CVE-2018-19277 in PhpSpreadsheet
Summary
by MITRE
securityScan() in PHPOffice PhpSpreadsheet through 1.5.0 allows a bypass of protection mechanisms for XXE via UTF-7 encoding in a .xlsx file
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Analysis
by VulDB Data Team • 09/04/2024
The vulnerability identified as CVE-2018-19277 resides within PHPOffice PhpSpreadsheet version 1.5.0 and earlier, presenting a critical security risk that undermines XML external entity protection mechanisms. This flaw specifically affects the securityScan() function which is designed to prevent malicious XML entities from being processed during spreadsheet file parsing. The vulnerability enables attackers to bypass these security controls through the exploitation of UTF-7 encoding techniques, allowing for unauthorized access to sensitive data and potential system compromise.
The technical implementation of this vulnerability stems from insufficient input validation within the spreadsheet parsing engine. When PhpSpreadsheet processes .xlsx files, it internally handles XML content that may contain external entity references. The securityScan() function should normally intercept and neutralize such references to prevent XML external entity attacks, but the UTF-7 encoding bypass allows malicious payloads to be silently processed without triggering the intended protection mechanisms. This encoding technique exploits the way certain XML parsers handle UTF-7 character sets, where the encoding can mask malicious content that would otherwise be detected by standard security filters.
The operational impact of this vulnerability extends beyond simple data exposure, as it can potentially enable remote code execution and privilege escalation within systems that process untrusted spreadsheet files. Attackers can craft specially formatted .xlsx files that contain UTF-7 encoded malicious XML entities, which then get processed by vulnerable applications. This creates a significant risk for organizations that frequently handle spreadsheet files from external sources, including financial institutions, government agencies, and enterprises that process large volumes of user-generated content. The vulnerability is particularly dangerous because it can be exploited silently without generating obvious error messages or alerts, making detection and incident response more challenging.
Organizations should immediately upgrade to PhpSpreadsheet version 1.5.1 or later, which includes proper UTF-7 encoding detection and mitigation. Additional defensive measures include implementing strict file validation policies that scan for suspicious encoding patterns, deploying network-based intrusion detection systems to monitor for known attack signatures, and establishing robust file upload sanitization processes. The vulnerability maps to CWE-611 (Improper Restriction of XML External Entity Reference) and aligns with ATT&CK technique T1203 (Exploitation for Client Execution) and T1059 (Command and Scripting Interpreter) within the MITRE ATT&CK framework. Security teams should also consider implementing application whitelisting policies for spreadsheet processing applications and conducting regular security assessments to identify similar encoding bypass vulnerabilities in other XML processing libraries.