CVE-2026-108357 in PhpSpreadsheet
Summary
by MITRE • 10/22/2026
PhpSpreadsheet is a pure PHP library for reading and writing spreadsheet files. From 3.3.0 until 3.10.8 and 5.10.0, Reader\Xml passes stored SpreadsheetML XML through XmlScanner::scan in Xml::canRead and Xml::trySimpleXMLLoadStringPrivate, while Xml::unentity decodes numeric entities only after each DOCTYPE pattern check. A double-encoded less-than character is reduced by the first callback to another encoded character and by the second callback to a literal less-than sign, so SimpleXML receives an unchecked DOCTYPE and entity declaration. Processing attacker-controlled SpreadsheetML XML can cause entity-expansion denial of service and can permit local file access or SSRF on PHP before 8.0 or when LIBXML_NOENT is enabled. This issue is fixed in versions 3.10.8 and 5.10.0.
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Analysis
by VulDB Data Team • 10/22/2026
PhpSpreadsheet serves as a widely adopted pure PHP library designed for the manipulation of spreadsheet files, supporting formats such as Excel and CSV. Within this ecosystem, the XML reader component plays a critical role in parsing SpreadsheetML documents. A significant security vulnerability was identified in versions ranging from 3.3.0 up to 3.10.8 and version 5.10.0. The core of the issue lies within the XmlScanner class and its interaction with SimpleXML during the validation phase where the library determines if a given file is a valid SpreadsheetML document. Specifically, in the methods canRead and trySimpleXMLLoadStringPrivate, the XML content undergoes scanning to detect potentially dangerous patterns such as DOCTYPE declarations which are often associated with external entity processing risks.
The technical flaw stems from an inconsistent handling of character encoding during this validation process. The library employs a callback mechanism to sanitize input by decoding numeric HTML entities. However, there is a critical ordering error in the execution flow. When an attacker provides double-encoded less-than characters, represented as &lt;, the first pass through the entity decoder reduces them to single-encoded forms like <. Subsequently, because the DOCTYPE pattern check occurs before the final decoding step for certain code paths or due to the specific implementation of unentity, this second layer of encoding is not fully neutralized before being passed to SimpleXML. Consequently, when the XML parser processes the document, it interprets these characters as literal less-than signs, effectively bypassing the security checks that were intended to block malicious DOCTYPE declarations and entity definitions.
This flaw leads directly to a class of vulnerabilities categorized under CWE-77: Improper Neutralization of Special Elements used in an Command (Command Injection) or more specifically CWE-611: Improper Restriction of XML External Entity Reference when considering the impact on external entities. By allowing unchecked DOCTYPE and entity declarations, an attacker can inject malicious XML structures into the parsed document. The operational impact is severe, primarily manifesting as a denial-of-service condition through exponential entity expansion attacks. Attackers can craft deeply nested or recursive entity definitions that cause the PHP process to consume excessive CPU and memory resources until it crashes or becomes unresponsive. This aligns with ATT&CK technique T1496: Resource Hijacking, where system resources are consumed to disrupt availability.
Beyond denial of service, this vulnerability permits more intrusive attacks depending on the server configuration. If the application is running on PHP versions prior to 8.0 or if the LIBXML_NOENT flag is enabled during parsing, the parser will resolve external entities defined in the malicious DOCTYPE. This allows for Local File Inclusion (LFI) where an attacker can read sensitive files from the server's filesystem by referencing them via file:// URIs in entity definitions. Furthermore, it enables Server-Side Request Forgery (SSRF), allowing the attacker to force the server to make HTTP requests to internal or external network resources that would otherwise be inaccessible. This corresponds to ATT&CK technique T1078: Valid Accounts if combined with credential theft, but more directly maps to T1563: Remote Service Exploitation for SSRF scenarios where the application acts as a proxy for unauthorized network access.
The resolution of this issue was implemented in PhpSpreadsheet versions 3.10.8 and 5.10.0 by correcting the order of operations within the XML parsing logic. The fix ensures that all entity decoding is completed before any pattern matching or DOCTYPE validation occurs, thereby preventing double-encoded bypasses from reaching the SimpleXML parser with unchecked content. Organizations utilizing affected versions must upgrade to these patched releases immediately to mitigate the risk of denial-of-service attacks and unauthorized data access. Additionally, it is recommended to review application configurations to ensure that LIBXML_NOENT is not enabled unless strictly necessary for legitimate functionality, as disabling this flag reduces the attack surface even if other vulnerabilities exist in XML parsing libraries. Regular security audits and input validation practices should be maintained to defend against evolving XML-based threats.