CVE-2024-48917 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, in a bypass of the previously reported `CVE-2024-47873`, the regexes from the `findCharSet` method, which is used for determining the current encoding can be bypassed by using a payload in the encoding UTF-7, and adding at end of the file a comment with the value `encoding="UTF-8"` with `"`, which is matched by the first regex, so that `encoding='UTF-7'` with single quotes `'` in the XML header is not matched by the second regex. 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 described in CVE-2024-48917 affects the PhpSpreadsheet library, a widely used PHP component for handling spreadsheet file operations. This library serves as a critical dependency for numerous web applications that process excel files, making the security implications particularly significant. The flaw resides within the XmlScanner class's scan method, which is designed to prevent XML External Entity (XXE) attacks by sanitizing input data. The vulnerability represents a sophisticated bypass of an existing security mechanism, specifically targeting the encoding detection logic that was previously thought to be sufficient for preventing XXE exploitation. The issue demonstrates how attackers can leverage specific encoding behaviors to circumvent security controls that were implemented to protect against malicious XML content.
The technical exploitation mechanism involves a clever manipulation of UTF-7 encoding combined with XML comment parsing behavior. An attacker can craft a payload where the XML declaration uses encoding="UTF-7" with single quotes, while simultaneously appending a comment at the end of the file containing encoding="UTF-8" with double quotes. This dual approach exploits a flaw in the regex validation logic within the findCharSet method, where the first regular expression matches the double-quoted UTF-8 declaration in the comment, causing the system to incorrectly identify the encoding as UTF-8 rather than UTF-7. The second regex, designed to detect and block UTF-7 encoding, fails to match due to the quote character difference, allowing the malicious UTF-7 encoding to pass through the sanitization process. This bypass technique directly relates to CWE-611, which addresses improper neutralization of direct references to external entities, and demonstrates how encoding confusion can lead to security vulnerabilities.
The operational impact of this vulnerability is substantial for any system utilizing PhpSpreadsheet for processing user-uploaded spreadsheet files. Attackers can exploit this weakness to perform XXE attacks, potentially gaining access to sensitive system information, executing arbitrary file reads, or even performing server-side request forgery attacks. The vulnerability affects multiple versions of the library including 1.9.4, 2.1.3, 2.3.2, and 3.4.0, indicating a widespread exposure across different release branches. Organizations relying on this library for processing spreadsheet data, particularly in applications that accept untrusted input, face significant risk of compromise. The attack vector is particularly dangerous because it can be executed through normal file upload operations, making it difficult to detect and prevent through traditional network monitoring approaches.
Security mitigations for this vulnerability require immediate patching of affected versions to the fixed releases mentioned in the advisory. The remediation strategy should also include implementing additional input validation layers and restricting file upload capabilities where possible. Organizations should consider implementing network-level protections such as web application firewalls that can detect and block suspicious XML patterns, particularly those involving encoding manipulation. The fix addresses the root cause by improving the encoding detection logic to properly handle all encoding variations and prevent the bypass technique. From an ATT&CK framework perspective, this vulnerability maps to T1213.002 for data from information repositories and T1059.007 for command and scripting interpreter, as it enables attackers to execute arbitrary code through XML external entity exploitation. Security teams should also conduct comprehensive vulnerability assessments to identify any other systems using affected versions of PhpSpreadsheet and implement proper security monitoring to detect potential exploitation attempts.