CVE-2026-92596

Summary

Nodemailer resolves an international (IDN / non-ASCII) recipient domain to a different Punycode xn-- label than every UTS‑46‑conformant parser (web browsers, the WHATWG URL Standard, Node's url.domainToASCII, Python's idna). Its address normalizer (_normalizeAddress in lib/mime-node/index.js) uses the bundled raw RFC‑3492 Punycode codec with no UTS‑46 mapping/normalization, so a domain that a standards‑compliant validator maps to a trusted domain is delivered by Nodemailer to a different, attacker‑registrable domain.

An application that applies a domain allow‑list / same‑domain check to a recipient using a normal IDN‑aware parser (or that shows the normalized recipient to a user for confirmation) and then relies on Nodemailer to deliver to that domain can be induced to send email to an unintended external domain. This is the same weakness class as CVE‑2025‑13033 (Interpretation Conflict, CWE‑436) but reached through IDN/Punycode rather than quoted local‑parts, and it is not addressed by the 7.0.7 fix.

Because the mismatch can be triggered with an invisible character (U+00AD SOFT HYPHEN) that UTS‑46 folds away to the exact trusted domain string, no visible look‑alike/homograph is required.

Details

lib/mime-node/index.js → _normalizeAddress(address) (around lines 1307–1346) splits the address at the last @ and normalizes the domain like this:

// lib/mime-node/index.js
try {
    if (/[\x80-￿]/.test(user)) {
        encodedDomain = punycode.toUnicode(domain.toLowerCase());   // line ~1338
    } else {
        encodedDomain = punycode.toASCII(domain.toLowerCase());     // line ~1340
    }
} catch (_err) {
    // keep domain as supplied
}
return `${this._normalizeLocalPart(user)}@${encodedDomain}`;         // line ~1346

punycode here is the project’s bundled codec (lib/punycode/), which is a pure RFC 3492 (Punycode) implementation. The only normalization applied to the domain is .toLowerCase(). It performs none of the UTS‑46 “IDNA2008 + compatibility processing” steps that browsers and DNS‑facing resolvers apply before Punycode encoding, specifically:

• removing Ignored code points such as U+00AD SOFT HYPHEN,
• Mapping full‑width / compatibility characters to their canonical ASCII forms,
• Unicode NFC normalization,
• validity checks.

As a result, for any domain containing a UTS‑46‑mapped or ‑ignored character, Nodemailer’s punycode.toASCII(...) produces a different A‑label than url.domainToASCII(...) (Node ≥ 7 / WHATWG), new URL('http://'+domain), browsers, and Python’s idna (uts46=True). Nodemailer then uses its A‑label as:

• the SMTP envelope recipient written to the wire as RCPT TO:<local@xn--…> (getEnvelope() → lib/smtp-connection/index.js _setEnvelope), and
• the address emitted in the To: / From: headers (_convertAddresses).

So the domain a standards‑compliant validator computes and the domain Nodemailer actually delivers to disagree, on a syntactically valid, validator‑accepted address. Concrete divergences (verified on 9.0.6):

| recipient (raw) | UTS‑46 parser (url.domainToASCII) | Nodemailer delivers to | |---|---|---| | victim@compa{U+00AD}ny.com (invisible soft hyphen) | company.com | xn--company-pka.com | | victim@ｃｏｍｐａｎｙ.com (full‑width) | company.com | xn--mi7cd4afch9d.com | | user@exámple.com (NFD a+U+0301) | xn--exmple-qta.com | xn--example-vge.com |

This is the “Punycode / IDN parser discrepancy” technique documented in PortSwigger’s Splitting the email atom research (which produced e.g. Joomla CVE‑2024‑21725 and fixes in the PHP idna_convert library). The fix for CVE‑2025‑13033 (nodemailer 7.0.7) hardened the quoted‑local‑part path only; this IDN path is independent and still present in 9.0.6 (latest) and, given the long‑standing use…

Nodemailer before 9.1.0 contains a quadratic time complexity vulnerability in the addressparser component that allows remote attackers to cause denial of service by supplying a crafted comma-separated address list. Attackers can send a single email with a large number of addresses to block the Node.js event loop for extended periods, consuming 100% CPU and freezing the process.

Nodemailer before 9.1.0 contains a quadratic time complexity vulnerability in the addressparser component that allows remote attackers to cause denial of service by supplying a crafted comma-separated address list. Attackers can send a single email with a large number of addresses to block the Node.js event loop for extended periods, consuming 100% CPU and freezing the process.

Nodemailer before 9.1.0 contains a quadratic time complexity vulnerability in the addressparser component that allows remote attackers to cause denial of service by supplying a crafted comma-separated address list. Attackers can send a single email with a large number of addresses to block the Node.js event loop for extended periods, consuming 100% CPU and freezing the process.

Summary

Nodemailer constructs List-* headers from the caller-provided list message option using internally prepared header values. The list.*.comment field is inserted into those prepared values without removing CR (\r) or LF (\n) characters. Because prepared headers bypass the normal header-value sanitizer and are passed to mimeFuncs.foldLines(), a CRLF sequence in a list comment is emitted as an actual header boundary in the generated RFC822 message.

An application that lets a lower-privileged or unauthenticated user influence list.help.comment, list.unsubscribe.comment, list.subscribe.comment, list.post.comment, list.owner.comment, list.archive.comment, or list.id.comment can therefore be made to generate messages containing attacker-chosen additional headers.

Details

Source-to-sink evidence:

• lib/mailer/mail-message.js:241-249 calls _getListHeaders(this.data.list) and adds each returned value with this.message.addHeader(listHeader.key, value).
• lib/mailer/mail-message.js:253-296 builds each list header value as { prepared: true, foldLines: true, value: ... }.
• For List-ID, lib/mailer/mail-message.js:272-279 copies value.comment into the generated header value. If mimeFuncs.isPlainText(comment) returns true, it wraps the comment in quotes rather than encoding or CRLF-normalizing it.
• For the other List-* headers, lib/mailer/mail-message.js:283-288 copies value.comment into (<comment>). If mimeFuncs.isPlainText(comment) returns true, the value is not encoded or CRLF-normalized.
• lib/mime-node/index.js:323-351 accepts the prepared header object.
• lib/mime-node/index.js:533-540 trusts options.prepared; when foldLines is set, it pushes mimeFuncs.foldLines(key + ': ' + value) directly into the header block.
• The normal header-value sanitizer path is bypassed because the value is marked prepared. By contrast, ordinary unprepared header values are normalized in the regular header-building path.
• lib/mailer/mail-message.js:299-308 removes whitespace and angle brackets from list.*.url, so the confirmed injection source is the comment field, not the URL field.

Default/common exposure evidence:

• lib/nodemailer.js:21-60 exposes the public createTransport(...).sendMail(...) flow used by the package.
• examples/full.js:106-123 documents list.unsubscribe.comment and list.id.comment as normal message options.
• The behavior is in shipped runtime code and does not require test-only code, non-default build steps, or undocumented internals.

False-positive screening and negative controls:

• SMTP command construction was separately reviewed. Envelope sender/recipients reject CRLF before SMTP commands, EHLO names strip CRLF, SIZE is numeric, and DSN fields are encoded; no SMTP command-injection variant was confirmed.
• Ordinary subject header input containing CRLF was normalized to a single Subject: header and did not create X-Injected in the local control case.
• Address display names and MIME filename/content-type parameters were reviewed by a focused MIME/header audit and were encoded or CRLF-normalized in local checks.
• prepared: true custom headers are an explicit low-level escape hatch, but this issue is different because Nodemailer itself creates prepared headers from the documented list.*.comment option.

Variant analysis:

Local testing confirmed the same root cause for comments in List-Help, List-Unsubscribe, List-Subscribe, List-Post, List-Owner, List-Archive, and List-ID. These should be fixed together by rejecting or normalizing CR/LF in list comments before prepared header generation, or by avoiding the prepared-header bypass for caller-controlled list values.

Affected version evidence and uncertainty:

• Confirmed vulnerable: nodemailer 8.0.8 at commit 15138a84c543c20aa399218534cdbbfa2ea1ce55.
• Git history shows _getListHeaders present in historical commits including 22fcff8 (v4.3.0) and…

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, '[email protected],'.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', '[email protected],'.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 [email protected]. 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 = '[email protected],'.repeat(n);        // n valid, comma-separated recipients
  const t0 = process.hrtime.bigint();
  addressparser(payload);          …

Summary

Nodemailer's email-address parser treats an RFC 5322 comment ( ... ) inside the domain as a point to concatenate the surrounding text, rather than as folding whitespace (CFWS) that terminates the domain. Consequently a recipient address such as [email protected](x)evil.com is parsed and delivered to good-corp.comevil.com (registrable domain comevil.com, attacker‑controlled), while a conformant RFC 5322 parser terminates the domain at the comment and reads good-corp.com.

An application that decides whether it is allowed to email a recipient by parsing/validating the recipient's domain — with a strict RFC 5322 parser (used without inspecting parse defects) or with a naive prefix/substring allow‑list — and then hands the raw address to Nodemailer for delivery, can be induced to send mail to a domain the attacker controls. This is an Interpretation Conflict (CWE‑436), the same class as CVE‑2025‑13033, reached through the RFC 5322 comment construct (the "Comments" technique in PortSwigger's Splitting the email atom research, which produced a Postfix fix).

Severity is Moderate: exploitation requires the app's domain check to disagree with Nodemailer (see Impact for exactly which parsers do and do not). Verified end‑to‑end against a real RFC 5321 SMTP server (nodemailer 9.0.6 → aiosmtpd).

Details

Root cause is in lib/addressparser/index.js.

1. The tokenizer registers the comment as an operator pair (Tokenizer.operators):

   '(': ')',            // line ~331
   

2. When the closing ) is immediately followed by a non‑break character (anything other than space / tab / CR / LF / , / ;), the tokenizer marks that operator token with noBreak = true:

   // Tokenizer.checkChar, lines ~398-399
   if (nextChr && ![' ', '\t', '\r', '\n', ',', ';'].includes(nextChr)) {
       this.node.noBreak = true;
   }
   

3. _handleAddress then glues the token that follows the comment onto the token that preceded it (dropping the comment):

   // _handleAddress, lines ~187-188
   if (prevToken && prevToken.noBreak && data[state].length) {
       data[state][data[state].length - 1] += token.value;   // <-- concatenation
   }
   

For the input [email protected](x)evil.com the tokens are text:"[email protected]", op:"(", text:"x", op:")" (flagged noBreak), text:"evil.com". Step 3 appends evil.com onto [email protected], producing the single domain good-corp.comevil.com. The comment content (x) is discarded into the display‑name field.

RFC 5322 defines a comment as CFWS — semantically folding whitespace — and it may not appear inside a dot-atom. A comment therefore separates tokens and terminates the domain; the conformant reading of good-corp.com(x)evil.com is the domain good-corp.com (with the trailing evil.com being invalid/ignored). Nodemailer instead concatenates the two atoms across the removed comment, yielding a different, attacker‑registrable domain.

Nodemailer uses the parsed address for both the SMTP envelope (getEnvelope() → RCPT TO) and the emitted To:/From: headers, so the entire message is routed to the concatenated domain.

Related grammar defect (bonus, lower impact): nested comments are legal in RFC 5322, but the tokenizer closes the comment at the first ) (chr === this.operatorExpecting, line ~392), so a valid nested comment such as [email protected](a(b)c) is mis‑balanced and mangled to x.comc). That particular output contains a stray ) and is rejected by a conformant MTA (501) — a bounce/robustness issue, not a misroute.

Suggested fix: treat a comment as folding whitespace that terminates the current token — i.e. do not propagate noBreak across a comment‑closing ) (restrict the noBreak optimization to quoted‑string closes), and support nested comments per RFC 5322. Equivalently, never emit a…

Summary

Nodemailer resolves an international (IDN / non-ASCII) recipient domain to a different Punycode xn-- label than every UTS‑46‑conformant parser (web browsers, the WHATWG URL Standard, Node's url.domainToASCII, Python's idna). Its address normalizer (_normalizeAddress in lib/mime-node/index.js) uses the bundled raw RFC‑3492 Punycode codec with no UTS‑46 mapping/normalization, so a domain that a standards‑compliant validator maps to a trusted domain is delivered by Nodemailer to a different, attacker‑registrable domain.

An application that applies a domain allow‑list / same‑domain check to a recipient using a normal IDN‑aware parser (or that shows the normalized recipient to a user for confirmation) and then relies on Nodemailer to deliver to that domain can be induced to send email to an unintended external domain. This is the same weakness class as CVE‑2025‑13033 (Interpretation Conflict, CWE‑436) but reached through IDN/Punycode rather than quoted local‑parts, and it is not addressed by the 7.0.7 fix.

Because the mismatch can be triggered with an invisible character (U+00AD SOFT HYPHEN) that UTS‑46 folds away to the exact trusted domain string, no visible look‑alike/homograph is required.

Details

lib/mime-node/index.js → _normalizeAddress(address) (around lines 1307–1346) splits the address at the last @ and normalizes the domain like this:

// lib/mime-node/index.js
try {
    if (/[\x80-￿]/.test(user)) {
        encodedDomain = punycode.toUnicode(domain.toLowerCase());   // line ~1338
    } else {
        encodedDomain = punycode.toASCII(domain.toLowerCase());     // line ~1340
    }
} catch (_err) {
    // keep domain as supplied
}
return `${this._normalizeLocalPart(user)}@${encodedDomain}`;         // line ~1346

punycode here is the project’s bundled codec (lib/punycode/), which is a pure RFC 3492 (Punycode) implementation. The only normalization applied to the domain is .toLowerCase(). It performs none of the UTS‑46 “IDNA2008 + compatibility processing” steps that browsers and DNS‑facing resolvers apply before Punycode encoding, specifically:

• removing Ignored code points such as U+00AD SOFT HYPHEN,
• Mapping full‑width / compatibility characters to their canonical ASCII forms,
• Unicode NFC normalization,
• validity checks.

As a result, for any domain containing a UTS‑46‑mapped or ‑ignored character, Nodemailer’s punycode.toASCII(...) produces a different A‑label than url.domainToASCII(...) (Node ≥ 7 / WHATWG), new URL('http://'+domain), browsers, and Python’s idna (uts46=True). Nodemailer then uses its A‑label as:

• the SMTP envelope recipient written to the wire as RCPT TO:<local@xn--…> (getEnvelope() → lib/smtp-connection/index.js _setEnvelope), and
• the address emitted in the To: / From: headers (_convertAddresses).

So the domain a standards‑compliant validator computes and the domain Nodemailer actually delivers to disagree, on a syntactically valid, validator‑accepted address. Concrete divergences (verified on 9.0.6):

| recipient (raw) | UTS‑46 parser (url.domainToASCII) | Nodemailer delivers to | |---|---|---| | victim@compa{U+00AD}ny.com (invisible soft hyphen) | company.com | xn--company-pka.com | | victim@ｃｏｍｐａｎｙ.com (full‑width) | company.com | xn--mi7cd4afch9d.com | | user@exámple.com (NFD a+U+0301) | xn--exmple-qta.com | xn--example-vge.com |

This is the “Punycode / IDN parser discrepancy” technique documented in PortSwigger’s Splitting the email atom research (which produced e.g. Joomla CVE‑2024‑21725 and fixes in the PHP idna_convert library). The fix for CVE‑2025‑13033 (nodemailer 7.0.7) hardened the quoted‑local‑part path only; this IDN path is independent and still present in 9.0.6 (latest) and, given the long‑standing use…

Nodemailer before 9.1.0 contains a quadratic time complexity vulnerability in the addressparser component that allows remote attackers to cause denial of service by supplying a crafted comma-separated address list. Attackers can send a single email with a large number of addresses to block the Node.js event loop for extended periods, consuming 100% CPU and freezing the process.

TuxCare rebuilt nodemailer to address 1 CVE. Fixed in 2.7.2-tuxcare.3.

TuxCare rebuilt nodemailer to address 4 CVEs. Fixed in 6.10.1-tuxcare.8.

TuxCare rebuilt nodemailer to address 4 CVEs. Fixed in 2.7.2-tuxcare.6.

TuxCare rebuilt nodemailer to address 4 CVEs. Fixed in 9.0.1-tuxcare.1.

TuxCare rebuilt nodemailer to address 1 CVE. Fixed in 9.0.5-tuxcare.1.

TuxCare rebuilt nodemailer to address 2 CVEs. Fixed in 9.0.3-tuxcare.1.

TuxCare rebuilt nodemailer to address 11 CVEs. Fixed in 9.0.6-tuxcare.1.

TuxCare rebuilt nodemailer to address 5 CVEs. Fixed in 9.0.5-tuxcare.2.

TuxCare rebuilt nodemailer to address 2 CVEs. Fixed in 9.0.2-tuxcare.1.

TuxCare rebuilt nodemailer to address 3 CVEs. Fixed in 9.0.0-tuxcare.3.

Root has patched GHSA-268h-hp4c-crq3 in the nodemailer package for Root:npm. Multiple fixed versions available.

Root has patched GHSA-2x7j-588g-ccc2 in the nodemailer package for Root:npm. Multiple fixed versions available.