CVE-2018-20718 in Pydio
Summary
by MITRE
In Pydio before 8.2.2, an attack is possible via PHP Object Injection because a user is allowed to use the $phpserial$a:0:{} syntax to store a preference. An attacker either needs a "public link" of a file, or access to any unprivileged user account for creation of such a link.
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Analysis
by VulDB Data Team • 05/01/2020
The vulnerability identified as CVE-2018-20718 represents a critical PHP Object Injection flaw affecting Pydio versions prior to 8.2.2. This security weakness stems from insufficient input validation and sanitization mechanisms within the application's preference storage system. The vulnerability allows attackers to manipulate serialized PHP objects through the $phpserial$a:0:{} syntax, which creates a serialized empty array object that can be exploited during deserialization processes. The attack vector specifically leverages the ability to store preferences using this syntax, enabling unauthorized code execution or system compromise.
The technical exploitation of this vulnerability occurs when an attacker gains access to a user account or obtains a public link to a file within the Pydio system. This access enables them to manipulate the preference storage mechanism and inject malicious serialized objects. The flaw operates at the serialization layer where user-supplied data is processed without proper validation, allowing attackers to craft serialized objects that, when later deserialized, execute arbitrary code or trigger unexpected behavior within the application. This type of vulnerability falls under CWE-502 which specifically addresses deserialization of untrusted data and aligns with ATT&CK technique T1203 for exploitation of software vulnerabilities.
The operational impact of this vulnerability extends beyond simple privilege escalation as it can lead to complete system compromise when combined with the ability to create public links. An attacker with access to any unprivileged user account can leverage this flaw to execute arbitrary commands on the server, potentially gaining administrative privileges or accessing sensitive data. The vulnerability's severity is amplified by its ability to be exploited through public links, meaning that even users without direct account access can potentially exploit the flaw if they can obtain the necessary link. This makes the vulnerability particularly dangerous in environments where public sharing is enabled, as it creates an attack surface that can be exploited by anyone with knowledge of the link structure.
Mitigation strategies for CVE-2018-20718 require immediate patching of the affected Pydio installations to version 8.2.2 or later, which addresses the serialization vulnerability through proper input validation and sanitization. Organizations should also implement additional security measures including restricting preference modification capabilities, implementing strict input validation for all user-supplied data, and monitoring for unusual preference storage activities. Network segmentation and access controls should be enforced to limit the potential impact of successful exploitation, while regular security audits should be conducted to identify similar vulnerabilities in other applications. The fix implemented in version 8.2.2 specifically addresses the deserialization process by ensuring that serialized data is properly validated before processing, preventing malicious objects from being executed during preference handling operations.