CVE-2021-33790: The RebornCore library before 4.7.3 allows remote code execution because it deserializes untrusted data in Obj

This week

CRITICAL 9.8REMOTEUPGRADE

CVE-2021-33790 is a critical-severity flaw (CVSS 9.8) affecting reborncore. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The 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.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	2.8% (top 13%)
CISA exploited list	No
Weakness	CWE-502
Published	2021-05-31
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: techreborn

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-05-31 04:15 UTCPublished CVE record published
2026-06-17 03:55 UTCUpdated Record last modified

techreborn reborncore UPGRADE

Affected: <= 3.13.8 · >= 3.19.0 and < 3.19.5 · >= 4.2.0 and < 4.2.10 · >= 4.7.0 and < 4.7.3

Fixed in: 3.19.5, 4.2.10, 4.7.3

Safe to downgrade to: a release before 3.19.0

1. Check installed version

apt-cache policy reborncore || rpm -q reborncore || reborncore --version

2. UPGRADE to 3.19.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade reborncore

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade reborncore

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.19.0 (Debian / Ubuntu)

apt-cache madison reborncore sudo apt install reborncore=<VERSION_BEFORE_3.19.0> sudo apt-mark hold reborncore

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list reborncore sudo dnf downgrade reborncore-<VERSION_BEFORE_3.19.0> sudo dnf versionlock add reborncore

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-33790?

reborncore: upgrade to 3.19.5, 4.2.10, 4.7.3. Exact commands for each OS are on this page.

Is CVE-2021-33790 being exploited?

CVE-2021-33790 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-33790 affect?

CVE-2021-33790 affects reborncore. Severity is critical (CVSS 9.8). The 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

How urgent is CVE-2021-33790?

Priority 61/100. Critical severity; Remotely exploitable, no login, no user action

References

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