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GHSA-5w5r-mf82-595p — capnp

Fix: capnproto/capnproto-rust@7b981f4

GHSA-5w5r-mf82-595p is a security vulnerability in capnp. A fix is available for capnp — see the affected versions and patch details below.

Cap'n Proto has Undefined Behavior in constant::Reader and StructSchema

Also known asRUSTSEC-2025-0143
Published
Jan 28, 2026
Updated
Feb 3, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Feb 3, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected
🦀capnp

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iocapnpall versions0.24.0cargo update -p capnp --precise 0.24.0

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for capnp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update capnp to 0.24.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-5w5r-mf82-595p is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-5w5r-mf82-595p can be triaged on real exposure rather than presence alone.

Tailored to GHSA-5w5r-mf82-595p. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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` ```rust pub fn get(&self) -> Result<<T as Owned>::Reader<'static>> { // ... // UNSAFE: access `words` without validation } ``` ## `StructSchema::new` ```rust 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
O3 Security · Impact-Aware SCA

Is GHSA-5w5r-mf82-595p in your dependencies?

O3 Security finds GHSA-5w5r-mf82-595p across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works
GHSA-5w5r-mf82-595p: capnp | O3 Security