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CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

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A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

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Total vulnerabilities in the database

Nodemailer: CRLF injection in Nodemailer List-* header comments allows arbitrary message header injection — nodemailer

Improper Neutralization of CRLF Sequences ('CRLF Injection')

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 related list-header work in 9b4f90a (v3.1.8), but older versions were not dynamically tested during this audit.
Affected range is therefore recorded as unknown beyond the confirmed current version.
No patched version was identified in this checkout.

Severity rationale:

AV: The vulnerable library path is reached through application-level message submission in typical networked applications that use Nodemailer.
AC: A single CRLF sequence in a documented message option triggers the issue.
PR: Conservative assumption that the attacker is a lower-privileged user of an application that exposes list metadata fields. Some applications could expose this to unauthenticated users, but that was not assumed.
UI: No maintainer or victim interaction is needed after the application accepts the message object.
S: The impact remains in the application/mail-generation security scope.
C/I: Injected headers can affect message metadata, mail-client/filter interpretation, and downstream mail-pipeline decisions. No SMTP envelope recipient injection or code execution was demonstrated.
A: No availability impact was demonstrated.

Final self-review:

Reproduction evidence was generated locally from this checkout with a safe in-memory streamTransport PoC and a negative Subject control case.
The PoC is non-destructive and does not send network traffic outside the process.
The observed output contains an actual CRLF-delimited injected header line.
Reachability, sanitizer bypass, package exposure, variants, and non-exploitable sibling paths were checked as described above.
The affected range is not overclaimed; only the current tested version is confirmed vulnerable.

PoC

From a clean checkout of nodemailer at commit 15138a84c543c20aa399218534cdbbfa2ea1ce55, run:

node &lt;&lt;&#039;NODE&#039;
&#039;use strict&#039;;
const nodemailer = require(&#039;./&#039;);
const headersEnd = raw =&gt; raw.slice(0, raw.indexOf(&#039;\r\n\r\n&#039;));
const hasStandaloneInjected = raw =&gt; /\r\nX-Injected: yes\)/.test(raw) || /\r\nX-Injected: yes\r\n/.test(raw);
(async () =&gt; {
  const transport = nodemailer.createTransport({ streamTransport: true, buffer: true });
  const positive = await transport.sendMail({
    from: &#039;[email protected]&#039;,
    to: &#039;[email protected]&#039;,
    subject: &#039;control&#039;,
    list: { unsubscribe: { url: &#039;https://example.test/u&#039;, comment: &#039;ok\r\nX-Injected: yes&#039; } },
    text: &#039;body&#039;
  });
  const positiveRaw = positive.message.toString(&#039;utf8&#039;);
  console.log(&#039;POSITIVE_HAS_INJECTED=&#039; + hasStandaloneInjected(positiveRaw));
  console.log(&#039;POSITIVE_LIST_LINE=&#039; + JSON.stringify(headersEnd(positiveRaw).split(&#039;\r\n&#039;).filter(line =&gt; /^List-Unsubscribe:|^X-Injected:/.test(line)).join(&#039;\n&#039;)));

  const control = await transport.sendMail({
    from: &#039;[email protected]&#039;,
    to: &#039;[email protected]&#039;,
    subject: &#039;safe\r\nX-Injected: no&#039;,
    text: &#039;body&#039;
  });
  const controlRaw = control.message.toString(&#039;utf8&#039;);
  console.log(&#039;CONTROL_HAS_INJECTED=&#039; + /\r\nX-Injected: no\r\n/.test(controlRaw));
  console.log(&#039;CONTROL_SUBJECT=&#039; + JSON.stringify(headersEnd(controlRaw).split(&#039;\r\n&#039;).filter(line =&gt; /^Subject:|^X-Injected:/.test(line)).join(&#039;\n&#039;)));

  const variantKeys = [&#039;help&#039;, &#039;unsubscribe&#039;, &#039;subscribe&#039;, &#039;post&#039;, &#039;owner&#039;, &#039;archive&#039;, &#039;id&#039;];
  const result = [];
  for (const key of variantKeys) {
    const info = await transport.sendMail({
      from: &#039;[email protected]&#039;,
      to: &#039;[email protected]&#039;,
      subject: &#039;variant &#039; + key,
      list: Object.assign({}, { [key]: { url: key === &#039;id&#039; ? &#039;example.test&#039; : &#039;https://example.test/&#039; + key, comment: &#039;c\r\nX-Variant-&#039; + key + &#039;: yes&#039; } }),
      text: &#039;body&#039;
    });
    result.push(key + &#039;=&#039; + new RegExp(&#039;\\r\\nX-Variant-&#039; + key + &#039;: yes&#039;).test(info.message.toString(&#039;utf8&#039;)));
  }
  console.log(&#039;VARIANTS=&#039; + result.join(&#039;,&#039;));
})().catch(err =&gt; { console.error(err &amp;&amp; err.stack || err); process.exit(1); });
NODE

Observed output in this environment:

POSITIVE_HAS_INJECTED=true
POSITIVE_LIST_LINE=&quot;List-Unsubscribe: &lt;https://example.test/u&gt; (ok\nX-Injected: yes)&quot;
CONTROL_HAS_INJECTED=false
CONTROL_SUBJECT=&quot;Subject: safe X-Injected: no&quot;
VARIANTS=help=true,unsubscribe=true,subscribe=true,post=true,owner=true,archive=true,id=true

Expected vulnerable output: POSITIVE_HAS_INJECTED=true and all listed variants ending in =true. Expected negative/control output: CONTROL_HAS_INJECTED=false, showing the ordinary Subject header path does not create a separate injected header.

Cleanup: none required; the PoC uses only in-memory message generation.

Impact

A lower-privileged attacker who can influence list.*.comment fields in an application using Nodemailer can inject arbitrary additional headers into generated email messages. This can alter message semantics and downstream mail-client or mail-filter behavior, including adding attacker-controlled metadata headers. The PoC confirms header-boundary injection in the generated RFC822 output; it does not demonstrate SMTP command injection, recipient injection, or code execution.

Suggested remediation

Normalize or reject CR and LF in list.*.comment before constructing prepared List-* headers. Prefer sharing the same CRLF-neutralization behavior used for ordinary header values, or avoid using prepared: true for caller-controlled list comment content. Add regression tests for CRLF in every documented list comment-bearing field and verify that generated messages do not contain attacker-controlled standalone headers.

Published: Jun 15, 2026
Updated: Jun 16, 2026
GHSA: GHSA-268h-hp4c-crq3
Severity: Medium
Exploit:
CISA KEV:

CVSS v3:

Severity: Medium
Score: 5.4
AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

CWEs:

CWE-93

Affected Software
References

Software	From	Fixed in
nodemailer	-	8.0.9

https://github.com/nodemailer/nodemailer/security/advisories/GHSA-268h-hp4c-crq3

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.