CVE-2017-5661 in FOP
Summary
by MITRE
In Apache FOP before 2.2, files lying on the filesystem of the server which uses FOP can be revealed to arbitrary users who send maliciously formed SVG files. The file types that can be shown depend on the user context in which the exploitable application is running. If the user is root a full compromise of the server - including confidential or sensitive files - would be possible. XXE can also be used to attack the availability of the server via denial of service as the references within a xml document can trivially trigger an amplification attack.
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Analysis
by VulDB Data Team • 10/19/2019
Apache FOP version 2.1 and earlier contains a critical information disclosure vulnerability that allows remote attackers to access arbitrary files on the server filesystem through maliciously crafted SVG files. This vulnerability stems from insufficient input validation and improper handling of external entity references within the SVG processing pipeline. The flaw exists because the application fails to properly sanitize SVG content that may contain references to local files, enabling attackers to traverse the filesystem and retrieve sensitive data. The vulnerability is particularly dangerous because it can be exploited by unauthenticated users who simply need to submit a malicious SVG file to the vulnerable FOP service. When processing such files, the system attempts to resolve external entity references without proper access controls, potentially exposing files that the application user context has access to. If the FOP process runs with elevated privileges such as root access, the compromise can extend to full system access including confidential databases, configuration files, and other sensitive system resources.
The technical implementation of this vulnerability aligns with CWE-20: Improper Input Validation and CWE-94: Improper Control of Generation of Code, as the system fails to properly validate and sanitize external references within SVG documents. The vulnerability enables arbitrary file read operations through XML External Entity (XXE) processing mechanisms, where the SVG files can contain malicious entity declarations that reference local files. This weakness allows attackers to perform path traversal attacks and access files that should normally be restricted. The attack vector specifically targets the SVG rendering engine within Apache FOP, which processes external entity references without adequate sandboxing or access control measures. The vulnerability also presents a significant availability risk as demonstrated by the potential for denial of service attacks through XML amplification techniques where the references within the XML document can trigger resource exhaustion. The XXE capabilities can be leveraged to create massive XML documents that consume excessive server resources, leading to service disruption.
The operational impact of this vulnerability extends beyond simple information disclosure to encompass complete system compromise when the FOP service operates with elevated privileges. Attackers can exploit this vulnerability to access sensitive configuration files, database credentials, application source code, and other confidential data stored on the server. The attack can be executed remotely without requiring authentication, making it particularly dangerous in multi-tenant environments or public-facing applications. Organizations running vulnerable versions of Apache FOP are exposed to data breaches that could result in regulatory compliance violations, financial losses, and reputational damage. The vulnerability can be exploited through various attack vectors including web applications that utilize FOP for document generation, email servers, or any system that accepts SVG uploads and processes them through the vulnerable library. The impact is further amplified by the fact that the vulnerability affects not just file content disclosure but also availability through potential denial of service attacks that can exhaust system resources.
Organizations should immediately upgrade to Apache FOP version 2.2 or later, which contains patches addressing this vulnerability. The upgrade process should include thorough testing to ensure compatibility with existing applications that depend on FOP functionality. Additional mitigations include implementing proper input validation and sanitization for all SVG content, restricting file system access for FOP processes through sandboxing techniques, and implementing network-level controls to limit access to FOP processing endpoints. Security configurations should enforce strict access controls and disable unnecessary external entity processing capabilities. Organizations should also implement monitoring and logging mechanisms to detect suspicious file upload activities and unusual resource consumption patterns. The vulnerability demonstrates the importance of proper input validation and the principle of least privilege in preventing privilege escalation attacks. Network segmentation and application firewalls can provide additional layers of defense by restricting access to FOP processing services and limiting the potential impact of successful exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to identify and remediate similar issues in other components of the application stack.