CVE-2019-12331 in PhpSpreadsheet
Summary
by MITRE
PHPOffice PhpSpreadsheet before 1.8.0 has an XXE issue. The XmlScanner decodes the sheet1.xml from an .xlsx to utf-8 if something else than UTF-8 is declared in the header. This was a security measurement to prevent CVE-2018-19277 but the fix is not sufficient. By double-encoding the the xml payload to utf-7 it is possible to bypass the check for the string ?<!ENTITY? and thus allowing for an xml external entity processing (XXE) attack.
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Analysis
by VulDB Data Team • 09/04/2024
The vulnerability CVE-2019-12331 affects PHPOffice PhpSpreadsheet versions prior to 1.8.0 and represents a critical XXE (XML External Entity) security flaw that undermines the software's ability to process spreadsheet files safely. This issue stems from the XmlScanner component's handling of XML data within .xlsx files, specifically how it processes character encoding declarations. The vulnerability exploits a flawed security measure that was originally implemented to address CVE-2018-19277, demonstrating how security fixes can sometimes introduce new attack vectors when not thoroughly vetted.
The technical flaw manifests in the XmlScanner's approach to encoding conversion where it automatically converts sheet1.xml content from non-UTF-8 encodings to UTF-8. This behavior was designed as a defensive mechanism against XML parsing vulnerabilities but contains a critical oversight. Attackers can bypass this protection through double-encoding techniques that manipulate the XML payload to UTF-7 encoding, effectively circumventing the string detection mechanism that looks for ?<!ENTITY? patterns. This bypass allows malicious actors to inject external entity references that can trigger XXE processing, potentially leading to unauthorized data access, server-side request forgery, or even remote code execution depending on the target environment.
The operational impact of this vulnerability extends beyond simple file processing risks as it affects any application or system that relies on PhpSpreadsheet for handling Microsoft Excel files. The XXE attack vector can be exploited through seemingly innocuous spreadsheet files that appear legitimate, making detection difficult and increasing the attack surface significantly. When combined with other attack techniques, this vulnerability can enable attackers to access internal network resources, exfiltrate sensitive data, or establish persistent access points within target environments, particularly affecting web applications that process user-uploaded Excel files without proper validation.
Mitigation strategies for CVE-2019-12331 require immediate application of the patched PhpSpreadsheet version 1.8.0 or later, which properly addresses the encoding bypass mechanism. Organizations should implement comprehensive file validation procedures that include strict content type checking, sandboxed processing environments, and network-level restrictions that prevent outbound connections from processing systems. Security teams should also consider implementing web application firewalls with XXE detection capabilities and establish monitoring protocols to identify suspicious XML processing activities. The vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and maps to ATT&CK technique T1059.007 (Command and Scripting Interpreter: PowerShell) when used in conjunction with other attack vectors, demonstrating how XXE vulnerabilities can serve as initial access points for broader exploitation campaigns.