# CVE-2021-33037: Incorrect Transfer-Encoding handling with HTTP/1.0

> Attackers can perform HTTP request smuggling against Apache Tomcat when used with a reverse proxy due to incorrect Transfer-Encoding header parsing.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **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:** 8 Oct 2026
- **Also known as:** BIT-tomcat-2021-33037, DEBIAN-CVE-2021-33037, GHSA-4vww-mc66-62m6, OESA-2021-1275, UBUNTU-CVE-2021-33037

## What is CVE-2021-33037?

CVE-2021-33037 is a medium-severity 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

Apache Tomcat 10.0.0-M1 to 10.0.6, 9.0.0.M1 to 9.0.46 and 8.5.0 to 8.5.66 did not correctly parse the HTTP transfer-encoding request header in some circumstances leading to the possibility to request smuggling when used with a reverse proxy. Specifically: - Tomcat incorrectly ignored the transfer encoding header if the client declared it would only accept an HTTP/1.0 response; - Tomcat honoured the identify encoding; and - Tomcat did not ensure that, if present, the chunked encoding was the final encoding.

Weakness: CWE-444 (Inconsistent Interpretation of HTTP Requests)

## Which versions are affected by CVE-2021-33037?

7 open source packages ship an affected version, including org.apache.tomcat:tomcat and tomcat. It affects 22 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `org.apache.tomcat:tomcat` | Maven | From 10.0.0-M1 | 10.0.7 |
| `org.apache.tomcat:tomcat` | Maven | From 10.0.0-M1 | 10.0.7 |
| `org.apache.tomcat:tomcat` | Maven | From 8.5.0 | 8.5.68 |
| `org.apache.tomcat:tomcat` | Maven | From 8.5.0 | 8.5.68 |
| `org.apache.tomcat:tomcat` | Maven | From 9.0.0-M1 | 9.0.48 |
| `org.apache.tomcat:tomcat` | Maven | From 9.0.0-M1 | 9.0.48 |
| `tomcat` | Bitnami | From 8.5.0 | 8.5.67, 9.0.47, 10.0.7 |

Products: Apache Tomcat, Apache Tomee, Debian Debian Linux, Mcafee Epolicy Orchestrator, Oracle Agile Product Lifecycle Management, Oracle Communications Cloud Native Core Policy, Oracle Communications Cloud Native Core Service Communication Proxy, Oracle Communications Diameter Signaling Router, Oracle Communications Instant Messaging Server, Oracle Communications Policy Management, Oracle Communications Pricing Design Center, Oracle Communications Session Report Manager, Oracle Communications Session Route Manager, Oracle Graph Server and Client, Oracle Healthcare Translational Research, Oracle Hospitality Cruise Shipboard Property Management System, Oracle Instantis Enterprisetrack, Oracle Managed File Transfer, Oracle Mysql Enterprise Monitor, Oracle Sd-Wan Edge and 2 more.

## How do I fix CVE-2021-33037?

Upgrade org.apache.tomcat:tomcat to a fixed version for your release line: 10.0.7, 8.5.68 or 9.0.48.

## Is CVE-2021-33037 being exploited?

There is no known exploitation of CVE-2021-33037 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-2021-33037 and fix it if it is affected.

What it is: CVE-2021-33037 is a medium-severity 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-2021-33037 in the wild and no public exploit code.

Affected packages and fixed versions:
- org.apache.tomcat:tomcat (Maven): affected from 10.0.0-M1; fixed in 10.0.7
- org.apache.tomcat:tomcat (Maven): affected from 10.0.0-M1; fixed in 10.0.7
- org.apache.tomcat:tomcat (Maven): affected from 8.5.0; fixed in 8.5.68
- org.apache.tomcat:tomcat (Maven): affected from 8.5.0; fixed in 8.5.68
- org.apache.tomcat:tomcat (Maven): affected from 9.0.0-M1; fixed in 9.0.48
- org.apache.tomcat:tomcat (Maven): affected from 9.0.0-M1; fixed in 9.0.48
- tomcat (Bitnami): affected from 8.5.0; fixed in 8.5.67, 9.0.47, 10.0.7

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-2021-33037/ (Markdown: https://insights.sbomify.com/CVE-2021-33037/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-33037
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-33037
- OSV: https://osv.dev/vulnerability/CVE-2021-33037

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-33037/
Summaries are generated and may contain mistakes; the official description above is the record.