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<header class="hero detail-hero"><div class="wrap">
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  <a class="back" href="/#labs">&larr; All work</a>
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  <div class="kicker">ADVERSARY EMULATION</div>
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  <h1>Purple-Team Lab</h1>
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  <p class="tagline">Adversary emulation that validates the detections instead of assuming they work.</p>
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  <div class="tags"><span>Atomic Red Team</span><span>Caldera</span><span>Wazuh FIM</span><span>MITRE ATT&amp;CK</span><span>Auditd</span><span>Detection-as-Code</span></div>
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  <div class="facts"><div class="stat"><div class="n">4</div><div class="k">Custom Wazuh rules (100410-100413)</div></div><div class="stat"><div class="n">6</div><div class="k">ATT&amp;CK techniques validated</div></div><div class="stat"><div class="n">5</div><div class="k">Detections firing at alert severity</div></div><div class="stat"><div class="n">12</div><div class="k">Highest rule level (authorized_keys)</div></div><div class="stat"><div class="n">18</div><div class="k">Invalid SSH logins driving brute-force rule 5712</div></div><div class="stat"><div class="n">6</div><div class="k">FIM paths watched in real time</div></div></div>
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<section><div class="wrap">
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  <figure class="shot"><img loading="lazy" src="/assets/purple-team-lab/01-detections-fired.png" alt="Emulated techniques detected in Wazuh: custom rules 100410-100413 plus the brute-force rule firing on the purple-target agent, each ATT&amp;CK-tagged."></figure><figcaption>Emulated techniques detected in Wazuh: custom rules 100410-100413 plus the brute-force rule firing on the purple-target agent, each ATT&amp;CK-tagged.</figcaption>
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  <div class="content">
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  <h2>Custom Wazuh rules</h2>
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<p>Four custom rules sit in the Wazuh <code>syscheck</code> group and key off real-time FIM events rather than syscall auditing. Each is ATT&amp;CK-tagged and assigned a level that reflects how it should be triaged: the <code>authorized_keys</code> rule is level 12 because an attacker who can append a key owns persistent access; the others are level 10. The full ruleset, verbatim, deployed to the manager as <code>rules/local_purple_rules.xml</code>:</p><pre><code>&lt;group name="linux,syscheck,purple-team-fim,"&gt;
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  &lt;rule id="100410" level="10"&gt;
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    &lt;if_group&gt;syscheck&lt;/if_group&gt;
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    &lt;field name="file" type="pcre2"&gt;^/etc/cron&lt;/field&gt;
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    &lt;description&gt;Cron file created or modified: $(file) - possible scheduled-task persistence&lt;/description&gt;
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    &lt;mitre&gt;
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      &lt;id&gt;T1053.003&lt;/id&gt;
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    &lt;/mitre&gt;
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  &lt;/rule&gt;
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  &lt;rule id="100411" level="10"&gt;
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    &lt;if_group&gt;syscheck&lt;/if_group&gt;
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    &lt;field name="file" type="pcre2"&gt;^/etc/systemd/system/.+\.service&lt;/field&gt;
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    &lt;description&gt;Systemd unit created or modified: $(file) - possible service persistence&lt;/description&gt;
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    &lt;mitre&gt;
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      &lt;id&gt;T1543.002&lt;/id&gt;
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    &lt;/mitre&gt;
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  &lt;/rule&gt;
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  &lt;rule id="100412" level="12"&gt;
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    &lt;if_group&gt;syscheck&lt;/if_group&gt;
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    &lt;field name="file" type="pcre2"&gt;\.ssh/authorized_keys$&lt;/field&gt;
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    &lt;description&gt;SSH authorized_keys modified: $(file) - possible persistence via account manipulation&lt;/description&gt;
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    &lt;mitre&gt;
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      &lt;id&gt;T1098.004&lt;/id&gt;
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    &lt;/mitre&gt;
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  &lt;/rule&gt;
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  &lt;rule id="100413" level="10"&gt;
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    &lt;if_group&gt;syscheck&lt;/if_group&gt;
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    &lt;field name="file" type="pcre2"&gt;^/usr/local/bin/&lt;/field&gt;
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    &lt;description&gt;Binary placed in /usr/local/bin: $(file) - possible persistence or tooling drop&lt;/description&gt;
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    &lt;mitre&gt;
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      &lt;id&gt;T1543&lt;/id&gt;
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    &lt;/mitre&gt;
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  &lt;/rule&gt;
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&lt;/group&gt;</code></pre><p>The rules depend on the agent's <code>syscheck</code> block watching those paths in real time. That FIM configuration is a one-liner deployed to the agent:</p><pre><code>&lt;directories realtime="yes" check_all="yes" report_changes="yes"&gt;/etc/cron.d,/etc/cron.daily,/etc/systemd/system,/root/.ssh,/home/zion/.ssh,/usr/local/bin&lt;/directories&gt;</code></pre>
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<h2>Adversary emulation</h2>
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<p>A single Atomic Red Team script runs the technique set against the <code>purple-target</code> VM &mdash; a dedicated Ubuntu 22.04 host rather than a container, because kernel-level telemetry needs a real kernel. It drops a cron job in <code>/etc/cron.d</code> (T1053.003), creates a systemd unit in <code>/etc/systemd/system</code> (T1543.002), appends an SSH key to <code>~/.ssh/authorized_keys</code> (T1098.004), plants an executable in <code>/usr/local/bin</code> (T1543), and fires 18 invalid SSH logins to trip the brute-force rule (T1110). The persistence actions, verbatim from the runner:</p><pre><code>echo '* * * * * root /usr/bin/id' | sudo tee /etc/cron.d/atomic-persist &gt;/dev/null
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printf '[Unit]\nDescription=atomic test\n[Service]\nExecStart=/usr/bin/id\n[Install]\nWantedBy=multi-user.target\n' \
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  | sudo tee /etc/systemd/system/atomic-evil.service &gt;/dev/null
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mkdir -p "$HOME/.ssh"
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echo 'ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAtomicRedTeamTestKeyDoNotUse attacker@evil' &gt;&gt; "$HOME/.ssh/authorized_keys"
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printf '#!/bin/bash\nid\n' | sudo tee /usr/local/bin/definitely-not-malware &gt;/dev/null
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sudo chmod +x /usr/local/bin/definitely-not-malware</code></pre><p>MITRE Caldera is provisioned via Docker Compose for graph-based emulation when an attack chain &mdash; rather than discrete atomics &mdash; is needed. It is built from upstream master and exposed on <code>:8888</code>:</p><pre><code>services:
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  caldera:
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    image: caldera:local
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    build:
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      context: https://github.com/mitre/caldera.git#master
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    ports:
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      - "8888:8888"
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      - "8443:8443"
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    environment:
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      - CALDERA_CONF=local
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    command: ["--insecure", "--build"]</code></pre>
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<h2>Coverage matrix</h2>
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<p>Every technique was executed on the endpoint and confirmed in the SIEM, mapped to ATT&amp;CK and assigned a level that reflects how it should be triaged. Five of six fire on detections written for this lab; the sixth is a baseline check that login telemetry is reaching the manager.</p><table><thead><tr><th>Technique</th><th>ATT&amp;CK</th><th>Atomic action</th><th>Telemetry</th><th>Rule</th><th>Level</th><th>Detected</th></tr></thead><tbody><tr><td>Brute Force</td><td>T1110</td><td>18 invalid SSH logins from one source</td><td>sshd auth log</td><td>5712 (built-in)</td><td>10</td><td>yes</td></tr><tr><td>Scheduled Task / Cron</td><td>T1053.003</td><td>write job to <code>/etc/cron.d/</code></td><td>FIM (real-time)</td><td>100410 (custom)</td><td>10</td><td>yes</td></tr><tr><td>Systemd Service</td><td>T1543.002</td><td>create unit in <code>/etc/systemd/system/</code></td><td>FIM (real-time)</td><td>100411 (custom)</td><td>10</td><td>yes</td></tr><tr><td>SSH Authorized Keys</td><td>T1098.004</td><td>append key to <code>~/.ssh/authorized_keys</code></td><td>FIM (real-time)</td><td>100412 (custom)</td><td>12</td><td>yes</td></tr><tr><td>Create / Modify System Process</td><td>T1543</td><td>drop binary in <code>/usr/local/bin/</code></td><td>FIM (real-time)</td><td>100413 (custom)</td><td>10</td><td>yes</td></tr><tr><td>Valid Accounts / Sudo</td><td>T1078 / T1548.003</td><td>login + sudo to root</td><td>sshd/PAM auth log</td><td>5501 / 5402 (built-in)</td><td>3</td><td>yes (baseline)</td></tr></tbody></table>
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<h2>Why FIM, not auditd</h2>
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<p>The persistence detections key off file integrity monitoring, not syscall auditing &mdash; and that is a deliberate choice, not a shortcut. FIM is Wazuh-native and works everywhere, including containers and hardened hosts where the kernel audit framework is not available to you. Auditd execve rules, by contrast, silently do nothing on a host that boots with audit disabled &mdash; which is the common case inside an LXC container, where the audit subsystem is namespaced away and rules load without error but never fire.</p><p>The auditd ruleset that would be layered on a host with working kernel auditing is included for completeness in <code>agent/auditd-purple.rules</code> &mdash; it watches execve, credential files, and the persistence directories &mdash; but the validated detections above do not depend on it:</p><pre><code>-a always,exit -F arch=b64 -S execve -S execveat -k exec
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-a always,exit -F arch=b32 -S execve -S execveat -k exec
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-w /etc/shadow -p r -k cred_access
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-w /etc/passwd -p wa -k passwd_change
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-w /etc/sudoers -p wa -k sudoers_change
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-w /etc/cron.d -p wa -k cron_persist
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-w /etc/systemd/system -p wa -k systemd_persist</code></pre><p>The honest consequence is a noted coverage gap: execve-based detections such as reverse-shell command lines (T1059.004) need that host syscall auditing the target kernel did not provide. The FIM-based persistence detections are independent of it, which is exactly why they survive the environments where auditd does not.</p>
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<h2>Running it</h2>
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<p>On the endpoint &mdash; an enrolled Wazuh agent with the FIM config from <code>agent/</code> applied &mdash; the entire technique set runs from one script. It logs each step with a timestamp and supports a <code>--cleanup</code> flag that removes the cron job, systemd unit, dropped binary, and the planted SSH key:</p><pre><code>sudo bash atomics/run_atomics.sh            # execute the technique set
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sudo bash atomics/run_atomics.sh --cleanup  # execute, then revert all artifacts</code></pre><p>The brute-force step loops 18 password-auth attempts against localhost with pubkey auth forced off, so each one lands in the sshd auth log as an invalid user:</p><pre><code>for i in $(seq 1 18); do
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  ssh -o BatchMode=yes -o ConnectTimeout=2 -o StrictHostKeyChecking=no \
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      -o PreferredAuthentications=password -o PubkeyAuthentication=no \
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      "evil_user_${i}@127.0.0.1" true 2>/dev/null
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done</code></pre><p>Then in the Wazuh dashboard, filter Threat Hunting &rarr; Events to confirm the hits:</p><pre><code>rule.id:(100410 or 100411 or 100412 or 100413 or 5712)</code></pre><p>Caldera, for graph-based emulation, is optional and comes up alongside:</p><pre><code>cd caldera &amp;&amp; docker compose up -d          # http://localhost:8888</code></pre>
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<h2>Detection-as-code</h2>
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<p>This lab is one half of a two-repo workflow. Detections are authored once as Sigma in a separate detection-as-code repository and compiled to each target SIEM; the Wazuh-native versions of the persistence rules proven here are the compiled output. The endpoint itself is the same instrumented host stood up in a companion SOC-automation lab, so the agent, indexer, and dashboard are not bespoke to this project &mdash; they are the standing detection stack this work validates against.</p><p>Keeping emulation and authoring in separate repos enforces the discipline: a rule does not get marked validated here until an atomic in <code>run_atomics.sh</code> has demonstrably triggered it on the live agent. The matrix is regenerated from real runs, and the screenshot in the repo is the evidence, not a mockup.</p>
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<h2>What it proves</h2>
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<p>Severity is the point as much as coverage is. The persistence detections fire at level 10&ndash;12 because they would page an analyst, while login and sudo events sit at level 3 as context, not alerts. A detection that fires at the wrong severity is as useless as one that does not fire at all.</p><ul><li><strong>Detections are tested, not assumed.</strong> Each custom rule has a corresponding atomic that demonstrably triggers it on the live agent &mdash; the matrix reflects an actual run, not an intended design.</li><li><strong>Gaps are noted honestly.</strong> Execve-based detections such as reverse-shell command lines (T1059.004) require host syscall auditing the target kernel did not provide; that limitation is documented rather than papered over, and the FIM persistence path is independent of it.</li><li><strong>Telemetry choice is justified.</strong> FIM was chosen over auditd specifically because it survives containers and hardened hosts where kernel auditing silently fails &mdash; a deliberate engineering decision with a stated tradeoff.</li><li><strong>It plugs into a real workflow.</strong> The rules proven here are the compiled output of a Sigma-first detection-as-code pipeline, validated against a standing Wazuh stack rather than a throwaway.</li></ul>
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  <p class="repo-line">Repository &middot; github.com/zionboggan/purple-team-lab</p>
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