CVE-2018-13415 in Plex Media Server
Summary
by MITRE
In Plex Media Server 1.13.2.5154, the XML parsing engine for SSDP/UPnP functionality is vulnerable to an XML External Entity Processing (XXE) attack. Remote, unauthenticated attackers can use this vulnerability to: (1) Access arbitrary files from the filesystem with the same permission as the user account running Plex, (2) Initiate SMB connections to capture a NetNTLM challenge/response and crack to cleartext password, or (3) Initiate SMB connections to relay a NetNTLM challenge/response and achieve Remote Command Execution in Windows domains.
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Analysis
by VulDB Data Team • 07/02/2024
The vulnerability identified as CVE-2018-13415 affects Plex Media Server version 1.13.2.5154 and represents a critical XML External Entity Processing flaw within the Simple Service Discovery Protocol/Universal Plug and Play functionality. This vulnerability exists in the XML parsing engine that handles UPnP communications, creating a pathway for remote attackers to exploit the system through malformed XML requests. The flaw specifically manifests when the server processes incoming SSDP/UPnP messages that contain external entity references, allowing malicious actors to manipulate the XML parser behavior. The vulnerability is classified under CWE-611 as an improper restriction of XML external entity reference, which directly relates to the insecure handling of external entities in XML processing. This issue is particularly concerning because it affects a widely deployed media server application that typically runs with elevated privileges on home and enterprise networks.
The technical exploitation of this XXE vulnerability enables attackers to perform several malicious activities without authentication, making it especially dangerous in networked environments. Remote attackers can leverage the vulnerability to read arbitrary files from the server's filesystem, potentially accessing sensitive configuration files, user data, or system information with the same privileges as the Plex Media Server process. The attack vector operates through carefully crafted XML payloads that reference external entities, causing the vulnerable XML parser to resolve these references and return file contents to the attacker. Additionally, the vulnerability permits attackers to establish SMB connections to external servers, enabling them to capture NetNTLM challenge/response hashes that can be cracked using password recovery tools to obtain cleartext passwords. This capability significantly increases the attack surface, as it allows adversaries to escalate privileges and move laterally within Windows domains.
The operational impact of this vulnerability extends beyond simple information disclosure, as it creates opportunities for full domain compromise through credential relay attacks. When attackers capture NetNTLM hashes through SMB connections initiated by the vulnerable Plex server, they can utilize these credentials for pass-the-hash or pass-the-ticket attacks against other systems within the domain. This represents a direct violation of the principle of least privilege, as the Plex server process may be running with administrative rights on the host system. The vulnerability also enables remote command execution capabilities when combined with SMB relay attacks, allowing attackers to execute arbitrary code on target systems without direct access. This threat model aligns with ATT&CK technique T1078.002 for valid accounts and T1566.001 for spearphishing attachments, as the exploitation can occur through network-based attacks that don't require physical access or prior compromise of the target system.
Organizations should immediately implement mitigations including updating to patched versions of Plex Media Server, as the vulnerability was addressed in subsequent releases. Network segmentation and firewall rules should be configured to restrict UPnP/SSDP traffic between the Plex server and external networks, preventing unauthorized access to the vulnerable functionality. The XML parsing configuration should be modified to disable external entity resolution entirely, which can be achieved through proper XML parser settings that prevent loading of external DTDs. System administrators should monitor for unusual SMB connections originating from Plex server instances and implement network detection rules to identify potential XXE exploitation attempts. Additionally, privilege separation should be considered where possible, ensuring that the Plex Media Server process runs with minimal required permissions rather than administrative privileges. Regular security assessments should include verification of XML processing configurations in all network services to prevent similar vulnerabilities from being introduced in other applications. The vulnerability demonstrates the importance of secure coding practices and proper input validation in network services, particularly those that process external data through XML parsers.