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fkie_cve-2026-110871

Nodemailer from 3.0.0 before 10.0.10 contains a denial of service vulnerability in MessageParser.parseHeaders that allows users to exhaust CPU through quadratic header unfolding. Attackers can supply large recipient lists or long headers that fold into tens of thousands of continuation lines, blocking the Node.js event loop during DKIM signing.

GHSA-39m8-27wv-hr27 — Nodemailer: Quadratic (O(n^2)) DKIM header unfolding lets a large folded header cause remote denial of service during s…

Affected: npm: nodemailer

## Summary

When nodemailer DKIM-signs an outbound message, its DKIM message parser unfolds RFC 5322 header continuation lines with a loop that concatenates each continuation line onto the preceding line and then re-tests the anchored regex `/^[ \t]/` against the growing merged line on every iteration. Re-testing the regex against the ever-growing accumulated line forces a full string flatten proportional to the accumulated length on each iteration, yielding O(n^2) CPU in the number of continuation lines. An application user who influences the size of a header that folds into many continuation lines (most naturally a large recipient list, but also a long custom header, Subject, or References) can drive tens of thousands of continuation lines and block the single-threaded Node.js event loop for seconds during signing, a repeatable denial of service.

## Root Cause

The invariant that signing a message performs work linear in the message size is violated in the header-unfolding step. In `src/dkim/message-parser.ts`, `parseHeaders()` splits the buffered header block into `lines` and iterates backward; for each line matching the continuation pattern `/^[ \t]/`, it does `lines[i - 1] += '\n' + lines[i]` and `lines.splice(i, 1)`. The dominant cost is not the array splice: iterating backward keeps each splice near the shrinking tail, so it shifts only the few trailing header lines. The quadratic cost is the anchored regex test `/^[ \t]/.test(lines[i])` (together with the `+=` conca…

UBUNTU-CVE-2026-110871

Affected: Ubuntu:20.04:LTS: node-nodemailer, Ubuntu:22.04:LTS: node-nodemailer, Ubuntu:24.04:LTS: node-nodemailer, Ubuntu:26.10: node-nodemailer, Ubuntu:26.04:LTS: node-nodemailer