RUSTSEC-2020-0009: `read_scalar` and `read_scalar_at` allow transmuting values without `unsafe` blocks

What to do

1. See which version you have: cargo tree -i flatbuffers (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p flatbuffers. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example flatbuffers = "2.0.0") 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

The read_scalar and read_scalar_at functions are unsound because they allow transmuting values without unsafe blocks.

The following example shows how to create a dangling reference:

fn main() {
    #[derive(Copy, Clone, PartialEq, Debug)]
    struct S(&'static str);
    impl flatbuffers::EndianScalar for S {
        fn to_little_endian(self) -> Self { self }
        fn from_little_endian(self) -> Self { self }
    }
    println!("{:?}", flatbuffers::read_scalar::<S>(&[1; std::mem::size_of::<S>()]));
}

References

• Package: crates.io/crates/flatbuffers
• Advisory: rustsec.org/advisories/RUSTSEC-2020-0009.html
• Report: github.com/google/flatbuffers/issues/5825

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

Other advisories for flatbuffers

• Vulnerability CriticalRUSTSEC-2019-0028 · 2019-10-20Unsound `impl Follow for bool`
• Vulnerability CriticalRUSTSEC-2021-0122 · 2021-10-31Generated code can read and write out of bounds in safe code

Full security history of flatbuffers