CVE-2021-43783 in Backstage
Summary
by MITRE • 11/29/2021
@backstage/plugin-scaffolder-backend is the backend for the default Backstage software templates. In affected versions a malicious actor with write access to a registered scaffolder template is able to manipulate the template in a way that writes files to arbitrary paths on the scaffolder-backend host instance. This vulnerability can in some situation also be exploited through user input when executing a template, meaning you do not need write access to the templates. This method will not allow the attacker to control the contents of the injected file however, unless the template is also crafted in a specific way that gives control of the file contents. This vulnerability is fixed in version `0.15.14` of the `@backstage/plugin-scaffolder-backend`. This attack is mitigated by restricting access and requiring reviews when registering or modifying scaffolder templates.
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Analysis
by VulDB Data Team • 01/03/2025
The CVE-2021-43783 vulnerability affects the @backstage/plugin-scaffolder-backend component which serves as the backend infrastructure for managing software templates within the Backstage platform. This vulnerability represents a critical path traversal and arbitrary file writing flaw that arises from insufficient input validation and access control mechanisms within the template processing pipeline. The vulnerability specifically targets the scaffolder backend host instance where template execution occurs, creating a potential attack vector for privilege escalation and system compromise.
The technical flaw stems from improper validation of template paths during the scaffolder template execution process. Attackers with write access to registered scaffolder templates can manipulate template content to write files to arbitrary locations on the backend host system. This occurs because the system fails to properly sanitize or restrict file paths that templates may attempt to create or modify. The vulnerability manifests through the template engine's handling of file system operations, where user-supplied template data is directly processed without adequate path validation. According to CWE-22, this represents a path traversal vulnerability where attacker-controlled input influences file system operations. The issue is particularly dangerous because it can be exploited through multiple attack vectors, including direct template modification and user input manipulation during template execution.
The operational impact of this vulnerability extends beyond simple file system manipulation to potentially enable full system compromise. An attacker could leverage this vulnerability to write malicious files to critical system locations such as configuration directories, binary execution paths, or system service locations. Even when file contents cannot be directly controlled through the attack vector, the ability to write files to arbitrary paths provides attackers with opportunities for privilege escalation, persistence mechanisms, and further exploitation. The vulnerability's severity is compounded by the fact that it can be exploited even without direct template write access through user input manipulation, making it particularly dangerous in environments where template execution involves untrusted input. This aligns with ATT&CK technique T1059.001 for command and scripting interpreter execution, where the attacker can manipulate the execution environment to write malicious payloads.
The vulnerability was addressed in version 0.15.14 of the @backstage/plugin-scaffolder-backend package through enhanced input validation and path sanitization mechanisms. The fix implements proper restrictions on file system operations within template execution contexts, preventing the exploitation of arbitrary path writing capabilities. Security mitigations should include implementing strict access controls for template registration and modification, requiring multi-factor authentication for template management, and establishing comprehensive code review processes for all template changes. Organizations should also implement network segmentation to limit access to the scaffolder backend hosts and deploy monitoring solutions to detect anomalous file system activity. The vulnerability highlights the importance of principle of least privilege in software development, where template execution contexts should not have unrestricted file system write capabilities. Additionally, the fix demonstrates the necessity of input validation and proper error handling in web application frameworks to prevent path traversal attacks. The vulnerability serves as a reminder that even seemingly benign template processing functionality can become a critical security risk when proper security controls are not implemented.