GHSA-g57g-f23g-4646: Input Validation

Summary

Nodemailer's address parser can produce an unexpected recipient address when an RFC 5322 comment follows the domain of an address whose local-part is a quoted string.

For example:

text "user"@example.com(x)evil.com

is parsed as:

text { address: "user@example.com evil.com", name: "" }

The resulting address therefore contains additional attacker-controlled domain text separated by a literal space.

The parsed .address value is subsequently propagated into the SMTP envelope:

text src/addressparser/index.ts ↓ recipient.address ↓ src/mime-node/index.ts ↓ envelope.to

The envelope construction uses the parsed address without another strict address validation step.

This appears to be a variant of the RFC 5322 comment parsing issue addressed by GHSA-cc9r-2j5m-2m83, but it follows a different parser path when the local-part is quoted.

Reproduction

Input:

text "user"@example.com(x)evil.com

Observed parser output:

text address: "user@example.com evil.com" name: ""

A second example:

text "a"@b.com(c)d.com(e)f.com

produces:

text address: "a@b.com d.com f.com" name: ""

For comparison, the corresponding unquoted form:

text user@example.com(x)evil.com

takes a different code path and is handled by the existing protection differently.

Technical Details

The issue is caused by different parsing behavior for quoted and unquoted local-parts.

The quoted-local-part variant allows the comment-separated trailing domain atoms to remain in the resulting .address value.

That value is then used when constructing the message envelope:

text envelope.to = recipients.map(to => to.address as string)

No additional strict recipient validation is performed at this boundary.

Security Impact

The confirmed impact is that attacker-controlled comment content can result in a malformed/ambiguous recipient address being accepted by the parser and propagated into envelope.to.

The reporter did not confirm successful delivery to an unintended recipient through a real SMTP server using this exact quoted-local-part variant.

The remaining question is how real SMTP servers and other Nodemailer transports handle an envelope recipient containing a value such as:

text user@example.com evil.com

An end-to-end SMTP test is required to determine whether this parser behavior results in an exploitable delivery or recipient-validation bypass.

Relationship to GHSA-cc9r-2j5m-2m83

This report is intended as a potential variant/follow-up to GHSA-cc9r-2j5m-2m83.

The existing advisory addresses RFC 5322 comment handling in email domains. This report identifies a separate parser path involving quoted local-parts that can preserve additional domain-like content in the normalized address.

Please evaluate whether this behavior is already covered by the existing fix or represents a remaining parser variant.

Suggested Fix

The parser should consistently reject or correctly terminate addresses containing trailing domain atoms after RFC 5322 comments, regardless of whether the local-part is quoted.

In particular:

text "user"@example.com(x)evil.com

should not result in:

text user@example.com evil.com

The envelope-generation layer should also avoid assuming that a parser-produced address is safe for SMTP delivery without appropriate validation.

Verification Status

Confirmed:

Parser accepts the quoted-local-part variant. Parser produces an address containing additional attacker-controlled text. The resulting address is propagated into envelope.to.

Not confirmed by reporter:

Successful delivery through a real SMTP server. Delivery to an unintended recipient. Exploitability against a specific downstream SMTP implementation.