CVE-2021-36770 : What You Need to Know

This article provides an overview of CVE-2021-36770, a vulnerability in Encode.pm in Perl through version 5.34.0 that allows local users to gain privileges. Find out the impact, technical details, and mitigation strategies below.

Understanding CVE-2021-36770

CVE-2021-36770 is a security vulnerability found in the Encode.pm module of Perl versions up to 5.34.0. It enables local users to elevate their privileges by utilizing a malicious Encode::ConfigLocal library present in the current directory.

What is CVE-2021-36770?

The vulnerability in Encode.pm allows attackers to gain elevated privileges by exploiting a Trojan horse Encode::ConfigLocal library. This issue is triggered by how the || operator evaluates the @INC array in a scalar context.

The Impact of CVE-2021-36770

The impact of CVE-2021-36770 is the potential privilege escalation of local users on the affected system. Attackers can abuse a vulnerable configuration to load a malicious library and execute arbitrary code.

Technical Details of CVE-2021-36770

CVE-2021-36770 originates from a flaw in the way the Encode.pm module handles dynamic module loading. Specifically, certain 2021 versions of Encode.pm (3.05 through 3.11) are susceptible to exploitation.

Vulnerability Description

The vulnerability lies in how the || operator evaluates @INC in a scalar context, where it mistakenly interprets @INC as having an integer value, paving the way for privilege escalation.

Affected Systems and Versions

Systems running Perl versions up to 5.34.0 with Encode.pm versions 3.05 through 3.11 are at risk. Proper version upgrades and patches are essential to mitigate this vulnerability.

Exploitation Mechanism

To exploit CVE-2021-36770, an attacker must plant a malicious Encode::ConfigLocal library in the working directory and manipulate the dynamic loading process to pre-empt loading of legitimate modules.

Mitigation and Prevention

It is crucial to take immediate action to protect systems from CVE-2021-36770. Follow these mitigation strategies to enhance the security posture of your environment:

Immediate Steps to Take

Update Perl to a non-vulnerable version (post-5.34.0) and ensure Encode.pm versions are above 3.11.
Regularly monitor for unauthorized modifications to the @INC array and system libraries.

Long-Term Security Practices

Implement strong access controls and least privilege principles to limit user capabilities.
Conduct security audits and code reviews to identify and address potential vulnerabilities proactively.

Patching and Updates

Stay informed about security advisories and promptly apply patches released by Perl maintainers, distribution vendors, and security organizations to safeguard your systems.