CVE-2026-40518 is a path traversal and arbitrary file write vulnerability in ByteDance DeerFlow versions before commit 2176b2b. The issue arises because the agent name validation can be bypassed, allowing attackers to supply traversal-style or absolute paths as the agent name. This manipulation influences directory creation and permits writing files outside the designated custom-agent directory, potentially resulting in arbitrary file write on the affected system, subject to filesystem permissions.

Successful exploitation allows an attacker with limited privileges to write files arbitrarily outside the intended directory structure, which can lead to unauthorized file modification or creation. The impact depends on the filesystem permissions but can potentially escalate to broader system compromise or persistent code execution.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or patch is currently documented. Until a patch is available, restrict access to the affected functionality and monitor for suspicious activity related to custom-agent creation.