CVE-2017-6921 in Drupal
Summary
by MITRE
In Drupal 8 prior to 8.3.4; The file REST resource does not properly validate some fields when manipulating files. A site is only affected by this if the site has the RESTful Web Services (rest) module enabled, the file REST resource is enabled and allows PATCH requests, and an attacker can get or register a user account on the site with permissions to upload files and to modify the file resource.
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Analysis
by VulDB Data Team • 12/29/2020
The vulnerability identified as CVE-2017-6921 represents a critical security flaw in Drupal 8 versions prior to 8.3.4 that specifically targets the file REST resource functionality. This issue stems from inadequate validation mechanisms within the RESTful Web Services module when processing file manipulation operations through PATCH requests. The vulnerability operates under specific prerequisites that must be met for exploitation to occur, making it somewhat targeted but still potentially dangerous for affected systems. The flaw resides in the validation logic that governs how file fields are processed when REST requests are made, creating an avenue for unauthorized file manipulation that could lead to significant security consequences.
The technical implementation of this vulnerability involves the RESTful Web Services module's handling of file resource operations, particularly when PATCH requests are processed through the file REST endpoint. When a site has the REST module enabled along with the file resource endpoint configured to accept PATCH requests, the system fails to properly validate certain file fields during manipulation operations. This validation failure allows attackers to potentially bypass normal file upload restrictions and manipulate file properties in ways that should otherwise be restricted. The vulnerability specifically affects scenarios where an attacker can obtain or register a user account with sufficient privileges to upload files and modify file resources, creating a pathway for exploitation that leverages legitimate REST API functionality against the system's own validation mechanisms.
The operational impact of this vulnerability extends beyond simple file manipulation and represents a serious threat to Drupal 8 installations that meet the specified conditions. An attacker who successfully exploits this vulnerability could potentially upload malicious files, modify existing file properties, or manipulate file metadata in ways that could compromise the entire system. The implications include potential code execution through malicious file uploads, data corruption, or the ability to escalate privileges within the system's file management capabilities. This vulnerability particularly affects organizations that rely heavily on REST APIs for file operations and have enabled the file REST resource with PATCH request capabilities, as these systems become vulnerable to manipulation attacks that exploit the validation gap.
Organizations affected by this vulnerability should prioritize immediate remediation through the official Drupal security update process, specifically upgrading to Drupal 8.3.4 or later versions where the validation issues have been addressed. The mitigation strategy should include disabling the file REST resource endpoint when PATCH requests are not required for operational purposes, or ensuring that proper access controls are implemented to restrict who can register accounts with file upload permissions. Additionally, implementing network-level controls such as firewalls that restrict access to REST endpoints and monitoring for unusual file manipulation patterns can provide additional layers of defense. This vulnerability aligns with CWE-20, which describes improper input validation, and represents a specific instance where REST API validation mechanisms fail to properly sanitize or validate file-related fields. The attack pattern associated with this vulnerability follows ATT&CK technique T1059.006 for executing malicious code through file uploads, and T1078 for gaining access through legitimate user accounts with appropriate privileges. Organizations should also consider implementing automated security scanning tools that can detect and alert on improper file handling within REST endpoints to prevent exploitation of similar validation flaws in other parts of their Drupal installations.