Unicode: security vulnerability (CVE-2021-42574)

What is affected, what to do

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.

An exploit is publicly available. Update Unicode to version 14.0.0.

Affected

• Unicode lt *..14.0.0
• Fedora eq 33..33
• Fedora eq 34..34
• Fedora eq 35..35
• Starwindsoftware Starwind Virtual San eq v8r13..v8r13

Fixed in

• Unicode 14.0.0

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Metrics

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

Read in glossary

From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

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. NOTE: the Unicode Consortium offers the following alternative approach to presenting this concern. An issue is noted in the nature of international text that can affect applications that implement support for The Unicode Standard and the Unicode Bidirectional Algorithm (all versions). Due to text display behavior when text includes left-to-right and right-to-left characters, the visual order of tokens may be different from their logical order. Additionally, control characters needed to fully support the requirements of bidirectional text can further obfuscate the logical order of tokens. Unless mitigated, an adversary could craft source code such that the ordering of tokens perceived by human reviewers does not match what will be processed by a compiler/interpreter/etc. The Unicode Consortium has documented this class of vulnerability in its document, Unicode Technical Report #36, Unicode Security Considerations. The Unicode Consortium also provides guidance on mitigations for this class of issues in Unicode Technical Standard #39, Unicode Security Mechanisms, and in Unicode Standard Annex #31, Unicode Identifier and Pattern Syntax. Also, the BIDI specification allows applications to tailor the implementation in ways that can mitigate misleading visual reordering in program text; see HL4 in Unicode Standard Annex #9, Unicode Bidirectional Algorithm.

Source: NVD (NIST), cvelistv5

Vendors and products

• Fedora Project Infrastructure Fedora
• Starwindsoftware Android
• HackerOne

References & sources

• unicode.orgversions/Unicode14.0.0Release notes, Vendor advisory
• trojansource.codesExploit, Technical description, Third-party advisory
• openwall.com[oss-security] 20211101 CVE-2021-42574: rustc 1.56.0 and bidirectional-override codepoints in source codeMailing list, Exploit, Mailing list, Mitigation, Third-party advisory
• openwall.com[oss-security] 20211101 Re: CVE-2021-42574: rustc 1.56.0 and bidirectional-override codepoints in source codeMailing list, Exploit, Mailing list, Third-party advisory
• openwall.com[oss-security] 20211101 Trojan Source AttacksMailing list, Mailing list, Third-party advisory
• openwall.com[oss-security] 20211102 Re: CVE-2021-42574: rustc 1.56.0 and bidirectional-override codepoints in source codeMailing list, Mailing list, Third-party advisory
• openwall.com[oss-security] 20211102 Re: Trojan Source AttacksMailing list, Mailing list
• lists.fedoraproject.orgFEDORA-2021-0578e23912Vendor advisory
• lists.fedoraproject.orgFEDORA-2021-7ad3a01f6aVendor advisory
• kb.cert.orgVU#999008Third-party advisory, Third-party advisory, US government resource
• lists.fedoraproject.orgFEDORA-2021-443139f67cVendor advisory
• scyon.nlpost/trojans-in-your-source-codeExploit, Mitigation, Third-party advisory
• unicode.orgreports/tr36Technical description, Vendor advisory
• unicode.orgreports/tr39Technical description, Vendor advisory
• unicode.orgreports/tr31Technical description, Vendor advisory
• unicode.orgreports/tr9/tr9-44.htmlTechnical description, Vendor advisory
• starwindsoftware.comsecurity/sw-20220804-0002Third-party advisory
• security.gentoo.orgGLSA-202210-09Vendor advisory, Third-party advisory