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CERTVU:999008

HistoryNov 09, 2021 - 12:00 a.m.

Compilers permit Unicode control and homoglyph characters

2021-11-0900:00:00

www.kb.cert.org

8.3 High

CVSS3

Attack Vector

NETWORK

Attack Complexity

HIGH

Privileges Required

NONE

User Interaction

REQUIRED

Scope

CHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H

5.1 Medium

CVSS2

Access Vector

NETWORK

Access Complexity

HIGH

Authentication

NONE

Confidentiality Impact

PARTIAL

Integrity Impact

PARTIAL

Availability Impact

PARTIAL

AV:N/AC:H/Au:N/C:P/I:P/A:P

0.003 Low

EPSS

Percentile

69.2%

JSON

Overview

Attacks that allow for unintended control of Unicode and homoglyphic characters, described by the researchers in this report leverage text encoding that may cause source code to be interpreted differently by a compiler than it appears visually to a human reviewer. Source code compilers, interpreters, and other development tools may permit Unicode control and homoglyph characters, changing the visually apparent meaning of source code.

Description

Internationalized text encodings require support for both left-to-right languages and also right-to-left languages. Unicode has built-in functions to allow for encoding of characters to account for bi-directional, or Bidi ordering. Included in these functions are characters that represent non-visual functions. These characters, as well as characters from other human language sets (i.e., English vs. Cyrillic) can also introduce ambiguities into the code base if improperly used.

This type of attack could potentially be used to compromise a code base by capitalizing on a gap in visually rendered source code as a human reviewer would see and the raw code that the compiler would evaluate.

Impact

The use of attacks that incorporate maliciously encoded source code may go undetected by human developers and by many automated coding tools. These attacks also work against many of the compilers currently in use. An attacker with the ability to influence source code could introduce undetected ambiguity into source code using this type of attack.

Solution

The simplest defense is to ban the use of text directionality control characters both in language specifications and in compilers implementing these languages.

Two CVEs were assigned to address the two types of attacks described in this report.

CVE-2021-42574 was created for tracking the Bidi attack. CVE-2021-42694 was created for tracking the homoglyph attack.

Acknowledgements

Thanks to the reporters, Nicholas Boucher and Ross Anderson of The University of Cambridge (UK).

This document was written by Chuck Yarbrough.

Vendor Information

999008

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Atlassian __ Affected

Notified: 2021-09-27 Updated: 2021-11-09

Statement Date: November 03, 2021

CVE-2021-42574	Affected
CVE-2021-42694	Affected VU#999008.1

Vendor Statement

We have not received a statement from the vendor.

References

Rust Security Response WG __ Affected

Notified: 2021-10-26 Updated: 2021-11-09

Statement Date: November 04, 2021

CVE-2021-42574	Affected
CVE-2021-42694	Not Affected VU#999008.1

Vendor Statement

Regarding CVE-2021-42574, the Rust project released Rust 1.56.1, featuring new lints to alert developers about the presence of bidirectional-override codepoints in their source code. No builtin mitigation is present in Rust 1.0.0 to Rust 1.56.0: we recommend users of those compiler versions to either upgrade to a newer compiler, or to perform out-of-band checks for the presence of those codepoints in their codebase.

Regarding CVE-2021-42694, Rust already includes protection from homoglyphs in identifiers. Rust 1.0.0 to Rust 1.52.1 doesn’t support non-ASCII identifiers, which prevents the issue completely. Rust 1.53.0 and later versions do support non-ASCII identifiers, but include lints to alert developers about the presence of homoglyphs or similar issues.

References

<https://blog.rust-lang.org/2021/11/01/cve-2021-42574.html>

The LLVM Security Group __ Affected

Notified: 2021-09-27 Updated: 2021-11-09

Statement Date: October 30, 2021

CVE-2021-42574	Unknown
CVE-2021-42694	Unknown VU#999008.1

Vendor Statement

In a future release the LLVM project will include new checkers as part of clang-tidy to detect occurences of both CVE-2021-42574 and CVE-2021-42694. In the meantime we recommend clang users to perform out-of-band checks for the presence of these security issues in their codebases.