{"id":47437,"date":"2026-06-02T14:22:11","date_gmt":"2026-06-02T08:52:11","guid":{"rendered":"https:\/\/www.getastra.com\/blog\/?p=47437"},"modified":"2026-06-02T14:39:04","modified_gmt":"2026-06-02T09:09:04","slug":"nodemailer-vulnerability","status":"publish","type":"post","link":"https:\/\/www.getastra.com\/blog\/vulnerability\/nodemailer-vulnerability\/","title":{"rendered":"Nodemailer Improper Certificate Validation Allows MITM"},"content":{"rendered":"
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Product Name:<\/strong>\u00a0Nodemailer (npm)
Vulnerability:<\/strong>\u00a0Improper TLS certificate validation vulnerability
Vulnerable Version:\u00a0<\/strong><= 8.0.7<\/p>\n\n<\/div>\n\n\n

On May 27, 2026, a security researcher at Astra Security found an improper TLS certificate validation vulnerability in Nodemailer\u2019s internal HTTPS fetch implementation, affecting versions up to 8.0.7.<\/strong><\/p>\n\n\n\n

If exploited, this vulnerability would allow an attacker positioned as a machine-in-the-middle (MITM)<\/strong> to intercept OAuth2 credential exchanges using self-signed or invalid<\/strong> TLS certificates<\/strong>.<\/p>\n\n\n\n

Nodemailer is one of the most widely used email libraries in the Node.js ecosystem, with approximately 15 million weekly downloads<\/strong> on npm and over 10,000 dependent packages<\/strong>. It is heavily integrated into authentication systems, SaaS platforms, transactional email services, and enterprise applications.\u00a0<\/p>\n\n\n\n

A successful exploitation and weaponization of this vulnerability could lead to widespread credential theft and compromise of numerous production deployments worldwide.<\/p>\n\n\n\n

<\/span>Technical Breakdown <\/span><\/h2>\n\n\n\n

The flaw resides in the library\u2019s internal HTTPS fetch implementation and completely bypasses certificate validation during OAuth2 token exchanges. <\/p>\n\n\n\n

Given that the NPM<\/a> ecosystem is already heavily targeted and weaponized by threat actors, this vulnerability represents a particularly high-risk vector for large-scale attacks.<\/p>\n\n\n\n

How was it discovered?<\/h3>\n\n\n\n

While analyzing Nodemailer\u2019s OAuth2 authentication flow, our security researcher noticed that the internal fetch client disabled TLS certificate validation through the use of rejectUnauthorized: false.<\/code><\/p>\n\n\n\n

To verify the impact, a test was conducted by redirecting the accessUrl<\/code> to a fake OAuth server with a self-signed certificate. When a token refresh was triggered, Nodemailer sent sensitive credentials (client_secret<\/code>, refresh_token<\/code> etc.)\u00a0 to the malicious server without any rejection. This confirmed a machine-in-the-middle (MITM) vulnerability<\/strong> that allows credential interception during the OAuth2 token exchange.<\/p>\n\n\n\n

How to replicate the vulnerability<\/h3>\n\n\n\n
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  1. Set up vulnerable Nodemailer code<\/li>\n\n\n\n
  2. Then set up a malicious OAuth2 endpoint using node.js\/express<\/code> server with a self-signed certificate.<\/li>\n\n\n\n
  3. Intercept traffic through DNS poisoning, proxy manipulation, network interception, or the machine-in-the-middle (MITM) technique.<\/li>\n\n\n\n
  4. Successful replication shows the client_secret<\/code> and refresh_token <\/code>(or authorization code) in the malicious server logs.<\/li>\n<\/ol>\n\n\n\n