Nodemailer: Quadratic (O(n^2)) SMTP client reply parsing lets a malicious server or MITM cause a pre-auth remote denial of service

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-110872: 0.36% estimated exploitation activity in the next 30 days; percentile 29.24 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The SMTP client response parser reassembles multiline server replies by appending each continuation line onto a single response-queue entry and re-testing whether that entry is still a partial (continuation) reply on every line. The partial-response test runs `str.slice(str.lastIndexOf('\n') + 1)` on the growing entry, which forces a full flatten of the entire accumulated string on each line. A malicious or compromised SMTP server (or an on-path attacker on a plaintext or pre-STARTTLS hop) that streams continuation lines and never sends a completion line drives O(n^2) CPU on the single-threaded Node.js event loop before any authentication, freezing the whole process for an attacker-controlled duration.

Root Cause The invariant that a server cannot cause superlinear client-side work while a reply is being reassembled is violated. In `src/smtp-connection/index.ts`, `_onData` splits inbound bytes into lines and, while the last queued entry is a continuation reply,…

Published by the source

27 Oct 2026, 15:04 UTC

Last modified by the source

27 Oct 2026, 15:15 UTC

Source last checked

28 Oct 2026, 06:00 UTC

1. Revision 2 · amended · 28 Oct 2026

   Amended from osv record GHSA-4g23-2xm8-66gc

2. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-4g23-2xm8-66gc

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# Patch Next advisory brief: CVE-2026-110872

Advisory: Nodemailer: Quadratic (O(n^2)) SMTP client reply parsing lets a malicious server or MITM cause a pre-auth remote denial of service
IDs: CVE-2026-110872, GHSA-4g23-2xm8-66gc
Record: https://patchnext.com/advisories/ghsa-4g23-2xm8-66gc (revision 2, source checked 28 Oct 2026, 06:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nodemailer 10.0.10. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm nodemailer: affected >= 3.0.0, < 10.0.10; fixed in 10.0.10

## Suggested inspection (read-only)
- `npm ls nodemailer --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nodemailer.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-4g23-2xm8-66gc (GitHub-reviewed advisory GHSA-4g23-2xm8-66gc lists affected ranges: npm nodemailer >=3.0.0 <=10.0.9)
- Verified: https://osv.dev/vulnerability/GHSA-4g23-2xm8-66gc (OSV record GHSA-4g23-2xm8-66gc lists affected ranges: npm nodemailer >=3.0.0 <10.0.10 (fixed))

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