CVE-2021-25641: Dubbo Zookeeper does not check serialization id

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 21.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-06-01 · updated 2026-06-17
Weakness: CWE-502

Description

Each Apache Dubbo server will set a serialization id to tell the clients which serialization protocol it is working on. But for Dubbo versions before 2.7.8 or 2.6.9, an attacker can choose which serialization id the Provider will use by tampering with the byte preamble flags, aka, not following the server's instruction. This means that if a weak deserializer such as the Kryo and FST are somehow in code scope (e.g. if Kryo is somehow a part of a dependency), a remote unauthenticated attacker can tell the Provider to use the weak deserializer, and then proceed to exploit it.

Affected products

Apache Software Foundation Apache Dubbo: from 2.5.0 before 2.6.9; from 2.7.0 before 2.7.8 — fixed in 2.6.9, 2.7.8

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache Dubbo 2.6.9, 2.7.8. Check the vendor advisory for mitigations.

References

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