CVE-2021-36770 in Middleware Common Libraries and Tools
Summary
by MITRE • 08/12/2021
Encode.pm, as distributed in Perl through 5.34.0, allows local users to gain privileges via a Trojan horse Encode::ConfigLocal library (in the current working directory) that preempts dynamic module loading. Exploitation requires an unusual configuration, and certain 2021 versions of Encode.pm (3.05 through 3.11). This issue occurs because the || operator evaluates @INC in a scalar context, and thus @INC has only an integer value.
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Analysis
by VulDB Data Team • 11/04/2025
The vulnerability identified as CVE-2021-36770 represents a privilege escalation risk within the Perl programming language ecosystem, specifically affecting versions through 5.34.0. This flaw resides in the Encode.pm module, which serves as a critical component for character encoding handling in Perl applications. The vulnerability exploits a subtle but significant issue in how Perl evaluates the @INC array during module loading processes, creating a pathway for malicious actors to execute arbitrary code with elevated privileges. The attack vector requires a carefully crafted Trojan horse library named Encode::ConfigLocal that must be placed in the current working directory of a targeted process, effectively preempting legitimate module loading mechanisms.
The technical root cause of this vulnerability stems from a fundamental flaw in Perl's dynamic module loading behavior when the || operator is used with INC in scalar context. This particular implementation error results in INC being evaluated as a single integer value rather than maintaining its intended array structure. The || operator, which typically functions as a logical OR, when applied to @INC in scalar context, converts the array into a scalar representation that loses all directory path information. This conversion effectively undermines the module loading security model by allowing attackers to manipulate the module search path through the current working directory, bypassing normal security controls that would normally prevent loading of malicious libraries from untrusted locations.
The operational impact of CVE-2021-36770 extends beyond simple privilege escalation to encompass broader system compromise potential. When exploited, this vulnerability allows local users to inject malicious code into Perl processes that rely on the Encode module, potentially leading to complete system compromise. The attack requires specific conditions including the presence of vulnerable Encode.pm versions between 3.05 and 3.11, along with a particular system configuration that makes the current working directory manipulation feasible. This makes the vulnerability somewhat niche but still dangerous as it can be leveraged by attackers who have already gained local access to a system. The vulnerability aligns with CWE-276, which addresses improper privileges, and represents a classic example of a Trojan horse attack where malicious code masquerades as legitimate system components.
From a cybersecurity perspective, this vulnerability demonstrates the importance of understanding subtle language features and their security implications in interpreted languages. The attack scenario requires an attacker to place a malicious library in the current working directory of a process that will subsequently load the Encode::ConfigLocal module, which is particularly concerning in environments where user-controllable processes might execute with elevated privileges. The vulnerability also highlights the need for robust module loading security models and proper validation of module paths, as the issue essentially allows attackers to manipulate Perl's module resolution mechanism through directory manipulation. Mitigation strategies should focus on updating to patched versions of Perl and Encode.pm, implementing proper directory permissions, and ensuring that the current working directory is not trusted as a module source. This vulnerability serves as a reminder that seemingly innocuous language operators can create significant security implications when combined with specific environmental conditions, and aligns with ATT&CK technique T1059.007 for execution through scripting languages.