Summary
esbuild enables any website to send any requests to the development server and read the response
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.
GHSA-67MH-4WV8-2F99 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.25.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is GHSA-67MH-4WV8-2F99? GHSA-67MH-4WV8-2F99 is a medium-severity security vulnerability in esbuild (npm), affecting versions <= 0.24.2. It is fixed in 0.25.0.
- How severe is GHSA-67MH-4WV8-2F99? GHSA-67MH-4WV8-2F99 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of esbuild are affected by GHSA-67MH-4WV8-2F99? esbuild (npm) versions <= 0.24.2 is affected.
- Is there a fix for GHSA-67MH-4WV8-2F99? Yes. GHSA-67MH-4WV8-2F99 is fixed in 0.25.0. Upgrade to this version or later.
- Is GHSA-67MH-4WV8-2F99 exploitable, and should I be worried? Whether GHSA-67MH-4WV8-2F99 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-67MH-4WV8-2F99 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-67MH-4WV8-2F99? Upgrade
esbuildto 0.25.0 or later.