CVE-2026-110871: Nodemailer quadratic complexity in DKIM header unfolding

CVE-2026-110871 · Severity: medium · CVSS 6.5 · Published 2026-10-14

Nodemailer is a Node.js library that sends emails, including signing them with DKIM for deliverability. When processing large recipient lists or long headers, the library's DKIM signing code runs in quadratic time, causing the email sending process to freeze for several seconds per message. An attacker who can influence the recipient list or headers in a mail-sending application can repeatedly stall that application's entire event loop, blocking all other work.

Technologies: Nodemailer, nodemailer (npm). Vendors: Nodemailer, npm.

Quick answers

Is CVE-2026-110871 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-110871 being exploited in the wild.
How severe is CVE-2026-110871?
Medium severity, with a CVSS score of 6.5, published 14 October 2026.
Which products and versions are affected?
Nodemailer 3.0.0 before 10.0.10; nodemailer (npm) >= 3.0.0, <= 10.0.9, SEMVER:introduced=3.0.0,fixed=10.0.10.
Is there a fix for CVE-2026-110871?
Yes. nodemailer (npm): upgrade to 10.0.10 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-110871 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-110871?
Check whether you run Nodemailer or nodemailer (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • nodemailer (npm): upgrade to 10.0.10 or later
    npm install nodemailer@10.0.10

Technical details

The vulnerability is a quadratic time complexity (O(n²)) in MessageParser.parseHeaders during RFC 5322 header continuation-line unfolding. The code re-tests an anchored regex against a growing accumulated line on each iteration, forcing full string flattening proportional to accumulated length. An attacker can supply a large recipient list or custom header that folds into tens of thousands of continuation lines, starving the single-threaded Node.js event loop for seconds during DKIM signing; the precondition is that DKIM signing is enabled on the transport.

Affected products

  • Nodemailer Nodemailer 3.0.0 before 10.0.10

Identifiers

  • CVE-2026-110871
  • GHSA-39m8-27wv-hr27

Timeline

  • 2026-09-30: disclosed
  • 2026-10-14: advisory

References

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