CVE-2026-111053 in FOSSBilling
Summary
by MITRE • 10/19/2026
FOSSBilling is an open source billing and client-management platform. From 0.1.0 until 0.8.7, the unauthenticated api/guest/system/template_exists endpoint permits path traversal because templateExists uses Path::join on the attacker-controlled file parameter without rejecting .. sequences or verifying that the resolved path remains inside the active theme directory. The trigger is an unauthenticated request to guest/system/template_exists containing traversal sequences in the file parameter. An attacker can submit traversal sequences in the file parameter and observe the returned boolean to determine whether arbitrary files or directories exist on the server. The oracle does not disclose file contents. This issue is fixed in version 0.8.7.
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Analysis
by VulDB Data Team • 10/20/2026
FOSSBilling, an open-source billing and client-management platform, contains a critical path traversal vulnerability within its unauthenticated API endpoint located at api/guest/system/template_exists. This flaw exists in versions ranging from 0.1.0 up to and including 0.8.7. The underlying technical cause stems from the improper handling of user-supplied input by the templateExists function, which utilizes Path::join on a file parameter controlled entirely by the attacker. Crucially, the implementation fails to reject directory traversal sequences such as double dots or validate that the resulting resolved path remains confined within the boundaries of the active theme directory. This lack of strict validation allows an external actor to manipulate the filesystem navigation logic inherent in the application's template resolution mechanism.
The operational impact of this vulnerability is primarily informational, manifesting as a file existence oracle rather than direct code execution or data exfiltration. Because the endpoint returns a boolean value indicating whether a specific path exists on the server, an attacker can systematically probe the underlying operating system to map out its directory structure and identify sensitive files. By submitting various traversal sequences in the file parameter of an unauthenticated request, an adversary can determine with high precision which arbitrary files or directories are present on the target host. While this oracle does not disclose the actual contents of these files, it significantly aids attackers in reconnaissance efforts by confirming the presence of configuration files, backup archives, or other critical system components that may be targeted for subsequent exploitation attempts using different attack vectors.
From a security classification perspective, this vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, as it involves accessing resources outside the intended scope due to insufficient sanitization of path inputs. Furthermore, in the context of the MITRE ATT&CK framework, this behavior corresponds to T1083: File and Directory Discovery, where adversaries use system commands or scripts to enumerate files and folders on compromised systems. The fact that the endpoint is unauthenticated exacerbates the risk profile, as it allows any remote actor with network access to perform these discovery operations without needing valid credentials, thereby lowering the barrier for initial reconnaissance phases of an attack chain.
To mitigate this vulnerability, organizations running FOSSBilling must upgrade immediately to version 0.8.7 or later, where the issue has been resolved through improved input validation and path canonicalization checks that ensure all requested files reside within the designated theme directory. For systems that cannot be upgraded instantly due to operational constraints, temporary mitigations should include restricting access to the api/guest/system/template_exists endpoint via web application firewall rules or network-level controls if possible. Additionally, implementing strict output encoding and ensuring that any future API endpoints follow secure coding practices by validating all user-supplied paths against a whitelist of allowed directories will help prevent similar path traversal issues in other parts of the application infrastructure.