---

Jenkins is a cornerstone for automation in CI/CD pipelines, widely trusted by developers everywhere. But sometimes, even its plugins can have major security missteps. In this long-form read, we’ll break down a recent vulnerability: CVE-2023-40340, affecting the NodeJS Plugin up to version 1.6.. This bug results in credentials showing up in plain text inside Jenkins build logs — a nightmare, especially if those logs are stored or shared!

Below, you’ll find a breakdown of what happened, why it’s bad, how it can be exploited, code snippets to see it in action, plus helpful links for more reading.

What is CVE-2023-40340?

CVE-2023-40340 is a vulnerability in the Jenkins NodeJS Plugin (versions 1.6. and earlier) where credentials meant to be masked out in log outputs — usually replaced with <b></b> to prevent leaking sensitive info — are NOT masked if set through the Npm config file in a Pipeline.

Impact:
Anyone with access to the Jenkins build logs could directly view authentication tokens, private registry passwords, or access keys — potentially giving them control over private packages or other protected resources.

How the Vulnerability Works

Normally, Jenkins “masks” credentials, so if someone tries to print them in a log, the output just shows [<b></b>] instead.

But in this case, when pipeline scripts supply private NPM credentials (like registry tokens) through NodeJS Plugin’s configuration — for example via the .npmrc file, or ENV variables — these values get written as plain text to the build logs.

That means secrets meant for npm (like //registry.npmjs.org/:_authToken=YOUR_SECRET_TOKEN) can end up *fully readable* if the job logs the NPM config, or if npm itself prints something sensitive due to an error or info command.

Here’s a simple Pipeline snippet. Let’s say you’re using a Jenkinsfile with the NodeJS Plugin

pipeline {
    agent any
    environment {
        NPM_TOKEN = credentials('npm-private-token') // Jenkins credentials store
    }
    stages {
        stage('Install Packages') {
            steps {
                // This creates a .npmrc with the token
                writeFile file: '.npmrc', text: "//registry.npmjs.org/:_authToken=${env.NPM_TOKEN}";
                // Now install dependencies
                sh 'npm install'
            }
        }
        stage('Debug Info') {
            steps {
                // Here’s the toxic part — this can leak secrets
                sh 'cat .npmrc'
            }
        }
    }
}


Normally, Jenkins would mask ${env.NPM_TOKEN} in the log output.

Sample output

[Pipeline] sh
+ cat .npmrc
//registry.npmjs.org/:_authToken=ACTUAL_SECRET_VALUE


Anyone can now grab ACTUAL_SECRET_VALUE and impersonate you at npmjs.org. Ouch.

Credential leaks are security disasters.

- Logs are often stored for months/years, backed up, and sometimes shipped to external log aggregators.

Normal Jenkins masking won’t happen if the plugin’s logic is wrong or incomplete.

A Jenkins admin might think logs are clean and safe to share, but this bug silently defeats that assumption.

Exploit Details

Exploitation is Dead Simple:  
Anyone able to see build logs (including logs shipped to Splunk or another SIEM) can simply search for lines like:

//registry.npmjs.org/:_authToken=XXXXXXXX


Or, if you have script injection abilities (user or attacker controls the Jenkinsfile), you can force a dump:

sh 'cat ~/.npmrc || cat .npmrc'

Or just run

sh 'npm config list'


If the masking breaks, the secret will show.

No special privileges or advanced attack skills needed.
Just viewing logs is enough!

Update the NodeJS Plugin to at least version 1.6.1 — this includes the patch.

NodeJS Plugin Changelog
- Scrub old log files in your Jenkins history and backups, since previous builds may have already leaked secrets.

`

//registry.npmjs.org/:_authToken=[a-zA-Z-9_\-]+

References

- Official Jenkins Security Advisory (August 2023)
- CVE Details: CVE-2023-40340
- NodeJS Plugin on GitHub
- Best Practices: Jenkins Credentials
- Node.js Security WG Advisories

Closing Thoughts

CVE-2023-40340 shows how even basic features like log masking can fail, causing credential leaks with real-world fallout — not just theoretical risk.  
If your Jenkins build scripts ever handled NPM tokens or other secrets in logs or shell commands, audit and rotate now! This kind of issue is a classic reminder: updates are for security, not just features.

Was this helpful?

Drop a comment or share your Jenkins war stories below!

Timeline

Published on: 08/16/2023 15:15:00 UTC
Last modified on: 08/22/2023 18:56:00 UTC