An issue was discovered in the better-macro crate through 2021-07-22 for Rust. It intentionally demonstrates that remote attackers can execute arbitrary code via proc-macros, and otherwise has no legitimate purpose.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-38196
• https://github.com/raycar5/better-macro/blob/24ff1702397b9c19bbfa4c660e2316cd77d3b900/src/lib.rs#L36-L38
• https://rustsec.org/advisories/RUSTSEC-2021-0077.html