Nodemailer: Quadratic backtracking in SMTP AUTH mechanism detection regexes allows pre-auth remote denial of service

{
  "id": "dfdead9b5673d1ac9c225ef03bbdab22f5ce15b305aae3d3f7257351a0e5b551",
  "url": "https://osv.dev/vulnerability/GHSA-4ffr-jq9g-5ffx",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Nodemailer: Quadratic backtracking in SMTP AUTH mechanism detection regexes allows pre-auth remote denial of service",
    "summary": "## 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 engine must try every partition of the whitespace run between `[^\\n]*` and `\\s+` before failing. The four regexes run unconditionally on every EHLO response, with no length cap, no shape validation, and no early rejection before the engine enters the ambiguous alternation. ```text Component: nodemailer (npm), SMTP client Function: SMTPConnection._actionEHLO Location: src/smtp-connection/index.ts, lines 2162, 2167, 2172, 2177 Patterns: /[ -]AUTH(?:(\\s+|=)[^\\n]*\\s+|\\s+|=)PLAIN/i /[ -]AUTH(?:(\\s+|=)",
    "evidence": "osv:GHSA-4ffr-jq9g-5ffx",
    "products": [
      {
        "name": {
          "value": "nodemailer",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "10.0.13",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "medium",
    "structured": {
      "id": "GHSA-4ffr-jq9g-5ffx",
      "aliases": [
        "CVE-2026-110869",
        "GHSA-4ffr-jq9g-5ffx"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "3.0.0"
                },
                {
                  "fixed": "10.0.13"
                }
              ]
            }
          ],
          "package": {
            "name": "nodemailer",
            "purl": "pkg:npm/nodemailer",
            "ecosystem": "npm"
          },
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-4ffr-jq9g-5ffx/GHSA-4ffr-jq9g-5ffx.json",
            "last_known_affected_version_range": "<= 10.0.12"
          }
        }
      ],
      "modified": "2026-10-27T15:15:07.820609988Z",
      "provider": "osv",
      "references": [
        "https://github.com/nodemailer/nodemailer/security/advisories/GHSA-4ffr-jq9g-5ffx",
        "https://github.com/nodemailer/nodemailer/commit/b5a896fcb02df1628329d7cd665ca08490b14b56",
        "https://github.com/nodemailer/nodemailer",
        "https://github.com/nodemailer/nodemailer/releases/tag/v10.0.13"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-1333"
        ],
        "severity": "MODERATE",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-27T15:04:39Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-110869",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 462821,
  "published_at": "2026-10-27T15:04:39+00:00"
}