Astaroth, stego C2, and why browser security helps — but won’t stop everything

Written By John Giglio

The recent Astaroth campaign blended classic steganography with modern cloud tactics—hiding its command-and-control data in GitHub-hosted images. Enterprise browsers like Edge, Island, Talon, and Chrome Enterprise can mitigate some of these risks, but they aren’t a silver bullet. This post breaks down the attack, explains where browser-based protections fall short, and ties prevention guidance to CIS Level 1 Controls and other “security hygiene” fundamentals.

TL;DR (for scrolling brains)

  • What happened: Astaroth used phishing → .lnk + mshta → downloader → AutoIt/shellcode → DLL injection. When its C2 was unreachable it fetched encrypted config hidden inside images on GitHub (steganography) and used that as fallback.

  • Why browsers matter: Managed or enterprise browsers (like Edge for Business, Island, Talon, or Chrome Enterprise) can block malicious downloads, log suspicious GitHub requests, enforce extension policies, and provide telemetry — but they can’t reliably see or stop OS-level keyloggers or injected code running outside the browser.

  • What to do: Combine enterprise browser controls, strong EDR/antivirus, network controls, GitHub monitoring, and incident playbooks. Below I give configuration examples, sample YARA/IDS rules, and cloud CLI snippets to harden infrastructure and automate detection. Each recommendation ties back to CIS Level 1 Controls—the essential cyber hygiene practices every organization should have in place.

The attack, step by step (technical)

  1. Phishing delivery — a convincing email (DocuSign or other lures) delivers a ZIP containing a Windows shortcut (.lnk).
    Related CIS Controls:

    • CIS Control 9.1: Ensure use of only fully supported browsers and email clients.

    • CIS Control 14.1: Establish and maintain a security awareness program to reduce phishing success.

  2. LNK → mshta — the .lnk runs mshta.exe which executes obfuscated JavaScript/HTA that downloads further payloads.
    Related CIS Controls:

    • CIS Control 8.2: Ensure only authorized scripts and binaries execute (application allowlisting).

    • CIS Control 10.1: Enable and configure anti-malware scanning for all file downloads.

  3. Payload chain & Stego fallback C2 — downloaded payloads inject DLLs and pull hidden configs from GitHub images.
    Related CIS Controls:

    • CIS Control 4.2: Configure automatic anti-malware scanning of all downloads and attachments.

    • CIS Control 13.3: Monitor network traffic for unusual connections to external services (like GitHub’s raw CDN).

  4. Persistence & monitoring — .lnk added to startup folder, keylogging hooks monitor banking sessions.
    Related CIS Controls:

    • CIS Control 8.1: Limit privileges and prevent unauthorized startup changes.

    • CIS Control 6.7: Regularly collect and review audit logs.

How enterprise browsers help — and where they stop

If you’re like me, you might be wondering, this seems like a browser-based attack, what about those fancy enterprise browsers, what can they do for me? Well, they can certainly help but they won’t stop this type of attack all together (unless of course they stop the payload from being downloaded, which……could happen). So here’s what Enterprise browsers like Chrome Enterprise Premium, Talon, Island, Edge Enterprise and others CAN do:

  • Block or warn on malicious downloads (Safe Browsing, SmartScreen, or vendor-specific protection). (CIS 10.1)

  • URL filtering / access policies to prevent visits to known malicious sites. (CIS 9.1, 13.6)

  • Extension management — limit risk from unapproved add-ons. (CIS 2.6)

  • DLP / clipboard protections — prevent sensitive data leakage. (CIS 3.3)

  • Device posture & telemetry — integrate with EDR/SIEM. (CIS 8.7, 13.11)

Where browsers cannot help
They can’t see injected DLLs, block LOLBin abuse, or stop OS-level keyloggers. These require endpoint protection (CIS 10.3) and logging (CIS 8.7) at the OS level. Of course, you could also use on OS like ChromeOS which entirely mitigates these types of attacks, but that’s a topic for a future blog post.

Enterprise browser configuration guidance (generalized)

  1. Enable advanced download protection (CIS 10.1)

    • Turn on enhanced phishing/malware scanning and reputation-based blocking.

  2. URL filtering (CIS 9.1, 13.6)

    • Blacklists are a whack-a-mole game. Instead, use allowlists, SWGs, or CASB integrations with real-time intel.

  3. Force downloads through a proxy / CASB (CIS 13.6)

    • Enforce HTTPS inspection and inline DLP/AV scanning.

  4. Extension management (CIS 2.6)

    • Only permit verified extensions; disable unapproved ones.

  5. Enable event reporting & device posture (CIS 8.7, 13.11)

    • Integrate with SIEM; flag anomalous traffic such as mass image downloads.

Important caveat: These policies are useful, but require coordinating deployment (force-installs, proxy certs, and EDR) and user comms. They help a lot — just not 100%.

Endpoint & Network Recommendations

  1. Deploy EDR/EPP everywhere (CIS 10.3)

    • Deploy a modern EDR (CrowdStrike, SentinelOne, Microsoft Defender for Endpoint, Bitdefender, etc.) with behavioral blocking and script control.

    • Ensure EDR is configured to:

      • Block or alert on unsigned DLL injections into system processes.

      • Detect use of LOLBins (mshta, rundll32, regsvr32) executing scripts from the user temp/Downloads dir.

      • Detect spawning of mshta by email clients or LNK files..

  2. Script control / application allowlisting (CIS 8.2)

    • Allowlist privileged execution locations (AppLocker, Windows Defender Application Control, or third-party allowlisting).

    • Disallow execution from user profile/Downloads directories.

  3. Network segmentation & egress control (CIS 13.3)

    • Egress filtering to prevent endpoints from freely connecting to unknown services (use a SWG or firewall).

      • Avoid IP-based blocks as your main control; rely on reputation-based detection.

    • Block or proxy direct outbound connections to suspicious infra like ngrok endpoints unless explicitly allowed.

  4. Logging & detection correlation (CIS 8.7)

    • Feed browser telemetry + EDR logs to a SIEM for correlation. Look for:

      • Repeated image downloads from GitHub by background tasks or non-browser processes.

      • Calls to mshta.exe with remote script URLs.

      • Creation of .lnk in Startup folders.

      • Processes with injected DLLs that hook keyboard APIs.

    • Keep detections tuned to reduce noise and capture the important chains.

Sample detection rules & signatures (starter kits)

Example YARA (host-level) — detect suspicious decoded image artifacts

This YARA is illustrative to flag files that contain a small, encrypted blob signature appended to images. Tune to your environment before using:

rule Stego_Config_Append { meta: author = "803Tech" description = "Detect PNG/JPG files with appended config-like blobs (heuristic)" date = "2025-10-13" strings: $png_magic = { 89 50 4E 47 0D 0A 1A 0A } // PNG header $jpg_magic = { FF D8 FF } // JPG header $stego_marker = "CFGv1" ascii // hypothetical marker seen in some samples (tune accordingly) condition: (uint16(0) == 0x504B or $png_magic or $jpg_magic) and ($stego_marker in file or filesize > 50000 and filesize mod 512 != 0) }

Note: McAfee didn’t publish the exact steg marker; this is a pattern you can adapt if you find a consistent marker in your telemetry.

Suricata Detection Rules & Explanation

What is Suricata?
Suricata is an open-source intrusion detection and prevention engine that can alert on suspicious network behavior. It helps fulfill CIS Control 13.3 (monitoring for unauthorized network traffic).

Sample Rule:

alert http any any -> any any (msg:"Possible stego C2 fetch from raw.githubusercontent.com by non-browser agent"; flow:established,to_server; http.host; content:"raw.githubusercontent.com"; http.user_agent; pcre:"/^(curl|python|Wget|Go-http-client)/i"; sid:1000001; rev:1;)

Cloud & Infrastructure Mitigations

The attack is endpoint/OS first, but you should also harden cloud environments and automate detection & containment.

Google Cloud — enable Security Command Center and block egress to a list of domains

Example: enable the Security Command Center API and create a simple firewall rule to block egress from a VM to a listed IP or domain.

Enable Security Command Center:

gcloud services enable securitycenter.googleapis.com

Note: GCP firewalls don’t block by domain name directly — only by IP address, IP range (CIDR), tags, or service accounts.

To block by domain, you’d need to use Cloud Armor, a proxy, or VPC Service Controls. So if you know the IP you can create a rule like this:

gcloud compute firewall-rules create block-egress-to-ip \ --direction=EGRESS \ --priority=1000 \ --action=DENY \ --rules=tcp:80,tcp:443 \ --destination-ranges=203.0.113.10/32 \ --network=default \ --target-tags=web-vm

Azure — Network Security Group (NSG) (CIS 13.4)
Blocking individual IPs is not scalable. Instead, monitor for patterns and automate rule creation using Defender for Cloud analytics.

My take — a mix of new & old that deserves respect

This is a combination of new and old. Very clever tactics combining the new and the old — using stego to hide C2 inside pictures that look innocent is an old-school technique that I don't think gets enough credit. Granted, GitHub removed the affected repos fairly quickly, but I’m sure they were there long enough to serve their intended purpose as a backup C2 server. These actors aren’t inventing brand new primitives — they’re combining proven techniques (phishing, LOLBins, process injection) with operational creativity (GitHub as resilient infrastructure). That combination is the real threat driver here.

Closing guidance — what to prioritize this week

Each item here ties directly back to CIS Level 1 Controls and good security hygiene:

  1. Enforce enhanced browser protection (CIS 10.1)

  2. Deploy EDR and behavioral blocking (CIS 10.3)

  3. Correlate browser + EDR + network telemetry (CIS 8.7, 13.3)

  4. Regularly hunt for stego artifacts and LOLBin activity (CIS 8.8)

  5. Report malicious repos and automate blocking via threat intel feeds (CIS 13.6)

Browser-based security raises the bar significantly — but stopping threats like Astaroth means combining enterprise browser management with strong endpoint protection, network visibility, and foundational CIS hygiene. That’s how you win this cat-and-mouse game.

John Giglio
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