CVE-2021-30639

A vulnerability in Apache Tomcat allows an attacker to remotely trigger a denial of service. An error introduced as part of a change to improve error handling during non-blocking I/O meant that the error flag associated with the Request object was not reset between requests. This meant that once a non-blocking I/O error occurred, all future requests handled by that request object would fail. Users were able to trigger non-blocking I/O errors, e.g. by dropping a connection, thereby creating the possibility of triggering a DoS. Applications that do not use non-blocking I/O are not exposed to this vulnerability. This issue affects Apache Tomcat 10.0.3 to 10.0.4; 9.0.44; 8.5.64.

CVE-2021-30639 has a CVSS score of 7.5 (High). 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 (10.0.5, 9.0.45, 8.5.65); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2021-30639? CVE-2021-30639 is a high-severity security vulnerability in org.apache.tomcat:tomcat (maven), affecting versions >= 10.0.3, < 10.0.5. It is fixed in 10.0.5, 9.0.45, 8.5.65.
2. How severe is CVE-2021-30639? CVE-2021-30639 has a CVSS score of 7.5 (High). 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-2021-30639? org.apache.tomcat:tomcat (maven) versions >= 10.0.3, < 10.0.5 is affected.
4. Is there a fix for CVE-2021-30639? Yes. CVE-2021-30639 is fixed in 10.0.5, 9.0.45, 8.5.65. Upgrade to this version or later.
5. Is CVE-2021-30639 exploitable, and should I be worried? Whether CVE-2021-30639 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-2021-30639 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-2021-30639?
   • Upgrade org.apache.tomcat:tomcat to 10.0.5 or later
   • Upgrade org.apache.tomcat:tomcat to 9.0.45 or later
   • Upgrade org.apache.tomcat:tomcat to 8.5.65 or later