RUSTSEC-2022-0007: A malicious coder can get unsound access to TCell or TLCell memory

What to do

1. See which version you have: cargo tree -i qcell (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p qcell. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example qcell = "0.4.3") 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

This is impossible to do by accident, but by carefully constructing marker types to be covariant, a malicious coder can cheat the singleton check in TCellOwner and TLCellOwner, giving unsound access to cell memory. This could take the form of getting two mutable references to the same memory, or a mutable reference and an immutable reference.

The fix is for the crate to internally force the marker type to be invariant. This blocks the conversion between covariant types which Rust normally allows.

References

• Package: crates.io/crates/qcell
• Advisory: rustsec.org/advisories/RUSTSEC-2022-0007.html
• Report: github.com/uazu/qcell/issues/20

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