RUSTSEC-2021-0031: split_at allows obtaining multiple mutable references to the same data

What to do

1. See which version you have: cargo tree -i nano_arena (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p nano_arena. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example nano_arena = "0.5.2") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

Affected versions of this crate assumed that Borrow<Idx> was guaranteed to return the same value on .borrow(). The borrowed index value was used to retrieve a mutable reference to a value.

If the Borrow<Idx> implementation returned a different index, the split arena would allow retrieving the index as a mutable reference creating two mutable references to the same element. This violates Rust's aliasing rules and allows for memory safety issues such as writing out of bounds and use-after-frees.

The flaw was corrected in commit 6b83f9d by storing the .borrow() value in a temporary variable.

References

• Package: crates.io/crates/nano_arena
• Advisory: rustsec.org/advisories/RUSTSEC-2021-0031.html
• Report: github.com/bennetthardwick/nano-arena/issues/1

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.