# CVE-2021-30639: DoS after non-blocking IO error

> An attacker can cause a denial of service in Apache Tomcat by triggering non-blocking I/O errors, affecting applications using this feature.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Jul 2021
- **Updated:** 3 Aug 2024
- **Also known as:** BIT-tomcat-2021-30639, GHSA-44qp-qhfv-c7f6

## What is CVE-2021-30639?

CVE-2021-30639 is a high-severity denial of service vulnerability in Apache Tomcat, published on 12 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-755 (Improper Handling of Exceptional Conditions)

## Which versions are affected by CVE-2021-30639?

7 open source packages ship an affected version, including org.apache.tomcat:tomcat and tomcat. It affects 3 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `org.apache.tomcat:tomcat` | Maven | All versions | 8.5.65 |
| `org.apache.tomcat:tomcat` | Maven | All versions | 8.5.65 |
| `org.apache.tomcat:tomcat` | Maven | From 10.0.3 | 10.0.5 |
| `org.apache.tomcat:tomcat` | Maven | From 10.0.3 | 10.0.5 |
| `org.apache.tomcat:tomcat` | Maven | From 9.0.0 | 9.0.45 |
| `org.apache.tomcat:tomcat` | Maven | From 9.0.0 | 9.0.45 |
| `tomcat` | Bitnami | From 8.5.64 | 8.5.65, 9.0.45, 10.0.4, 10.0.5 |

Products: Apache Tomcat, Mcafee Epolicy Orchestrator, Oracle Big Data Spatial and Graph.

## How do I fix CVE-2021-30639?

Upgrade org.apache.tomcat:tomcat to a fixed version for your release line: 8.5.65, 10.0.5 or 9.0.45.

## Is CVE-2021-30639 being exploited?

There is no known exploitation of CVE-2021-30639 in the wild and no public exploit code.

## Other vulnerabilities in Apache Tomcat

- [CVE-2025-24813](https://insights.sbomify.com/CVE-2025-24813/): Apache Tomcat Path Equivalence (critical 10.0, exploited in the wild)
- [CVE-2016-8735](https://insights.sbomify.com/CVE-2016-8735/): Apache Tomcat Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2017-12617](https://insights.sbomify.com/CVE-2017-12617/): Apache Tomcat Remote Code Execution (high 8.1, exploited in the wild)
- [CVE-2017-12615](https://insights.sbomify.com/CVE-2017-12615/): Apache Tomcat on Windows Remote Code Execution (high 8.1, exploited in the wild)
- [CVE-2026-34486](https://insights.sbomify.com/CVE-2026-34486/): Apache Tomcat Missing Encryption of Sensitive Data (high 7.5, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-30639 and fix it if it is affected.

What it is: CVE-2021-30639 is a high-severity denial of service vulnerability in Apache Tomcat, published on 12 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-30639 in the wild and no public exploit code.

Affected packages and fixed versions:
- org.apache.tomcat:tomcat (Maven): affected all versions; fixed in 8.5.65
- org.apache.tomcat:tomcat (Maven): affected all versions; fixed in 8.5.65
- org.apache.tomcat:tomcat (Maven): affected from 10.0.3; fixed in 10.0.5
- org.apache.tomcat:tomcat (Maven): affected from 10.0.3; fixed in 10.0.5
- org.apache.tomcat:tomcat (Maven): affected from 9.0.0; fixed in 9.0.45
- org.apache.tomcat:tomcat (Maven): affected from 9.0.0; fixed in 9.0.45
- tomcat (Bitnami): affected from 8.5.64; fixed in 8.5.65, 9.0.45, 10.0.4, 10.0.5

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-30639/ (Markdown: https://insights.sbomify.com/CVE-2021-30639/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-30639
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-30639
- OSV: https://osv.dev/vulnerability/CVE-2021-30639

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-30639/
Summaries are generated and may contain mistakes; the official description above is the record.