CVE-2026-40518 is a path traversal and arbitrary file write vulnerability in ByteDance DeerFlow versions before commit 2176b2b. The issue arises from improper validation of the custom-agent name in bootstrap mode, allowing attackers to use path traversal sequences or absolute paths to influence directory creation and write files outside the designated custom-agent directory. This can result in arbitrary file writes on the affected system, subject to filesystem permissions. No official remediation or patch information is currently available.

Successful exploitation allows an attacker with limited privileges to write files arbitrarily outside the intended directory structure, potentially leading to unauthorized file modification or code execution depending on the filesystem permissions and context in which DeerFlow runs.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until a fix is available, restrict access to the affected functionality and monitor for suspicious activity related to custom-agent creation. Avoid running DeerFlow with elevated filesystem permissions where possible.