string-format-kitnpm
Advisory published Updated
string-format-kit is a confirmed malicious npm package (MAL-2026-11108) that steals credentials and exfiltrates sensitive data (malicious versions 1.0.1, 1.0.2). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in string-format-kit (npm)
What this malware does
The package's postinstall.js lifecycle script executes a shell pipeline via child_process.exec on npm install. The pipeline reads ~/.ssh/id_rsa, ~/.ssh/authorized_keys, ~/.ssh/known_hosts, ~/.aws/credentials, ~/.bash_history, the full process environment, and host reconnaissance output (whoami, hostname, id, sudo -ln, uname, /etc/os-release, /proc/1/cgroup, /.dockerenv, ps aux, ip addr, ls -la /), base64-encodes the aggregate, and transmits it via curl over plain HTTP to a hardcoded Burp Collaborator subdomain at pzs5w7ntzhsnepwk564lyfdci3oucl0a.oastify.com/d. The declared package purpose (string formatting) has no legitimate need to read the installer's SSH private keys or cloud credentials. The name resembles legitimate string-formatting libraries, consistent with typosquatting to lure installations.
The OpenSSF Package Analysis project identified 'string-format-kit' @ 1.0.2 (npm) as malicious.
It is considered malicious because:
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The package communicates with a domain associated with malicious activity.
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The package executes one or more commands associated with malicious behavior.
Malicious versions
Indicators of compromise (SHA-256)
Detection & response playbook
Credential / info stealerFind it
Search your lockfiles and build artifacts for string-format-kit (2 malicious versions).
If you installed it — respond
string-format-kit is built to steal secrets, so assume every credential the build or runtime could read is compromised. Remove it from your project and lockfile, then rotate ALL exposed secrets — npm/registry tokens, cloud keys, CI/CD secrets, SSH keys, and any .env values — from a known-clean machine. Audit logs for unauthorized use of those credentials.
Did it already run?
If string-format-kit 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
Campaign
References
Credits
- Amazon Inspector · finder
- OpenSSF: Package Analysis · finder
Detect & block this
O3 blocks string-format-kit-class packages before install and in CI — and if it already ran, its runtime egress monitoring catches the credential exfiltration and severs the channel.