CVE-2021-21290 is a medium-severity (CVSS 5.5) CWE-378 vulnerability in io.netty:netty-codec-http. 1 public exploit reference exists, so weaponization risk is real. A fix is available for io.netty:netty-codec-http — see the affected versions and patch details below.
Local Information Disclosure Vulnerability in Netty on Unix-Like systems
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21290 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
io.netty:netty-codec-http☕org.jboss.netty:netty☕io.netty:nettyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Impact
When netty's multipart decoders are used local information disclosure can occur via the local system temporary directory if temporary storing uploads on the disk is enabled.
The CVSSv3.1 score of this vulnerability is calculated to be a 6.2/10
Vulnerability Details
On unix-like systems, the temporary directory is shared between all user. As such, writing to this directory using APIs that do not explicitly set the file/directory permissions can lead to information disclosure. Of note, this does not impact modern MacOS Operating Systems.
The method File.createTempFile on unix-like systems creates a random file, but, by default will create this file with the permissions -rw-r--r--. Thus, if sensitive information is written to this file, other local users can read this information.
This is the case in netty's AbstractDiskHttpData is vulnerable.
AbstractDiskHttpData is used as a part of the DefaultHttpDataFactory class which is used by HttpPostRequestDecoder / HttpPostMultiPartRequestDecoder.
You may be affected by this vulnerability your project contains the following code patterns:
channelPipeline.addLast(new HttpPostRequestDecoder(...));
channelPipeline.addLast(new HttpPostMultiPartRequestDecoder(...));
Patches
This has been patched in version 4.1.59.Final.
Workarounds
Specify your own java.io.tmpdir when you start the JVM or use DefaultHttpDataFactory.setBaseDir(...) to set the directory to something that is only readable by the current user.
References
- CWE-378: Creation of Temporary File With Insecure Permissions
- CWE-379: Creation of Temporary File in Directory with Insecure Permissions
Similar Vulnerabilities
Similar, but not the same.
- JUnit 4 - https://github.com/junit-team/junit4/security/advisories/GHSA-269g-pwp5-87pp
- Google Guava - https://github.com/google/guava/issues/4011
- Apache Ant - https://nvd.nist.gov/vuln/detail/CVE-2020-1945
- JetBrains Kotlin Compiler - https://nvd.nist.gov/vuln/detail/CVE-2020-15824
For more information
If you have any questions or comments about this advisory:
Original Report
Hi Netty Security Team,
I've been working on some security research leveraging custom CodeQL queries to detect local information disclosure vulnerabilities in java applications. This was the result from running this query against the netty project: https://lgtm.com/query/7723301787255288599/
Netty contains three local information disclosure vulnerabilities, so far as I can tell.
One is here, where the private key for the certificate is written to a temporary file.
One is here, where the certificate is written to a temporary file.
The final one is here, where the 'AbstractDiskHttpData' creates a temporary file if the getBaseDirectory() method returns null. I believe that 'AbstractDiskHttpData' is used as a part of the file upload support? If this is the case, any files uploaded would be similarly vulnerable.
All of these vulnerabilities exist because
File.createTempFile(String, String)will create a temporary file in the system temporary directory if the 'java.io.tmpdir' system property is not explicitly set. It is my understanding that when java creates a file, by default, and using this method, the permissions on that file utilize the umask. In a majority of cases, this means that the file that java creates has the permissions:-rw-r--r--, thus, any other local user on that system can read the contents of that file.Impacted OS:
- Any OS where the system temporary directory is shared between multiple users. This is not the case for MacOS or Windows.
Mitigation.
Moving to the
FilesAPI instead will fix this vulnerability. https://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html#createTempFile-java.nio.file.Path-java.lang.String-java.lang.String-java.nio.file.attribute.FileAttribute...-This API will explicitly set the posix file permissions to something safe, by default.
I recently disclosed a similar vulnerability in JUnit 4: https://github.com/junit-team/junit4/security/advisories/GHSA-269g-pwp5-87pp
If you're also curious, this vulnerability in Jetty was also mine, also involving temporary directories, but is not the same vulnerability as in this case. https://github.com/eclipse/jetty.project/security/advisories/GHSA-g3wg-6mcf-8jj6
I would appreciate it if we could perform disclosure of this vulnerability leveraging the GitHub security advisories feature here. GitHub has a nice credit system that I appreciate, plus the disclosures, as you can see from the sampling above, end up looking very nice. https://github.com/netty/netty/security/advisories
This vulnerability disclosure follows Google's 90-day vulnerability disclosure policy (I'm not an employee of Google, I just like their policy). Full disclosure will occur either at the end of the 90-day deadline or whenever a patch is made widely available, whichever occurs first.
Cheers, Jonathan Leitschuh
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.netty:netty-codec-http | ≥ 4.0.0&&< 4.1.59.Final | 4.1.59.Finalio.netty:netty-codec-http:4.1.59.Final |
| ☕Maven | org.jboss.netty:netty | all versions | No fix |
| ☕Maven | io.netty:netty | all versions | No fix |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-http, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-codec-http to 4.1.59.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21290 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21290 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21290. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
| Product | Fixed in | Advisory |
|---|---|---|
| AMQ Clients 2.y for RHEL 7 | qpid-proton-0:0.33.0-6.el7_9 | RHSA-2021:1511 |
| Red Hat AMQ 7.8.2 | netty | RHSA-2021:2689 |
| Red Hat AMQ 7.9.0 | see advisory | RHSA-2021:3700 |
| Red Hat AMQ Online 1.7.0 GA | netty | RHSA-2021:0986 |
| Red Hat AMQ Streams 1.8.0 | netty | RHSA-2021:3225 |
| Red Hat build of Eclipse Vert.x 4.0.3 | netty | RHSA-2021:0943 |
| Red Hat build of Quarkus 2.2.3 | netty | RHSA-2021:3880 |
| Red Hat Data Grid 8.2.0 | netty | RHSA-2021:2139 |
Frequently Asked Questions
Is CVE-2021-21290 in your dependencies?
O3 Security finds CVE-2021-21290 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.