CVE-2026-3089 in Sync Server
Summary
by MITRE • 03/09/2026
Actual Sync Server allows authenticated users to upload files through POST /sync/upload-user-file. In versions prior to 26.3.0, improper validation of the user-controlled x-actual-file-id header means that traversal segments (../) can escape the intended directory and write files outside userFiles.This issue affects prior versions of Actual Sync Server 26.3.0.
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Analysis
by VulDB Data Team • 03/12/2026
The vulnerability identified as CVE-2026-3089 represents a critical directory traversal flaw within the Actual Sync Server authentication system. This security weakness exists in versions prior to 26.3.0 and stems from inadequate input validation mechanisms that fail to properly sanitize user-controlled headers. The specific technical flaw manifests through the POST endpoint /sync/upload-user-file which accepts the x-actual-file-id header from authenticated users. When this header contains traversal sequences such as ../ characters, the system fails to adequately validate or sanitize these inputs, allowing malicious actors to manipulate file paths and write files outside the designated userFiles directory structure. This vulnerability directly maps to CWE-22, which describes improper limitation of a pathname to a restricted directory, commonly known as directory traversal or path traversal attacks.
The operational impact of this vulnerability extends beyond simple unauthorized file access, creating a potential gateway for more severe compromise scenarios within the sync server environment. An authenticated attacker could leverage this flaw to overwrite critical system files, inject malicious code into the server's file structure, or potentially escalate privileges by placing malicious files in directories with higher permissions. The vulnerability affects the fundamental integrity of the file storage system, as it allows attackers to bypass intended access controls and write files anywhere within the server's filesystem hierarchy where the application has write permissions. This represents a significant threat to data confidentiality and system integrity, particularly in environments where the sync server handles sensitive user data or operates with elevated privileges. The issue specifically targets the userFiles directory boundary protection, which is a common security control mechanism designed to isolate user data and prevent unauthorized access to system resources.
Mitigation strategies for CVE-2026-3089 should prioritize immediate implementation of version updates to 26.3.0 or later, as this release contains the necessary fixes for the directory traversal vulnerability. Organizations should also implement additional defensive measures including input validation at multiple layers of the application architecture, strict header sanitization, and comprehensive path normalization before file operations are executed. Network segmentation and access controls should be reviewed to limit the impact of potential exploitation, while monitoring systems should be configured to detect unusual file creation patterns or unauthorized access attempts. The vulnerability demonstrates the importance of proper input validation and the principle of least privilege in system design, as outlined in various cybersecurity frameworks including the NIST Cybersecurity Framework and ISO 27001 standards. Security teams should also consider implementing automated vulnerability scanning tools that can detect similar path traversal patterns in other applications and systems within their environment, as this type of vulnerability frequently appears in web applications that handle file uploads or user-controlled paths.