CVE-2026-100702

Nodemailer before 10.0.2 fails to properly flatten deeply nested arrays in recipient fields such as to, cc, and bcc, allowing attackers to cause stack exhaustion. Attackers can supply a deeply nested JSON recipient array that triggers recursive Array.toString() conversion, exhausting the call stack and terminating the Node.js process.

Nodemailer before 10.0.2 fails to properly flatten deeply nested arrays in recipient fields such as to, cc, and bcc, allowing attackers to cause stack exhaustion. Attackers can supply a deeply nested JSON recipient array that triggers recursive Array.toString() conversion, exhausting the call stack and terminating the Node.js process.

Nodemailer before 10.0.2 fails to properly flatten deeply nested arrays in recipient fields such as to, cc, and bcc, allowing attackers to cause stack exhaustion. Attackers can supply a deeply nested JSON recipient array that triggers recursive Array.toString() conversion, exhausting the call stack and terminating the Node.js process.

Submission metadata

| Field | Value | |---|---| | Ecosystem | npm | | Package | nodemailer | | Repository | https://github.com/nodemailer/nodemailer | | Tested commit | 40d52215aac65b811d7e131bc916f68605efd9d2 | | Current tested version | 10.0.1 | | Confirmed vulnerable versions | 2.7.2, 3.0.0, 7.0.11, 9.1.1, 10.0.1 | | Proposed affected range | >= 2.7.2, <= 10.0.1 | | Patched version | 10.0.2 |

Summary

Nodemailer 10.0.1 does not safely process deeply nested arrays supplied through recipient fields such as to, cc, and bcc.

The public MimeNodeAddressInput type recursively permits arrays, but MimeNode._parseAddresses() flattens only the outermost array. A remaining nested array is passed to addressparser(), whose Tokenizer coerces the input with .toString(). Native Array.prototype.toString() recursively processes every nested array through join() and toString() until the V8 call stack is exhausted.

A valid 10,021-byte JSON recipient value containing one address wrapped in 5,000 arrays causes:

RangeError: Maximum call stack size exceeded

The exception occurs through the normal sendMail() API before the maxRecipients limit is evaluated. If the integrating application does not catch the synchronous exception around the complete sendMail() invocation, the Node.js worker or server process terminates.

No SMTP server, remote host, large attachment, or successful email delivery is required.

This is distinct from GHSA-rcmh-qjqh-p98v / CVE-2025-14874. The previous vulnerability concerned recursive parsing of RFC 5322 group strings. This report uses Nodemailer's structured recipient-array input and never reaches the parser's MAX_NESTED_GROUP_DEPTH protection.

Security impact

An attacker who can control a recipient field passed to Nodemailer can trigger stack exhaustion using a roughly 10 KB JSON value. Potentially affected integrations include:

• Email-sending HTTP APIs that accept structured recipient values.
• Notification workers consuming JSON jobs from a queue.
• Template or automation systems that forward parsed recipient data to sendMail().
• Multi-tenant applications that allow users to configure message recipients.

When the exception is not caught, a single request or queue item can terminate the process handling mail. A process manager may restart the worker, but repeated malicious inputs can keep workers in a restart loop and make the service unavailable.

Applications that explicitly validate recipient values as flat strings or flat arrays before calling Nodemailer are not exposed through this path. Applications that wrap the complete synchronous sendMail() invocation in exception handling can prevent process termination, although an attacker can still repeatedly force failed jobs.

Attack prerequisites

The vulnerability is reachable when:

1. An application accepts attacker-controlled or partially attacker-controlled recipient data.
2. The application preserves the array structure after parsing JSON.
3. The resulting value is passed to a Nodemailer recipient field such as to, cc, or bcc.
4. The application does not impose its own nesting-depth limit before calling Nodemailer.

The proof of concept uses jsonTransport only to avoid requiring an SMTP server. The vulnerable address normalization and envelope construction occur before transport-specific delivery, so the root cause is not limited to jsonTransport.

Technical details

1. The public input type accepts recursively nested arrays

At src/mime-node/index.ts:67, recipient input is recursively defined:

export type MimeNodeAddressInput = string | MimeNodeAddress | MimeNodeAddressInput[];

Pinned source:

https://github.com/nodemailer/nodemailer/blob/40d52215aac65b811d7e131bc916f68605efd9d2/src/mime-node/index.ts#L67

This permits values equivalent to:

[[[[['[email protected]']]]]]

2. _parseAddresses() removes only the outermost…

Nodemailer before 10.0.2 fails to properly flatten deeply nested arrays in recipient fields such as to, cc, and bcc, allowing attackers to cause stack exhaustion. Attackers can supply a deeply nested JSON recipient array that triggers recursive Array.toString() conversion, exhausting the call stack and terminating the Node.js process.

Nodemailer before 10.0.2 fails to properly flatten deeply nested arrays in recipient fields such as to, cc, and bcc, allowing attackers to cause stack exhaustion. Attackers can supply a deeply nested JSON recipient array that triggers recursive Array.toString() conversion, exhausting the call stack and terminating the Node.js process.