RUSTSEC-2023-0035: Adverserial use of `make_bitflags!` macro can cause undefined behavior

What to do

1. See which version you have: cargo tree -i enumflags2 (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p enumflags2. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example enumflags2 = "0.7.7") 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 macro relied on an expression of the form Enum::Variant always being a variant of the enum. However, it may also be an associated integer constant, in which case there's no guarantee that the value of said constant consists only of bits valid for this bitflag type.

Thus, code like this could create an invalid BitFlags<Test>, which would cause iterating over it to trigger undefined behavior. As the debug formatter internally iterates over the value, it is also affected.

use enumflags2::{bitflags, make_bitflags};

#[bitflags]
#[repr(u8)]
#[derive(Copy, Clone, Debug)]
enum Test {
    A = 1,
    B = 2,
}

impl Test {
    const C: u8 = 69;
}

fn main() {
    let x = make_bitflags!(Test::{C});
    // printing or iterating over x is UB
}

References

• Package: crates.io/crates/enumflags2
• Advisory: rustsec.org/advisories/RUSTSEC-2023-0035.html
• Web: github.com/meithecatte/enumflags2/releases/tag/v0.7.7

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