What does the mbo-letters-cl malware do?

The package mbo-letters-cl was found to contain malicious code. The OpenSSF Package Analysis project identified 'mbo-letters-cl' @ 1.9.992 (npm) as malicious. It is considered malicious because: - The package communicates with a domain associated with malicious activity. - The package executes one or more commands associated with malicious behavior.

Which versions of mbo-letters-cl are malicious?

The following npm versions of mbo-letters-cl are flagged malicious: 1.9.99, 1.9.992. Treat any of these as compromised.

How do I remediate mbo-letters-cl if I installed it?

Remove mbo-letters-cl from your project and lockfile, then assume any secrets accessible to the build or runtime were exposed: rotate API keys, tokens, and credentials, and audit for unexpected outbound activity or persistence.

I already installed mbo-letters-cl — how do I know if it already compromised me?

If mbo-letters-cl was installed, its post-install payload may already have run. Removing the package does not undo that — check outbound connections and credential use from the install window onward.

How do I find mbo-letters-cl across my projects, lockfiles, and CI pipelines?

Search your lockfiles and build artifacts for mbo-letters-cl (2 malicious versions).

Malicious package

mbo-letters-clnpm

Advisory published Jan 30, 2026Updated Feb 2, 2026

mbo-letters-cl is a confirmed malicious npm package (MAL-2026-611) that executes malicious code on install (malicious versions 1.9.99, 1.9.992). Do not install it — remove it immediately and rotate any exposed credentials.

Malicious code in mbo-letters-cl (npm)

MAL-2026-611

Immediate action

Remove the package, then rotate any secrets the build/runtime could reach.

npm uninstall mbo-letters-cl

What this malware does

The package mbo-letters-cl was found to contain malicious code.

The OpenSSF Package Analysis project identified 'mbo-letters-cl' @ 1.9.992 (npm) as malicious.

It is considered malicious because:

The package communicates with a domain associated with malicious activity.
The package executes one or more commands associated with malicious behavior.

Malicious versions

2 flagged

1.9.991.9.992

Indicators of compromise (SHA-256)

5f9cd1b0fbc8ed1c3c4975adcaea6990c6ac35de212e800a257b2a5a6164f281

7a038797c4387911dbfd31bb002720eaaf80b56da2385373ce047ce681f3d574

d367723532d1a3f90f88a1d23230230dafb47b5f26a68af29aafda36084f5417

Detection & response playbook

Malicious package

Find it
Search your lockfiles and build artifacts for mbo-letters-cl (2 malicious versions).
If you installed it — respond
Remove mbo-letters-cl from your project and lockfile, then assume any secrets accessible to the build or runtime were exposed: rotate API keys, tokens, and credentials, and audit for unexpected outbound activity or persistence.
Did it already run?
If mbo-letters-cl was installed, its post-install payload may already have run. Removing the package does not undo that — check outbound connections and credential use from the install window onward.

Frequently asked questions

No. mbo-letters-cl on npm has been identified as a malicious package (versions 1.9.99, 1.9.992 flagged). It should be removed immediately — do not install or keep it in your dependency tree.

Credits

Amazon Inspector · finder
OpenSSF: Package Analysis · finder

Detect & block this

O3 blocks mbo-letters-cl-class packages before install and in CI — and if it already ran, its runtime egress monitoring catches the malicious outbound activity and severs the channel.

Malware detection Runtime egress monitoring

Explore

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