MCP Server Path Traversal via Unvalidated Tool Arguments
First seen Sep 8, 2026 · Updated Sep 8, 2026 · CVSS 8.8
An MCP server fails to sanitize filesystem path arguments passed to its tools, allowing an attacker to read, create, overwrite, or delete files outside the intended project directory. This is a classic path traversal vulnerability exposed through an AI agent tool interface, giving attackers a direct route to filesystem compromise via crafted tool calls.
Technical Analysis
The MCP server exposes a file-handling tool that accepts a path argument without validating it against the intended base directory, allowing directory traversal sequences (e.g. '../') to escape the sandboxed project root. An attacker able to influence tool call arguments—whether directly, via a malicious prompt, or through a compromised upstream agent—can trigger arbitrary file read, write, overwrite, or delete operations with the privileges of the MCP server process. This crosses the trust boundary between the LLM/agent's intended sandbox and the host filesystem, turning a scoped tool capability into a full filesystem access primitive. Because MCP tool arguments are often derived from model-generated text, prompt injection or manipulated context can be used to smuggle traversal payloads into otherwise legitimate-looking file operations.
Affected Systems
MCP server (unspecified vendor, versions before 0.30.0); protocols: MCP
Detection Signatures
- Tool call arguments containing '../' or '..\\' sequences
- Path arguments resolving outside configured project/root directory
- Unexpected file access/modification logs for files outside expected project scope
- MCP server logs showing absolute paths or encoded traversal sequences (e.g. %2e%2e%2f) in tool parameters
- Unusual creation/deletion of .md files outside repository boundaries
Remediation Steps
- 1
Upgrade to patched version
Update the MCP server to version 0.30.0 or later, which includes path validation fixes.
- 2
Canonicalize and validate paths
Ensure all file path arguments are resolved to an absolute canonical path and verified to reside within an allow-listed base directory before any filesystem operation.
- 3
Sandbox the server process
Run the MCP server with least-privilege OS permissions and filesystem restrictions (e.g., chroot, containers, restricted service accounts) so path traversal cannot reach sensitive files even if validation is bypassed.
- 4
Audit tool argument sources
Review how tool arguments are generated and passed from the LLM/agent context to detect potential prompt-injection vectors that could supply malicious paths.
- 5
Enable file operation monitoring
Log and alert on file reads/writes/deletes occurring outside the expected project directory scope.
CVE / Advisory IDs
Industries Most Exposed
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