What is GHSA-5w5r-mf82-595p?

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 ::Reader > { // ... // 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

How severe is GHSA-5w5r-mf82-595p?

No CVSS score has been assigned to GHSA-5w5r-mf82-595p yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-5w5r-mf82-595p?

GHSA-5w5r-mf82-595p affects the following packages: capnp (crates.io). Ecosystems affected: crates.io.

How do I fix GHSA-5w5r-mf82-595p?

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.

How do I detect GHSA-5w5r-mf82-595p in my crates.io dependencies?

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.

How do I mitigate GHSA-5w5r-mf82-595p if there is no patch (or I can't update yet)?

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.

Is GHSA-5w5r-mf82-595p actively exploited in the wild?

No public exploit code has been indexed for GHSA-5w5r-mf82-595p yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was GHSA-5w5r-mf82-595p published, and has it been updated?

GHSA-5w5r-mf82-595p was published on January 28, 2026 and was last updated on February 3, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🦀 crates.io

Not in CISA KEV

Cap'n Proto has Undefined Behavior in constant::Reader and StructSchemaGHSA-5w5r-mf82-595p

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.

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

Ecosystem	Package	Vulnerable range	Fix
🦀crates.io	`capnp`	all versions	0.24.0`cargo update -p capnp --precise 0.24.0`

Detection & mitigation playbook

Open-source dependency

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.
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.
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.

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

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Cap'n Proto has Undefined Behavior in constant::Reader and StructSchemaGHSA-5w5r-mf82-595p

Real-World Exposure

Description

Reader::get

StructSchema::new