RUSTSEC-2021-0019: Multiple soundness issues

What to do

1. See which version you have: cargo tree -i xcb (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p xcb. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example xcb = "1.0.0") 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

Calls std::str::from_utf8_unchecked() without any checks

The function xcb::xproto::GetAtomNameReply::name() calls std::str::from_utf8_unchecked() on the raw bytes that were received from the X11 server without any validity checks. The X11 server only prevents interior null bytes, but otherwise allows any X11 client to create an atom for arbitrary bytes.

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/96

xcb::xproto::GetPropertyReply::value() allows arbitrary return types

The function xcb::xproto::GetPropertyReply::value() returns a slice of type T where T is an unconstrained type parameter. The raw bytes received from the X11 server are interpreted as the requested type.

The users of the xcb crate are advised to only call this function with the intended types. These are u8, u16, and u32.

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/95

Out of bounds read in xcb::xproto::change_property()

xcb::xproto::change_property has (among others) the arguments format: u8 and data: &[T]. The intended use is one of the following cases:

However, this constraint is not enforced. For example, it is possible to call the function with format = 32 and T = u8. In this case, a read beyond the end of the data slice is performed and the bytes are sent to the X11 server.

• format = 8 and T = u8
• format = 16 and T = u16
• format = 32 and T = u32

The users of the xcb crate are advised to only call this function with one of the intended argument combinations.

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/94

'Safe' wrapper around std::mem::transmute()

The function xcb::base::cast_event() takes a reference to a xcb::base::GenericEvent and returns a reference to an arbitrary type, as requested by the caller (or found via type interference). The function is implemented as a direct call to std::mem::transmute(). Since the return type is not constrained, this allows transmution to an incorrect type or a type that is larger than the X11 event that was passed in.

X11 events are mostly always 32 bytes large and this function works as intended.

Users are advised to only cast to the event structs provided by the xcb crate (and hope for the best).

This issue is tracked here: https://github.com/rust-x-bindings/rust-xcb/issues/78

References

• Package: crates.io/crates/xcb
• Advisory: rustsec.org/advisories/RUSTSEC-2021-0019.html
• Report: github.com/RustSec/advisory-db/issues/653
• Report: github.com/rust-x-bindings/rust-xcb/issues/78
• Report: github.com/rust-x-bindings/rust-xcb/issues/94
• Report: github.com/rust-x-bindings/rust-xcb/issues/95
• Report: github.com/rust-x-bindings/rust-xcb/issues/96

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 xcb

• Unsound MediumRUSTSEC-2020-0097 · 2020-12-10Soundness issue with base::Error
• UnsoundRUSTSEC-2025-0051 · 2025-08-05`xcb::Connection::connect_to_fd*` functions violate I/O safety

Full security history of xcb