CVE-2021-21290: Local Information Disclosure Vulnerability in Netty on Unix-Like systems due temporary files

Vulnerability summary

CVE-2021-21290 is a medium (CVSS 6.2) vulnerability in debian debian_linux. EPSS estimates a 1.80% chance of exploitation in the next 30 days, higher than 78.5% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 08-02-2021
   PublishedVulnerability first disclosed.
2. 08-10-2026
   Last modifiedVulnerability information updated.

Description

Netty is an open-source, asynchronous event-driven network application framework for rapid development of maintainable high performance protocol servers & clients. In Netty before version 4.1.59.Final there is a vulnerability on Unix-like systems involving an insecure temp file. 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. 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. This has been fixed in version 4.1.59.Final. As a workaround, one may 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.

• CWE-378Creation of Temporary File With Insecure PermissionsOpening temporary files without appropriate measures or controls can leave the file, its contents and any function that it impacts vulnerable to attack.
• CWE-379Creation of Temporary File in Directory with Insecure PermissionsThe product creates a temporary file in a directory whose permissions allow unintended actors to determine the file's existence or otherwise access that file.
• CWE-668Exposure of Resource to Wrong SphereThe product exposes a resource to the wrong control sphere, providing unintended actors with inappropriate access to the resource.