GHSA-67mh-4wv8-2f99
MEDIUMesbuild enables any website to send any requests to the development server and read the response
Blast Radius
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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:
- The attacker serves a malicious web page (
http://malicious.example.com). - The user accesses the malicious web page.
- 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. - 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 aassetsdirectory 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
/esbuildSSE 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
- Download reproduction.zip
- Extract it and move to that directory
- Run
npm i - Run
npm run watch - 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.
Impact
Users using the serve feature may get the source code stolen by malicious websites.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | esbuild | all versions | 0.25.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for esbuild. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update esbuild to 0.25.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-67mh-4wv8-2f99 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-67mh-4wv8-2f99 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-67mh-4wv8-2f99. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-67mh-4wv8-2f99 in your dependencies?
O3 detects GHSA-67mh-4wv8-2f99 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.