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.
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.
Impact Assessment
This high-severity vulnerability could allow attackers to gain unauthorized access, execute arbitrary code, or compromise data integrity. Prompt remediation is strongly recommended.
Quick Facts
CVSS v3.1 Metrics Breakdown
How to Fix This Vulnerability
Remediation Steps
- 1Upgrade the package to version 0.25.0 or later
- 2Review and audit affected systems for potential exploitation
- 3Implement additional security controls if immediate patching is not possible
- 4Monitor for suspicious activity related to this vulnerability
Mitigation Strategies
Immediate Patching
HighApply security patches to version 0.25.0 immediately
Network Segmentation
HighIsolate affected systems from untrusted networks until patching is complete
Monitoring and Detection
MediumImplement enhanced monitoring for exploitation attempts and unusual behavior