Summary
Cap'n Proto has Undefined Behavior in constant::Reader and StructSchema
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.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is GHSA-5W5R-MF82-595P? GHSA-5W5R-MF82-595P is a critical-severity security vulnerability in capnp (rust), affecting versions < 0.24.0. It is fixed in 0.24.0.
- Which versions of capnp are affected by GHSA-5W5R-MF82-595P? capnp (rust) versions < 0.24.0 is affected.
- Is there a fix for GHSA-5W5R-MF82-595P? Yes. GHSA-5W5R-MF82-595P is fixed in 0.24.0. Upgrade to this version or later.
- Is GHSA-5W5R-MF82-595P exploitable, and should I be worried? Whether GHSA-5W5R-MF82-595P is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-5W5R-MF82-595P is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-5W5R-MF82-595P? Upgrade
capnpto 0.24.0 or later.