CVE-2021-33790
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedThe RebornCore library before 4.7.3 allows remote code execution because it deserializes untrusted data in ObjectInputStream.readObject as part of reborncore.common.network.ExtendedPacketBuffer. An attacker can instantiate any class on the classpath with any data. A class usable for exploitation might or might not be present, depending on what Minecraft modifications are installed.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe RebornCore library before version 4.7.3 contains an insecure deserialization vulnerability in the ExtendedPacketBuffer class. The code uses ObjectInputStream.readObject() to deserialize data from network packets without validating the input, allowing an attacker to instantiate arbitrary classes on the classpath with controlled data. This is a classic Java deserialization flaw where exploitation depends on the presence of 'gadget chain' classes (available via Minecraft mods) that can be chained to achieve remote code execution.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data<= 3.13.8>= 3.19.0, < 3.19.5>= 4.2.0, < 4.2.10>= 4.7.0, < 4.7.3CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify installed RebornCore versionLocate the RebornCore mod JAR file in your mods folder and inspect its mod.toml, mcmod.info, or version manifest. Common paths: .minecraft/mods/ or .minecraft/versions/<profile>/mods/. Alternatively, check the filename itself which often contains the version number (e.g., RebornCore-4.7.2.jar).Affected if The version is 3.13.8 or earlier; OR 3.19.0 through 3.19.4; OR 4.2.0 through 4.2.9; OR 4.7.0 through 4.7.2. Any of these ranges indicate a vulnerable version.
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Confirm ExtendedPacketBuffer class is presentInspect the RebornCore JAR contents using a zip tool or via command: unzip -l RebornCore-<version>.jar | grep -i ExtendedPacketBuffer. This class handles packet deserialization and contains the vulnerable readObject() call.Affected if The ExtendedPacketBuffer class exists in the JAR, meaning the insecure deserialization code path is present in your environment.
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Verify network client/server functionality is activeDetermine if your Minecraft instance connects to external servers or hosts multiplayer games. Check server.properties for 'online-mode=true' or inspect network connections. The vulnerability is exploitable through network packets sent to or received from other clients/servers.Affected if Your client connects to multiplayer servers OR you host a server where untrusted clients can connect. This provides the attack vector for malicious deserialized objects.
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Scan for known gadget chain classesSearch your mods folder for classes commonly used in deserialization gadget chains. Check for libraries like CommonsCollections, CommonsBeanUtils, or Groovy in any loaded JARs: find . -name '*.jar' -exec unzip -l {} \; 2>/dev/null | grep -iE '(CommonsCollections|CommonsBeanUtils|groovy|jdk8u181|jboss|jmx)'Affected if Your modpack contains gadget chain classes that can be chained with the vulnerable readObject() to achieve code execution. Presence significantly increases exploitability but absence does not guarantee safety as custom gadget chains may exist.
You are affected if your installed RebornCore version falls within any of the vulnerable ranges AND network functionality is active, regardless of gadget chain presence since exploitation may still be possible.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped3.19.54.2.104.7.3
Upgrade RebornCore to version 4.7.3 or later which implements secure deserialization. If upgrading is not feasible, implement a custom ObjectInputStream with class-name whitelisting to restrict deserialization to safe, expected classes only.
RebornCore 4.7.3 (or latest stable 4.x release; alternatively 3.19.5+ for 3.x branch)
- 1. Identify the currently installed RebornCore version in your Minecraft modpack or project
- 2. If using RebornCore 3.x: upgrade to version 3.19.5 or later
- 3. If using RebornCore 4.2.x: upgrade to version 4.2.10 or later
- 4. If using RebornCore 4.7.x: upgrade to version 4.7.3 or later
- 5. Rebuild and redeploy the affected modpack or application
- 6. Verify the upgrade resolves the deserialization vulnerability by testing network packet handling
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-33790 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-33790 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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