Nodemailer: Denial of service via quadratic SMTP reply parsing
Summary
Nodemailer, an npm library for sending e-mail from Node.js, parses multiline SMTP server replies with quadratic complexity. A malicious SMTP server or an on-path attacker can stream continuation lines without a completion line, which blocks the Node.js event loop and exhausts memory of the sending process. The attack works before authentication, so no client credentials are required, and it affects every other task in the affected process.
Affected
- nodemailer >= 3.0.0 and <= 10.0.9
- Linux distribution packages (node-nodemailer) are under evaluation; status varies by distribution.
Fixed in
- nodemailer 10.0.10 (fix commit f2d82fa84d47015a7bbb4253da61eeb778c658b1)
- Newer releases also fix other Nodemailer DoS advisories; the latest 10.x release is the practical target.
Workaround
- No vendor workaround is known.
- Restrict SMTP hosts to a fixed, trusted list and do not let users or tenants set SMTP hosts.
- Prefer implicit TLS to known relays, since the attack window includes the plaintext hop before STARTTLS.
Exploitation
- CISA SSVC: exploitation status 'poc', automatable: no, technical impact: partial.
- A public proof-of-concept description exists.
- No exploitation in the wild has been reported; the CISA KEV catalogue has not been checked.
Our recommendation
- Upgrade nodemailer to 10.0.10 or later (preferably the latest 10.x release) in all applications and check lock files for transitive dependencies.
- Until patched, restrict SMTP hosts to trusted servers and avoid user-defined SMTP endpoints.
- Monitor the memory and event-loop behaviour of services that send e-mail.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- GitHub Security Advisory GHSA-4g23-2xm8-66gc github.com
- VulnCheck advisory www.vulncheck.com
- Ubuntu security tracker CVE-2026-110872 ubuntu.com
- OSV GHSA-4g23-2xm8-66gc osv.dev
- GitLab advisory GHSA-4g23-2xm8-66gc advisories.gitlab.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 16:11.
Original description (NVD)
Nodemailer from 3.0.0 before 10.0.10 contains a quadratic complexity denial of service vulnerability in the SMTP client reply parser in SMTPConnection._onData and isPartialResponse. A malicious SMTP server or on-path attacker can stream continuation lines without a completion line before authentication, blocking the Node.js event loop and exhausting memory.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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