CVE-2021-42574 in Specification
Summary
by MITRE • 11/01/2021
An issue was discovered in the Bidirectional Algorithm in the Unicode Specification through 14.0. It permits the visual reordering of characters via control sequences, which can be used to craft source code that renders different logic than the logical ordering of tokens ingested by compilers and interpreters. Adversaries can leverage this to encode source code for compilers accepting Unicode such that targeted vulnerabilities are introduced invisibly to human reviewers.
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Analysis
by VulDB Data Team • 05/02/2025
The vulnerability described in CVE-2021-42574 resides within the Unicode Bidirectional Algorithm specification through version 14.0, representing a sophisticated class of security flaws that exploit the fundamental way text is processed and displayed in computing systems. This issue specifically targets the handling of bidirectional text rendering where characters can be visually reordered through control sequences, creating a deceptive environment where the visual representation of code differs significantly from its actual logical execution order. The flaw operates at the intersection of text processing standards and software compilation, creating an attack surface that has profound implications for code review processes and security auditing procedures. This vulnerability is particularly concerning because it leverages the very standards that enable international text processing to create invisible security threats that bypass traditional code analysis methods.
The technical mechanism behind this vulnerability involves the Unicode Bidirectional Algorithm's handling of control characters that dictate how text should be displayed when mixed with right-to-left languages. When source code contains Unicode characters with bidirectional control sequences, these control codes can manipulate the visual presentation of the code while preserving the logical execution order that compilers and interpreters process. Attackers can strategically place these control sequences within source code to create visual representations that appear benign or even correct to human reviewers while simultaneously introducing malicious logic that executes differently than what is visually apparent. This creates a scenario where the code's visual appearance can be manipulated to hide actual security vulnerabilities, making the flaw particularly dangerous in environments where code review is the primary security control mechanism.
The operational impact of CVE-2021-42574 extends far beyond simple code obfuscation, as it fundamentally undermines the trust model that security professionals rely upon during code analysis and vulnerability assessment. The vulnerability enables attackers to craft source code that appears to perform legitimate operations while actually executing malicious logic that can lead to privilege escalation, data exfiltration, or other security breaches. This type of attack is particularly insidious because it can bypass static code analysis tools that rely on visual inspection and traditional pattern matching techniques, as the tools see the visual representation but not the underlying logical execution flow. The implications for software development security are severe, as this vulnerability can be exploited in any system that accepts source code input and processes it through compilers or interpreters that handle Unicode text processing, affecting everything from web applications to embedded systems.
The security implications of this vulnerability align with several established threat modeling frameworks, including the ATT&CK framework's techniques for code injection and privilege escalation through obfuscation. This flaw represents a specific instance of CWE-116 improper encoding or escaping of control characters, where the control sequences are not properly sanitized or handled during text processing. Organizations using affected software must implement comprehensive mitigation strategies that include updating Unicode libraries, implementing stricter input validation for source code, and developing new code review processes that account for bidirectional text manipulation. The vulnerability also highlights the importance of considering internationalization and localization standards as security considerations, as the same features that enable global text processing can be weaponized to create invisible security threats. Mitigation efforts should focus on preventing the injection of bidirectional control characters in source code, implementing automated detection mechanisms, and ensuring that all text processing systems properly handle Unicode control sequences in a security-conscious manner.