GHSA-5w5r-mf82-595p
Cap'n Proto has Undefined Behavior in constant::Reader and StructSchema
Blast Radius
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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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | capnp | all versions | 0.24.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for capnp. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether GHSA-5w5r-mf82-595p is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
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
Is GHSA-5w5r-mf82-595p in your dependencies?
O3 detects GHSA-5w5r-mf82-595p across crates.io dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.