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Perl/Calfbot is a Perl-based spam-sending malware component associated with Operation Windigo, a large profit-driven intrusion ecosystem centered on compromised Linux and Unix servers. Within that operation, Perl/Calfbot was used to distribute unsolicited bulk email at very large scale, with observed infrastructure supporting tens of millions of spam messages per day. It operated as one element of a broader toolset that also included Linux/Ebury, an OpenSSH backdoor and credential stealer; Linux/Cdorked, a web traffic redirection component; and Linux/Onimiki, a malicious DNS component.
Perl/Calfbot receives spam-sending jobs from command-and-control infrastructure, processes those jobs, and reports delivery statistics including successful and failed message attempts. Analysis of the wider operation linked Perl/Calfbot to shared infrastructure and overlapping compromised hosts used by other Windigo components. Spam delivered through the ecosystem was monetized alongside web redirection and drive-by malware delivery.
The malware was notable for cross-platform portability inherent to Perl deployments. It was observed affecting Linux and other Unix-like environments, and reporting also noted operation on FreeBSD, OpenBSD, macOS, and Windows when Perl was available through Cygwin. In the Windigo campaign, compromised servers hosting Perl/Calfbot were part of a long-running criminal infrastructure that relied primarily on stolen credentials rather than exploitation of novel server vulnerabilities. Perl/Calfbot is best understood as the spam bot element of that larger server-side malware operation.
Mallory pivots from this family to the IOCs, detections, and named campaigns that touch your stack, and pages you when something new lands.
11 distinct techniques documented for this family, organized by ATT&CK tactic.
The following domains are used to exfiltrate HTTP POST requests over HTTP or HTTPS.
Linux/Onimiki is a DNS server backdoor. These rules will alert on either inbound or outbound DNS requests with the specific Linux/Cdorked URL pattern... Since Perl/Calfbot uses HTTPS... these rules will match specific DNS requests.
First, the complete source code of the targeted web server is downloaded, along with a malicious “diff patch” coming from a server under the attacker’s control.
32 indicators attributed across vendor reports, sandbox runs, and researcher write-ups. Full values are available in Mallory.
IPs, domains, and DNS infrastructure linked to this family.
File hashes (MD5, SHA-1, SHA-256) from samples and reports.
Other indicator types observed in public reporting.
5 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Spam-sending component used in Operation Windigo; capable of operating across multiple OSes where Perl is available (including via Cygwin on Windows).
A lightweight Perl-based spam bot that runs primarily in memory, communicates with C2 over HTTPS using a DGA, validates C2 ownership, and sends large volumes of spam through compromised servers using local mail tools.
Perl/Calfbot is a Perl-based spam bot used in the Windigo operation to process TESTSEND and SEND jobs, validate deliverability, and report spam delivery statistics back to its C2 infrastructure.
A lightweight Perl-based spam bot that runs primarily in memory, communicates with C2 over HTTPS using a DGA, validates C2 ownership, and sends large volumes of spam through compromised servers using local mail tools.
Match every observed IP, domain, and hash against your live telemetry.
Named campaigns wielding this family, with evidence pinned to each claim.
CVEs this family uses for access and lateral movement.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Every documented technique, ranked by evidence weight.
Reddit, Mastodon, and CTI community discussion around this family.