CVE-2026-86439 in knowns
Summary
by MITRE • 09/08/2026
knowns versions before 0.30.0 fail to validate filesystem paths in MCP tool arguments, allowing attackers to read, create, overwrite and delete files outside the project directory. Attackers can supply path arguments containing directory traversal sequences to access arbitrary Markdown files accessible to the server process.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified in versions of the software prior to 0.30.0 represents a critical failure in input validation specifically concerning filesystem path handling within Model Context Protocol tool arguments. This flaw stems from an insufficient sanitization mechanism that fails to properly restrict or validate user-supplied paths before they are processed by the underlying file system operations. When attackers supply path arguments containing directory traversal sequences, such as dot-dot-slash patterns, the application does not adequately neutralize these characters. Consequently, the software interprets these malicious inputs literally rather than treating them as invalid requests, allowing the execution of file system commands outside the intended project directory boundaries. This lack of strict validation creates a direct pathway for unauthorized access to arbitrary files accessible by the server process, fundamentally breaking the isolation expected between the application and its operational environment.
From an offensive security perspective, this vulnerability enables several severe impacts including information disclosure, data integrity compromise, and potential denial of service through file deletion. An attacker can exploit this flaw to read sensitive configuration files, source code, or other Markdown documents that reside outside the designated project root but remain accessible via standard operating system permissions. Furthermore, the ability to create, overwrite, and delete arbitrary files allows for more sophisticated attacks such as planting malicious scripts, modifying application logic through file injection, or disrupting service availability by removing critical data stores. The operational impact is significant because it undermines the security boundary of the server process, potentially leading to full system compromise if combined with other vulnerabilities like command injection or privilege escalation techniques.
This issue aligns closely with Common Weakness Enumeration category CWE-22: Improper Limitation of a Pathname to a Restricted Directory, which describes flaws where software does not properly neutralize special elements within file paths that could resolve to unintended destinations. Additionally, the exploitation technique maps directly to MITRE ATT&CK tactic TA0001: Initial Access and specifically the sub-technique T1530: Data from Cloud Storage or local equivalent mechanisms involving path traversal for data exfiltration. The attack vector is typically classified as remote if exposed via network services but may also apply locally depending on deployment context, falling under ATT&CK technique T1083: File and Directory Discovery when used to enumerate accessible resources beyond the allowed scope.
To mitigate this vulnerability, it is imperative to upgrade immediately to version 0.30.0 or later where these validation checks have been implemented. For environments unable to patch instantly, defensive coding practices must be employed within any custom wrappers or middleware interacting with MCP tool arguments. This includes implementing strict allow-listing of permitted directories and rejecting any input containing relative path traversal sequences such as dot-dot-slash combinations. Additionally, employing canonicalization techniques to resolve paths before validation can help detect attempts to bypass restrictions using encoded characters or redundant separators. Implementing least-privilege principles for the server process ensures that even if an attacker succeeds in accessing files outside the project directory, their ability to modify system-critical resources remains limited by operating system permissions. Regular security audits and static analysis tools configured to detect path traversal weaknesses should also be integrated into the development lifecycle to prevent recurrence of such input validation failures.