add post about CVE-2021-42574

Author: pietroalbini

posts/2021-11-01-cve-2021-42574.md

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+---
+layout: post
+title: "Security advisory for rustc (CVE-2021-42574)"
+author: The Rust Security Response WG
+---
+
+> This is a cross-post of [the official security advisory][advisory]. The
+> official advisory contains a signed version with our PGP key, as well.
+
+[advisory]: https://groups.google.com/g/rustlang-security-announcements/c/bKPH8XYMvJU
+
+The Rust Security Response WG was notified of a security concern affecting
+source code containing "bidirectional override" Unicode codepoints: in some
+cases the use of those codepoints could lead to the reviewed code being
+different than the compiled code.
+
+This is a vulnerability in the Unicode specification, and its assigned
+identifier is [CVE-2021-42574]. While the vulnerability itself is not a rustc
+flaw, we're taking proactive measures to mitigate its impact on Rust
+developers.
+
+## Overview
+
+Unicode has support for both left-to-right and right-to-left languages, and to
+aid writing left-to-right words inside a right-to-left sentence (or vice versa)
+it also features invisible codepoints called "bidirectional override".
+
+These codepoints are normally used across the Internet to embed a word inside a
+sentence of another language (with a different text direction), but it was
+reported to us that they could be used to manipulate how source code is
+displayed in some editors and code review tools, leading to the reviewed code
+being different than the compiled code. This is especially bad if the whole
+team relies on bidirectional-aware tooling.
+
+As an example, the following snippet (with `{U+NNNN}` replaced with the Unicode
+codepoint `NNNN`):
+
+```rust
+if access_level != "user{U+202E} {U+2066}// Check if admin{U+2069} {U+2066}" {
+```
+
+...would be rendered by bidirectional-aware tools as:
+
+```rust
+if access_level != "user" { // Check if admin
+```
+
+## Affected Versions
+
+Rust 1.56.1 introduces two new lints to detect and reject code containing the
+affected codepoints. Rust 1.0.0 through Rust 1.56.0 do not include such lints,
+leaving your source code vulnerable to this attack if you do not perform
+out-of-band checks for the presence of those codepoints.
+
+To assess the security of the ecosystem we analyzed all crate versions ever
+published on crates.io (as of 2021-10-17), and only 5 crates have the affected
+codepoints in their source code, with none of the occurrences being malicious.
+
+## Mitigations
+
+We will be releasing Rust 1.56.1 today, 2021-11-01, with two new
+deny-by-default lints detecting the affected codepoints, respectively in string
+literals and in comments. The lints will prevent source code files containing
+those codepoints from being compiled, protecting you from the attack.
+
+If your code has legitimate uses for the codepoints we recommend replacing them
+with the related escape sequence. The error messages will suggest the right
+escapes to use.
+
+If you can't upgrade your compiler version, or your codebase also includes
+non-Rust source code files, we recommend periodically checking that the
+following codepoints are not present in your repository and your dependencies:
+U+202A, U+202B, U+202C, U+202D, U+202E, U+2066, U+2067, U+2068, U+2069.
+
+## Timeline of events
+
+* 2021-07-25: we received the report and started working on a fix.
+* 2021-09-14: the date for the embargo lift (2021-11-01) is communicated to us.
+* 2021-10-17: performed an analysis of all the source code ever published to
+  crates.io to check for the presence of this attack.
+* 2021-11-01: embargo lifts, the vulnerability is disclosed and Rust 1.56.1 is
+  released.
+
+## Acknowledgments
+
+Thanks to [Nicholas Boucher][1] and [Ross Anderson][2] from the University of
+Cambridge for disclosing this to us according to our [security policy][3]!
+
+We also want to thank the members of the Rust project who contributed to the
+mitigations for this issue. Thanks to Esteban Küber for developing the lints,
+Pietro Albini for leading the security response, and many others for their
+involvement, insights and feedback: Josh Stone, Josh Triplett, Manish
+Goregaokar, Mara Bos, Mark Rousskov, Niko Matsakis, and Steve Klabnik.
+
+## Appendix: Homoglyph attacks
+
+As part of their research, Nicholas Boucher and Ross Anderson also uncovered a
+similar security issue identified as [CVE-2021-42694] involving homoglyphs inside
+identifiers. Rust already includes mitigations for that attack since Rust
+1.53.0. Rust 1.0.0 through Rust 1.52.1 is not affected due to the lack of
+support for non-ASCII identifiers in those releases.
+
+[1]: https://github.com/nickboucher
+[2]: https://www.cl.cam.ac.uk/~rja14
+[3]: https://www.rust-lang.org/policies/security
+
+[CVE-2021-42574]: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-42574
+[CVE-2021-42694]: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-42694