Malicious code in mutex-core (npm)
The npm package mutex-core, published under a name resembling the popular async-mutex library, contained malicious code including a staged loader that decrypts and executes hidden payloads. The package included obfuscated code that conditionally triggers AES-256-GCM decryption and execution of encrypted binary content when specific conditions are met.
- Disclosed
- Last updated
- Blast radius
- All consumers of the mutex-core npm package
- Ecosystems
- Attack vectors
- Affected entities
- mutex-corenpm package masquerading as async-mutex
The npm package mutex-core was published as a typosquatting variant of the legitimate async-mutex library. Beyond its advertised mutex functionality, the package contained a staged loader mechanism embedded in lib/Mutex.js.
The malicious behavior is triggered in the Mutex.prototype.runExclusive method, which checks whether a callback's meta.jobId hashes to a hardcoded SHA256 digest (b7e54f328f8b2298252479f2a357e13e23701227d1f990a15c598a74c3de5fe5). Upon match, the code invokes a decryption routine from lib/common.js to AES-256-GCM-decrypt the sibling file lib/withMutex.min.js—a ~17KB base64-encoded literal that decodes to ~12.7KB of opaque encrypted bytes—using key material derived from scryptSync applied to the jobId. The decrypted plaintext is written to disk and executed via spawned node process with detached stdio, all wrapped in exception handling.
The lib/common.js file is string-array-obfuscated (numeric-indexed lookup with rotation) and serves only the decrypt-and-drop functionality; it is not referenced by any legitimate mutex behavior. The encrypted payload contents remain opaque to package consumers, making the executed code invisible to inspection.
The incident was identified and credited to the OpenSSF's malicious-packages repository.
Indicators of compromise
- Packages
- mutex-core
- Hashes
- b7e54f328f8b2298252479f2a357e13e23701227d1f990a15c598a74c3de5fe5
Remediation
- Immediately remove mutex-core from all dependencies and package-lock files
- Audit all systems that installed mutex-core for signs of unauthorized code execution or persistence mechanisms
- Replace mutex-core with the legitimate async-mutex package
- Review npm audit logs and CI/CD logs for any execution of mutex-core during the period it was installed
- Consider rotating credentials and secrets on any systems where mutex-core was installed, as the encrypted payload may have exfiltrated sensitive data
Sources
- GitHub Advisory GHSA-94q2-wjj8-835j · GitHub Advisory Database
Cite this entry
"Malicious code in mutex-core (npm)." supplychainattack.org, Supply Chain Attack Incident Catalog. Disclosed August 19, 2026; last updated August 19, 2026. https://supplychainattack.org/incident/malicious-code-in-mutex-core-npm-1z0975
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