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GHSA-5w5r-mf82-595p

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

9.3 CriticalFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
High
Integrity
High
Availability
High

What each source reports

GitHub Advisories Score used9.3

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.

Record
GHSA-5w5r-mf82-595p
CVSS 4.0
9.3 CriticalCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-758
Updated
2026-11-02
OSV9.3

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.

Record
GHSA-5w5r-mf82-595p
CVSS 4.0
9.3 CriticalCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-758
Updated
2026-11-02
OSVRUSTSEC-2025-0143No score

Unsound APIs of public `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.

Record
RUSTSEC-2025-0143
Updated
2026-11-02

Affected packages

Affected packages
PackageAffectedFixed
capnp
crates.io
before 0.24.0
0.24.0
capnp
crates.io
0.0.0-0 up to 0.24.0
0.24.0

History

History
RecordedChangedSources
first indexedGitHub Advisories, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

GHSA-5w5r-mf82-595p is a critical vulnerability (CVSS 9.3) in capnp, published December 24, 2025. No source lists public exploit code. Fixed in capnp 0.24.0.

References

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