive security testing of third-party libraries in production environments.
The fix implemented in versions 1.9.4, 2.1.3, 2.3.2, and 3.4.0 addresses the core issue by strengthening the regex patterns and improving the encoding detection mechanisms within the XmlScanner class. These patches ensure that UCS-4 and other encoding bypass techniques are properly detected and neutralized, thereby restoring the intended protection against XXE attacks. Security practitioners should conduct thorough testing of updated libraries in their environments and verify that no other similar vulnerabilities exist in their dependency chain. The vulnerability also serves as a reminder of the critical importance of maintaining up-to-date third-party components and implementing proper security controls around XML processing operations in web applications.