# CVE-2026-40518: ByteDance DeerFlow Path Traversal and Arbitrary File Write via Bootstrap Mode

> Attackers can bypass agent name validation in bootstrap mode to write arbitrary files outside the intended directory, potentially achieving code execution.

- **Severity:** High 7.1 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 17 Apr 2026
- **Updated:** 14 Aug 2026
- **Also known as:** GHSA-54pv-34f9-m4mq

## What is CVE-2026-40518?

CVE-2026-40518 is a high-severity path traversal vulnerability in bytedance deer-flow, published on 17 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

ByteDance DeerFlow before commit 2176b2b contains a path traversal and arbitrary file write vulnerability in bootstrap-mode custom-agent creation where the agent name validation is bypassed. Attackers can supply traversal-style values or absolute paths as the agent name to influence directory creation and write files outside the intended custom-agent directory, potentially achieving arbitrary file write on the system subject to filesystem permissions.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2026-40518?

It affects 1 product.


Products: Bytedance Deerflow.

## How do I fix CVE-2026-40518?

No fix has been published for CVE-2026-40518 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-40518 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-40518 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40518 and fix it if it is affected.

What it is: CVE-2026-40518 is a high-severity path traversal vulnerability in bytedance deer-flow, published on 17 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-40518 exists.

Affected product: bytedance deer-flow. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-40518/ (Markdown: https://insights.sbomify.com/CVE-2026-40518/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40518
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40518
- OSV: https://osv.dev/vulnerability/CVE-2026-40518

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40518/
Summaries are generated and may contain mistakes; the official description above is the record.