CVE-2021-25641

Deserializer tampering in Apache Dubbo

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.

Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.

CVE-2021-25641 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no 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 (2.7.8, 2.6.9); upgrading removes the vulnerable code path.

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1. What is CVE-2021-25641? CVE-2021-25641 is a critical-severity insecure deserialization vulnerability in org.apache.dubbo:dubbo (maven), affecting versions >= 2.5.0, < 2.7.8. It is fixed in 2.7.8, 2.6.9. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
2. How severe is CVE-2021-25641? CVE-2021-25641 has a CVSS score of 9.8 (Critical). 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.
3. Which packages are affected by CVE-2021-25641?
   • org.apache.dubbo:dubbo (maven) (versions >= 2.5.0, < 2.7.8)
   • com.alibaba:dubbo (maven) (versions >= 2.5.0, < 2.6.9)
4. Is there a fix for CVE-2021-25641? Yes. CVE-2021-25641 is fixed in 2.7.8, 2.6.9. Upgrade to this version or later.
5. Is CVE-2021-25641 exploitable, and should I be worried? Whether CVE-2021-25641 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
6. What actually determines whether CVE-2021-25641 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.
7. How do I fix CVE-2021-25641?
   • Upgrade org.apache.dubbo:dubbo to 2.7.8 or later
   • Upgrade com.alibaba:dubbo to 2.6.9 or later

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