ByteDance DeerFlow Path Traversal Allows Arbitrary File Writes

The National Vulnerability Database has detailed CVE-2026-40518, a high-severity path traversal and arbitrary file write vulnerability affecting ByteDance DeerFlow versions prior to commit 2176b2b. This flaw stems from insufficient validation of the agent name during custom-agent creation in bootstrap-mode, allowing attackers to bypass existing checks.

Attackers can exploit this by supplying path traversal sequences (e.g., ../../) or absolute paths as the agent name. This manipulation influences directory creation, enabling files to be written outside the intended custom-agent directory. Depending on filesystem permissions, this could lead to arbitrary file write capabilities on the underlying system.

With a CVSS score of 7.1 (HIGH), this vulnerability presents a significant risk. Arbitrary file write is a critical primitive, often a precursor to remote code execution. Defenders must prioritize patching DeerFlow instances to commit 2176b2b or later to mitigate this risk. The attacker’s calculus here is clear: gain a foothold and escalate privileges by writing malicious files to sensitive locations.