RUSTSEC-2025-0143: Unsound APIs of public `constant::Reader` and `StructSchema`

What to do

1. See which version you have: cargo tree -i capnp (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p capnp. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example capnp = "0.24.0") 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 safe API functions constant::Reader::get and StructSchema::new rely on PointerReader::get_root_unchecked, which can cause undefined behavior (UB) by constructing arbitrary words or schemas.

Reader::get

pub fn get(&self) -> Result<<T as Owned>::Reader<'static>> {
    // ...
    // UNSAFE: access `words` without validation
}

StructSchema::new

pub fn new(builder: RawBrandedStructSchema) -> StructSchema {
    // ...
    // UNSAFE: access encoded nodes without validation
}

This vulnerability allows safe Rust code to trigger UB, which violates Rust's safety guarantees.

The issue is resolved in version 0.24.0 by making constructor functions unsafe and mark the fields of struct as visible only in the crate.

References

• Package: crates.io/crates/capnp
• Advisory: rustsec.org/advisories/RUSTSEC-2025-0143.html
• Report: github.com/capnproto/capnproto-rust/issues/605

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 capnp

• VulnerabilityRUSTSEC-2022-0068 · 2022-11-30out-of-bounds read possible when setting list-of-pointers

Full security history of capnp