CVE-2021-42574

DISPUTED 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.