Executive brief
Nodemailer, a widely-used email sending library for Node.js, has a flaw in how it caches DNS lookups for SMTP connections. When multiple email transports connect to the same server using different security identities (SNI values), the library reuses cached security settings from the first connection. An attacker in a multi-tenant service can exploit this to intercept email credentials by priming the cache and forcing victim connections to accept the attacker's certificate.
Technical details
The vulnerability is a cache poisoning issue in the process-global DNS cache (keyed only by hostname) that also stores caller-specific TLS servername values. When a second direct TLS/SMTPS transport (secure: true) looks up the same non-IP host, the cached servername from the first transport overwrites the current transport's configured value via Object.assign(), causing incorrect SNI and certificate validation against the wrong identity. An authenticated attacker in a multi-tenant deployment can prime the cache with their malicious servername, causing victim transports to connect to attacker-controlled virtual hosts and leak SMTP credentials despite rejectUnauthorized: true.
Affected products
- Nodemailer Nodemailer 5.0.0 through 10.0.1
Timeline
- 2026-09-26: disclosed
- 2026-09-26: patched: Fixed in version 10.0.2