Key points
- Nodemailer versions before 10.0.13 contain a regular expression denial of service vulnerability rated 8.7 on the CVSS scale.
- Attackers can exploit overlapping whitespace patterns in AUTH mechanism detection to block the Node.js event loop for extended periods.
- The issue is triggered when a malicious server replies to EHLO commands with specific long whitespace runs and non-matching tokens.
Nodemailer versions earlier than 10.0.13 are affected by a high-severity security flaw that can freeze Node.js applications. The National Vulnerability Database records this issue as CVE-2026-110869 with a CVSS score of 8.7. The vulnerability resides in the SMTPConnection._actionEHLO function, which handles initial server communication. Users running any version of Nodemailer prior to 10.0.13 must update to resolve this exposure. The source material does not confirm whether a fix or workaround exists other than the version update.
In plain English
The flaw involves how the software processes text patterns during email authentication. Specifically, four regular expressions used to detect authentication mechanisms contain overlapping whitespace sub-patterns. When these patterns encounter certain inputs, they backtrack quadratically, consuming excessive processing power. This behavior effectively halts the application’s ability to perform other tasks. The result is a denial of service condition that blocks the Node.js event loop, preventing the program from processing subsequent events efficiently.
The background
According to the NVD record, the weakness is classified under CWE-1333. The trigger occurs when a Node.js application attempts to connect to an SMTP server. If that server is malicious or compromised by a man-in-the-middle attacker, it can send a crafted response to the EHLO command. This response includes a 250-AUTH status followed by a long run of whitespace and a token that does not match any expected authentication method.
What changes now
This specific input sequence causes the regular expression engine to engage in excessive backtracking. The process can block the Node.js event loop for durations ranging from tens of seconds to several hours. During this time, the application becomes unresponsive to further events. This impact is significant for any system relying on Nodemailer, as it can halt critical communication workflows. The vulnerability exists in all versions of Nodemailer released before the 10.0.13 update.
Who needs to act first
Teams should prioritize updating systems that handle external email connections or process messages from untrusted sources. Check which servers expose Nodemailer to public-facing SMTP endpoints first. Roll out the update to these high-exposure environments before internal tools. Test the new version in a staging environment to ensure compatibility with existing email templates and authentication configurations. Back up current application configurations before applying changes. Monitor application logs for unusual latency or hanging processes after the update to confirm the event loop remains responsive.
What to do and how to stay safe: Nodemailer
- Verify which versions of Nodemailer are installed across your development and production environments.
- Update all instances to version 10.0.13 or later once the vendor provides an update.
- Monitor application performance metrics for signs of event loop blocking or increased latency.
- Review SMTP server configurations to ensure they are not connecting to untrusted or unknown mail servers.
Step-by-step guide: Out-of-Band Patches: Meaning, Risks, and Deployment Reality
General security guidance from the Patch Herald newsroom. It is not confirmed advice from the organisations named in this story.
Frequently asked questions
Which versions of Nodemailer are affected by this vulnerability?
All versions of Nodemailer prior to 10.0.13 are affected by this regular expression denial of service flaw.
What is the severity rating for CVE-2026-110869?
The National Vulnerability Database rates this vulnerability as high severity with a CVSS score of 8.7.
How does an attacker exploit this vulnerability?
A malicious SMTP server can reply to EHLO commands with a long whitespace run and a non-matching token to trigger excessive backtracking.
