CVE-2021-43980

The simplified implementation of blocking reads and writes introduced in Tomcat 10 and back-ported to Tomcat 9.0.47 onwards exposed a long standing (but extremely hard to trigger) concurrency bug in Apache Tomcat 10.1.0 to 10.1.0-M12, 10.0.0-M1 to 10.0.18, 9.0.0-M1 to 9.0.60 and 8.5.0 to 8.5.77 that could cause client connections to share an Http11Processor instance resulting in responses, or part responses, to be received by the wrong client.

Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing. Typical impact: TOCTOU exploits, data corruption, or privilege escalation.

CVE-2021-43980 has a CVSS score of 3.7 (Low). 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 (8.5.78, 9.0.62, 10.0.20, 10.1.0-M14); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2021-43980? CVE-2021-43980 is a low-severity race condition vulnerability in org.apache.tomcat:tomcat (maven), affecting versions >= 8.5.0, < 8.5.78. It is fixed in 8.5.78, 9.0.62, 10.0.20, 10.1.0-M14. Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing.
2. How severe is CVE-2021-43980? CVE-2021-43980 has a CVSS score of 3.7 (Low). 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 versions of org.apache.tomcat:tomcat are affected by CVE-2021-43980? org.apache.tomcat:tomcat (maven) versions >= 8.5.0, < 8.5.78 is affected.
4. Is there a fix for CVE-2021-43980? Yes. CVE-2021-43980 is fixed in 8.5.78, 9.0.62, 10.0.20, 10.1.0-M14. Upgrade to this version or later.
5. Is CVE-2021-43980 exploitable, and should I be worried? Whether CVE-2021-43980 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-2021-43980 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-2021-43980?
   • Upgrade org.apache.tomcat:tomcat to 8.5.78 or later
   • Upgrade org.apache.tomcat:tomcat to 9.0.62 or later
   • Upgrade org.apache.tomcat:tomcat to 10.0.20 or later
   • Upgrade org.apache.tomcat:tomcat to 10.1.0-M14 or later