GHSA-6vj9-mwq6-2f5v

Nodemailer: Process-global DNS cache reuses TLS `servername` across transports, enabling cross-tenant SMTP credential disclosure

Published

2026-09-28

Modified

2026-09-28

Sources

github-advisory

Summary

### Summary Nodemailer's process-global DNS cache is keyed only by `host`, but each cache entry also stores the caller-specific TLS `servername`. When two direct SMTPS transports use the same DNS host with different `tls.servername` values, the first transport's server name is returned to the second transport and overwrites its explicitly configured value. As a result, Nodemailer sends the wrong SNI value and verifies the peer certificate against the wrong identity. In a multi-tenant service or SNI-routed SMTP gateway, one tenant can prime the cache so that a victim transport connects to the attacker's TLS virtual host, accepts the attacker's certificate with `rejectUnauthorized: true`, and sends the victim's SMTP credentials to it. ## Affected component - **Ecosystem:** npm - **Package:** `nodemailer` - **Repository:** https://github.com/nodemailer/nodemailer - **Tested version:** `10.0.1` - **Tested commit:** `40d52215aac65b811d7e131bc916f68605efd9d2` - **Runtime-confirmed vulnerable versions:** `5.0.0` and `10.0.1` - **Affected versions:** `>= 5.0.0, (); ``` Although the cache key contains only `host`, the cached value contains both DNS addresses and the request-specific TLS identity: ```ts const value: DnsCacheValue = { addresses: allAddresses, servername: options.servername || host }; dnsCache.set(host, { value, expires: Date.now() + (options.dnsTtl || DNS_TTL) }); ``` On a cache hit, `resolveHostname()` returns the cached `servername` without considering the current call's `options.servername`: ```ts if (!cached.expires || cached.expires >= now) { return callback( null, formatDNSValue(cached.value, { cached: true }) ); } ``` `formatDNSValue()` copies that stale value into the result: ```ts return Object.assign( { servername: value.servername, host, _addresses: addresses }, extra || {} ); ``` For a direct TLS connection, `SMTPConnection.connect()` initially copies the current transport's TLS configuration into `opts`. `_resolveAndConnect()` then overwrites every truthy field with the cached resolver result, including `opts.servername`: ```ts Object.assign(opts, this.options.tls || {}); if (this.servername && !opts.servername) { opts.servername = this.servername; } return this._resolveAndConnect(opts, resolved => { this._connectToHost(opts, this.secureConnection); }); ``` ```ts for (const key of Object.keys(resolved!)) { if (key.charAt(0) !== '_' && (resolved as { [key: string]: any })[key]) { (opts as { [key: string]: any })[key] = (resolved as { [key: string]: any })[key]; } } ``` The resulting `opts` object is passed to `tls.connect()`. Node therefore sends the cached server name as SNI and verifies the certificate against that cached name, rather than against the server name explicitly configured for the current transport. The default DNS cache TTL is five minutes: ```ts const DNS_TTL = 5 * 60 * 1000; ``` ### Code path ```text Tenant A: createTransport({ host: H, secure: true, tls: { servername: attackerName } }) -> SMTPConnection.connect() -> _resolveAndConnect(opts) -> shared.resolveHostname({ host: H, servername: attackerName }) -> dnsCache.set(H, { addresses, servername: attackerName }) Victim: createTran

Affected packages

Remediation: Upgrade to 10.0.2 or later.

References

• https://github.com/advisories/GHSA-6vj9-mwq6-2f5v