When Hackers Patch Your Servers: How Attackers “Fix” ActiveMQ CVE-2023-46604 to Lock Out Rivals

If you thought patching always meant “good guy at work,” think again. In a twist that sounds like a plotline from a cyber-thriller, researchers have observed threat actors exploiting a critical vulnerability—and then patching it themselves. Why? To lock out rival attackers, evade common detection, and keep quiet, long-term access to your environment.

This isn’t hypothetical. Red Canary recently reported activity targeting Apache ActiveMQ servers in cloud-based Linux environments. After achieving access, the adversaries replaced vulnerable JAR files with patched versions, effectively fixing CVE-2023-46604—only after they’d already installed other persistence and command-and-control (C2) mechanisms.

Here’s why that matters: It flips a common security assumption on its head. Even if your vulnerability scanner reports a “clean” bill of health, an adversary could be quietly operating behind the scenes, using your infrastructure and covering their tracks. In this guide, we break down what happened, how the attack unfolded, and what you can do to harden your systems against this “patch-and-persist” tactic.

The Short Version: What Happened and Why It’s a Big Deal

Attackers exploited a known ActiveMQ remote code execution (RCE) bug, CVE-2023-46604.
After gaining access, they deployed persistence tools and modified SSH settings to allow root logins.
They used a new downloader nicknamed “DripDropper,” which talks to a Dropbox account and pulls down additional malicious components.
Then, they applied a legitimate patch to ActiveMQ on the compromised hosts to:
Prevent other threat actors from using the same flaw.
Reduce the odds of discovery via vulnerability scans and opportunistic exploitation.
In short: they “borrowed” your patch process to entrench themselves.

If you rely solely on vulnerability scans to signal compromise, this tactic can blindside you.

A Quick Primer: What Is CVE-2023-46604 in Apache ActiveMQ?

Apache ActiveMQ is a popular open-source message broker used in distributed systems to handle messaging between services and applications. CVE-2023-46604 is a critical RCE vulnerability disclosed in October 2023. It stems from inadequate validation of throwable class types in OpenWire commands, which can allow attackers to execute arbitrary code on vulnerable systems.

Key points about CVE-2023-46604: – Severity: Critical (Remote Code Execution) – Affected: Apache ActiveMQ with vulnerable OpenWire handling – Fix: Official patches were released in 2023 – Status: Still widely targeted nearly two years later for ransomware, cryptomining, and persistent access

For technical details, check the NVD entry: NVD: CVE-2023-46604.

Why is it still a problem? Because patching lags, especially in complex cloud environments. And opportunistic attackers scan constantly for exposed services.

The New Twist: “Patch-and-Persist” as an Adversary Tactic

Red Canary’s August reporting described attackers who exploited ActiveMQ, established persistence, and then replaced vulnerable ActiveMQ JAR files with patched versions. That last step is the twist.

Why would a threat actor patch your server?

Exclusivity: Once they’re in, they don’t want competitors (ransomware crews, cryptominers) coming in through the same door.
Stealth: Vulnerability scanners often flag known CVEs. If the CVE appears fixed, defenders may move on.
Stability: They’ve already set up backup access (e.g., SSH root login, backdoors, C2 implants). Fixing the original bug won’t disrupt their foothold.
Noise reduction: Failed exploit attempts by other actors can generate alerts. Patching cuts down alerting “noise.”

Think of it like burglars who break in, install a keypad of their own, and quietly change the locks. You won’t notice the window they came through is “fixed,” because it actually is. But they’re still in your house.

For context and ongoing analysis, follow Red Canary’s research updates: Red Canary Blog.

Inside the Attack Chain: From ActiveMQ Exploit to Dropbox C2

Let’s break down the activity observed across several cloud-based Linux endpoints.

1) Initial access via ActiveMQ RCE – The attackers targeted vulnerable Apache ActiveMQ servers exposed to the internet. – After achieving code execution, they moved quickly to establish persistence and C2.

2) C2 tooling varied by endpoint – Sliver implant: The open-source Sliver framework is a common Red Team tool abused by attackers. Learn more here: Bishop Fox Sliver. – Cloudflare Tunnel: Used to create secure outbound tunnels that punch through inbound firewall restrictions. Docs: Cloudflare Tunnel.

3) SSH hardening—turned upside down – In at least one case, the attacker modified sshd_config to enable root login, giving highest-privilege remote access. – sshd is the OpenSSH server daemon handling authentication, encryption, terminals, and more. Reference: sshd_config man page.

4) DripDropper: a previously unknown downloader – The attackers downloaded “DripDropper,” an encrypted executable built with PyInstaller. More on PyInstaller: PyInstaller. – It communicated with an attacker-controlled Dropbox account using a hardcoded bearer token. – Dropbox developer docs: Dropbox Developers. – DripDropper wrote at least two malicious files and performed actions like: – Monitoring processes – Polling the Dropbox account for instructions – Preparing for long-term persistence by altering the default login shell for user accounts

5) Then came the “patch” – After establishing other forms of access, the adversary replaced vulnerable ActiveMQ JAR files with updated ones, applying a legitimate fix for CVE-2023-46604.

The net effect: defenders looking only for unpatched ActiveMQ might miss the compromise entirely.

Why Cloud-Based Linux Endpoints Are Attractive Targets

Cloud servers are often: – Internet-facing by design (APIs, message brokers, dashboards, admin consoles) – Ephemeral and autoscaled, which makes configuration drift and patch lag more likely – Managed by infrastructure code spread across teams, increasing the chance of blind spots – Less likely to run the same level of endpoint protection as desktops

And because many cloud services require outbound internet access, C2 tools can blend into “normal” egress—particularly if they use reputable platforms like Dropbox.

Defender Mindset Shift: From “Am I Vulnerable?” to “Am I Compromised?”

This campaign is a reminder that vulnerability status is not the same as integrity. A clean scan doesn’t guarantee a clean system. Attackers know how we measure risk and will use those assumptions against us.

So, pivot your mindset: – Don’t just check if CVE-2023-46604 is present; check if ActiveMQ was recently modified. – Don’t just block inbound; monitor outbound for suspicious C2. – Don’t just harden sshd; alert on changes to sshd_config, authorized_keys, shells, and PAM.

Let me explain why this helps: By shifting from “is a known bug present?” to “is anything abnormal in how the service behaves or is configured?”, you catch the tactic—even when the vulnerability is technically patched.

Detection Opportunities Across the Kill Chain

You don’t need to catch everything. You just need to catch something early enough. Here are practical detection choke points:

Initial access and post-exploit – Unexpected changes to ActiveMQ installation: – File integrity monitoring (FIM) on ActiveMQ JAR paths – Recent modifications to JARs not tied to your formal patching pipeline – Suspicious child processes spawned by ActiveMQ or Java – For example: ActiveMQ launching shell, curl/wget, Python, or uncommon binaries

SSH and privilege misuse – Alerts on changes to sshd_config, authorized_keys, or PermitRootLogin – Sudden enabling of root login via SSH – Unusual SSH authentication patterns or new keys appearing in root or service accounts

C2 and tooling indicators – Outbound connections to: – Cloudflare Tunnel endpoints (cloudflared processes or connections) – Dropbox domains that don’t match your business use – Unrecognized binaries signed with PyInstaller or containing “pyinstaller” strings in metadata – Presence of Sliver-like processes, service names, or TLS fingerprints

Persistence and living-off-the-land – Changes to default login shells for users – Cron jobs, systemd services, or rc scripts added/modified post-exploitation – Unusual use of compression/archiving tools followed by network exfiltration

Log sources to leverage – Linux auditd and syslog/journald – Application logs (ActiveMQ) – EDR telemetry for Linux – Cloud provider flow logs and DNS logs – Web application firewall (WAF) logs, if applicable

Pro tip: Build “golden signals” for critical services. For ActiveMQ, track: – Binary and JAR integrity – Startup parameters and environment variables – Service account privileges – Network connections initiated by the service

How to Protect Web Services on Cloud-Based Linux Systems

The Red Canary team highlighted several hardening steps. Here’s how to operationalize them with a modern cloud defender’s mindset.

1) Enforce policy-based controls with configuration management – Use Ansible, Puppet, or your chosen IaC tooling to detect and auto-remediate config drift. – Heal misconfigurations quickly—especially those adversaries might introduce (e.g., enabling root SSH login). – Helpful references: – Ansible – Puppet – CIS Benchmarks for Linux hardening: CIS Benchmarks

2) Run services as non-root – Configure ActiveMQ, web servers, and C2-sensitive services under dedicated, least-privileged accounts. – Use systemd unit hardening options (e.g., NoNewPrivileges, ProtectSystem, ReadWritePaths) to limit blast radius.

3) Enforce mandatory authentication everywhere – Disable anonymous or unauthenticated endpoints. – Require strong authentication for admin consoles and management planes. – For SSH: – Disable password auth; use keys with passphrases. – Enforce MFA where feasible (e.g., hardware tokens, short-lived cert-based access). – Reference: OpenSSH sshd_config.

4) Patch and prioritize known exploited vulnerabilities (KEV) – Prioritize services listed in CISA’s KEV catalog with tight SLAs. – Track exposure and patch velocity across environments. – Link: CISA Known Exploited Vulnerabilities Catalog.

5) Restrict network exposure – Lock down ingress rules to known IP ranges, Zero Trust networks, or VPNs. – Don’t expose message brokers or admin UIs directly to the internet unless absolutely necessary. – Consider Cloudflare, Fastly, or other providers for additional gatekeeping—while monitoring for abuse of outbound tunnels.

6) Apply least privilege to public-facing services – Limit system capabilities, filesystem access, and network privileges for internet-facing daemons. – Carefully scope IAM permissions for instances, containers, and service accounts. – Avoid “god mode” roles for convenience.

7) Control and observe egress – Egress filtering is your friend. Establish rules that block outbound to known file-sharing C2 patterns unless explicitly allowed. – Inspect DNS logs for suspicious domains and unusual hostnames. – Use a secure web gateway or DNS filtering for container and VM workloads.

8) Establish file integrity monitoring and change control – Hash and monitor critical binaries and JARs (including ActiveMQ components). – Alert on changes that don’t align with your CI/CD or patch pipeline.

9) Threat hunt for cloud-friendly C2 – Hunt for: – cloudflared processes and config artifacts – Dropbox usage from servers that don’t have business need – Sliver-related process names or network indicators – Maintain allowlists for sanctioned admin tools to reduce noise.

10) Bake security into build and deploy – Containerize services like ActiveMQ with hardened base images. – Run images as non-root; use read-only root filesystems where possible. – Keep images and AMIs patched and re-immortalize compromised nodes (rebuild rather than manually clean).

Don’t Be Fooled by “All Green” Scans: Common Pitfalls to Avoid

Overreliance on vulnerability scanners: They spot known flaws, not subtle persistence. Pair them with EDR, FIM, and behavioral detections.
Leaving services exposed “just for now”: Temporary exceptions become permanent risks. Treat internet exposure as a change that needs justification and review.
Ignoring egress: Without outbound controls, adversaries can “phone home” through trusted platforms.
Assuming a patch equals safety: If you didn’t apply the patch, assume the system might be tampered with—even if it’s fully up to date now.

Incident Response Playbook: Suspect You’ve Been “Patched by the Attacker”?

If the timeline or evidence suggests a post-exploitation patch, act decisively:

1) Contain – Isolate affected instances at the network level. Don’t power off immediately; preserve memory and disk state as needed. – Disable or heavily restrict outbound internet access from suspicious hosts.

2) Triage and collect – Collect volatile data (process lists, open connections), SSH logs, system logs, ActiveMQ logs, and FIM alerts. – Snapshot disks where possible; keep chain of custody if needed.

3) Hunt for persistence – Look for: – sshd_config changes, new SSH keys, or root login enabled – New systemd services, cron jobs, rc files – Modified default shells for user accounts – Unknown binaries in /tmp, /var/tmp, or user home directories – cloudflared, Sliver-related artifacts, or Dropbox-based tooling

4) Rebuild from clean sources – Assume deep tampering. Rebuild compromised nodes from trusted, patched images. – Re-deploy application components (e.g., ActiveMQ) from your CI/CD pipeline.

5) Rotate secrets and credentials – Rotate SSH keys, tokens, API keys, and cloud credentials used on affected systems. – Invalidate bearer tokens and reissue service account keys.

6) Retrospective search – Search logs, EDR telemetry, and cloud flow logs for similar activity across your fleet. – Identify patient zero and the full blast radius.

7) Lessons and hardening – Close exposure gaps, enforce IAM least privilege, and tighten egress. – Integrate detections for the specific techniques observed.

“DripDropper,” Dropbox C2, and the Broader Trend of Abuse of Legitimate Services

Using mainstream platforms for C2 isn’t new. Attackers leverage cloud storage, collaboration tools, and CDNs to blend in. Dropbox is one example; GitHub, Google Drive, and others have been abused too. The takeaway isn’t “ban productivity suites,” but:

Build allowlists and conditional access based on user, device, and context.
Monitor for anomalous usage from server workloads that typically don’t need these services.
Use SSL inspection judiciously in server-to-internet paths where feasible and compliant, or rely on metadata-based detections and DNS visibility.

When you see a PyInstaller-packed downloader communicating with an external service and altering shells and configs, treat it as a high-severity event—even if your vuln scanner shows “no critical findings.”

Actionable Checklist: Make This Week Count

Identify all internet-exposed ActiveMQ instances. Confirm versions and patch levels.
Validate the provenance of your ActiveMQ JAR files. Compare hashes against your official build artifacts.
Review sshd_config across your fleet for drift (PermitRootLogin, PasswordAuthentication).
Alert on new SSH keys added to root or service accounts.
Hunt for cloudflared processes and unexpected Dropbox connections from servers.
Enforce egress rules that require justification for outbound to file-sharing domains.
Add CVE-2023-46604 to your “never again” patch list and track with SLAs tied to CISA KEV.
Automate hardening with Ansible or Puppet, including self-healing for critical configs.

Frequently Asked Questions

Q: What is CVE-2023-46604 in ActiveMQ? A: It’s a critical remote code execution vulnerability in Apache ActiveMQ related to improper validation of throwable class types in OpenWire commands. Attackers can run arbitrary code on vulnerable servers. Details: NVD: CVE-2023-46604.

Q: Why would attackers patch a vulnerability after exploiting it? A: To keep exclusive access, reduce detection from vulnerability scans, and prevent noisy exploitation attempts by others. It’s a stealth and stability play—once they’ve established persistence through other means, closing the original hole helps them blend in.

Q: Does a patched ActiveMQ mean I’m safe? A: Not necessarily. If you didn’t apply the patch and the system shows signs of tampering, treat it as a potential compromise. Verify binary integrity, check for persistence mechanisms, and review logs for suspicious behavior.

Q: What is DripDropper? A: DripDropper is a previously unknown downloader observed by researchers in this campaign. It was packaged with PyInstaller and communicated with an attacker-controlled Dropbox account using a hardcoded bearer token to fetch instructions and deploy additional payloads.

Q: How can I detect Sliver C2 in my environment? A: Look for unusual processes, network beacons, TLS characteristics, and binaries associated with Sliver. Correlate with suspicious parent processes (e.g., ActiveMQ spawning unexpected shells), new services, or persistence artifacts. Reference: Sliver on GitHub.

Q: Should I block Dropbox entirely on servers? A: If servers don’t have a business need for Dropbox, blocking or tightly restricting it is reasonable. Otherwise, use egress controls, DNS filtering, and anomaly detection to spot misuse.

Q: How do I secure sshd on Linux? A: Use key-based auth, disable root login, disable password authentication, limit allowed users/groups, and monitor config changes. Reference: sshd_config.

Q: Is this kind of “patch-and-persist” tactic new? A: The idea has appeared before in different forms, but its use against widely deployed, cloud-exposed services reinforces how common and pragmatic this tactic has become. It’s a reminder that attackers adapt to our controls.

Q: What immediate steps should I take if I run ActiveMQ? A: Confirm exposure, verify patch levels, validate JAR integrity, review logs for post-exploitation activity, and ensure the service runs as a non-root user. Monitor for suspicious outbound connections and persistence techniques.

The Takeaway

Attackers don’t just exploit vulnerabilities—they increasingly manage them. By patching after compromise, they mute the easiest detection pathway and buy time. Don’t let that happen on your watch.

Shift your focus from “Am I vulnerable?” to “Am I behaving normally?” Pair rapid patching with behavioral detections, strict egress, least privilege, and configuration management that self-heals drift. If you harden services like ActiveMQ and sshd the right way—and validate the integrity of what’s running—you’ll catch the intruder even when the window they used is quietly repaired.

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