# GHSA-67mh-4wv8-2f99: esbuild enables any website to send any requests to the development server and read the response

> Misconfigured CORS headers in esbuild's development server allow malicious websites to read local source code and assets via cross-origin requests.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 10 Feb 2025

## What is GHSA-67mh-4wv8-2f99?

GHSA-67mh-4wv8-2f99 is a vulnerability in esbuild, published on 10 Feb 2025. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

### Summary

esbuild allows any websites to send any request to the development server and read the response due to default CORS settings.

### Details

esbuild sets `Access-Control-Allow-Origin: *` header to all requests, including the SSE connection, which allows any websites to send any request to the development server and read the response.

https://github.com/evanw/esbuild/blob/df815ac27b84f8b34374c9182a93c94718f8a630/pkg/api/serve_other.go#L121
https://github.com/evanw/esbuild/blob/df815ac27b84f8b34374c9182a93c94718f8a630/pkg/api/serve_other.go#L363

**Attack scenario**:

1. The attacker serves a malicious web page (`http://malicious.example.com`).
1. The user accesses the malicious web page.
1. The attacker sends a `fetch('http://127.0.0.1:8000/main.js')` request by JS in that malicious web page. This request is normally blocked by same-origin policy, but that's not the case for the reasons above.
1. The attacker gets the content of `http://127.0.0.1:8000/main.js`.

In this scenario, I assumed that the attacker knows the URL of the bundle output file name. But the attacker can also get that information by

- Fetching `/index.html`: normally you have a script tag here
- Fetching `/assets`: it's common to have a `assets` directory when you have JS files and CSS files in a different directory and the directory listing feature tells the attacker the list of files
- Connecting `/esbuild` SSE endpoint: the SSE endpoint sends the URL path of the changed files when the file is changed (`new EventSource('/esbuild').addEventListener('change', e => console.log(e.type, e.data))`)
- Fetching URLs in the known file: once the attacker knows one file, the attacker can know the URLs imported from that file

The scenario above fetches the compiled content, but if the victim has the source map option enabled, the attacker can also get the non-compiled content by fetching the source map file.

### PoC

1. Download [reproduction.zip](https://github.com/user-attachments/files/18561484/reproduction.zip)
2. Extract it and move to that directory
1. Run `npm i`
1. Run `npm run watch`
1. Run `fetch('http://127.0.0.1:8000/app.js').then(r => r.text()).then(content => console.log(content))` in a different website's dev tools.

![image](https://github.com/user-attachments/assets/08fc2e4d-e1ec-44ca-b0ea-78a73c3c40e9)

### Impact

Users using the serve feature may get the source code stolen by malicious websites.

## Which versions are affected by GHSA-67mh-4wv8-2f99?

2 open source packages ship an affected version, including esbuild.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `esbuild` | npm | All versions | 0.25.0 |
| `esbuild` | npm | All versions | 0.25.0 |

## How do I fix GHSA-67mh-4wv8-2f99?

Upgrade esbuild to 0.25.0 or later.

## Is GHSA-67mh-4wv8-2f99 being exploited?

There is no known exploitation of GHSA-67mh-4wv8-2f99 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-67mh-4wv8-2f99 and fix it if it is affected.

What it is: GHSA-67mh-4wv8-2f99 is a vulnerability in esbuild, published on 10 Feb 2025. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of GHSA-67mh-4wv8-2f99 in the wild and no public exploit code.

Affected packages and fixed versions:
- esbuild (npm): affected all versions; fixed in 0.25.0
- esbuild (npm): affected all versions; fixed in 0.25.0

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/GHSA-67mh-4wv8-2f99/ (Markdown: https://insights.sbomify.com/GHSA-67mh-4wv8-2f99/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-67mh-4wv8-2f99

---
From sbomify Insights: https://insights.sbomify.com/GHSA-67mh-4wv8-2f99/
Summaries are generated and may contain mistakes; the official description above is the record.