Nodemailer: Quadratic backtracking in SMTP AUTH mechanism detection regexes allows 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-2026-110869: 0.37% estimated exploitation activity in the next 30 days; percentile 30.58 (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 Nodemailer's SMTP client applies four regular expressions to a peer's EHLO response to detect the supported SASL mechanisms. Each pattern uses an alternation with two overlapping whitespace-matching sub-patterns, which produces quadratic (O(n^2)) backtracking whenever the mechanism name is absent from the line. A malicious or MITM SMTP server that answers EHLO with `250-AUTH` followed by a long whitespace-only run and a non-matching token blocks the Node.js event loop for tens of seconds to hours during a pre-authentication handshake. No authentication to the target SMTP server is required, and any application that connects to an attacker-influenced host is affected.

Root Cause Each of the four AUTH mechanism regexes in `SMTPConnection._actionEHLO` contains the sub-pattern `(?:(\s+|=)[^\n]*\s+|\s+|=)` before the mechanism literal. `[^\n]*` and the trailing `\s+` both match whitespace and can consume the same characters, so when the mechanism literal is absent, the regex…

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-4ffr-jq9g-5ffx

2. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-4ffr-jq9g-5ffx

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# Patch Next advisory brief: CVE-2026-110869

Advisory: Nodemailer: Quadratic backtracking in SMTP AUTH mechanism detection regexes allows pre-auth remote denial of service
IDs: CVE-2026-110869, GHSA-4ffr-jq9g-5ffx
Record: https://patchnext.com/advisories/ghsa-4ffr-jq9g-5ffx (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 10.0.13. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm nodemailer: affected >= 3.0.0, < 10.0.13; fixed in 10.0.13

## 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-4ffr-jq9g-5ffx (GitHub-reviewed advisory GHSA-4ffr-jq9g-5ffx lists affected ranges: npm nodemailer >=3.0.0 <=10.0.12)
- Verified: https://osv.dev/vulnerability/GHSA-4ffr-jq9g-5ffx (OSV record GHSA-4ffr-jq9g-5ffx lists affected ranges: npm nodemailer >=3.0.0 <10.0.13 (fixed))

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