GHSA-5W5R-MF82-595P

Vulnerability from github – Published: 2026-01-28 16:06 – Updated: 2026-01-28 16:06
VLAI
Summary
Cap'n Proto has Undefined Behavior in constant::Reader and StructSchema
Details

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.

Severity
9.3 (Critical)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Show details on source website
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{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "capnp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.24.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-758"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-01-28T16:06:09Z",
    "nvd_published_at": null,
    "severity": "CRITICAL"
  },
  "details": "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.\n\n## `Reader::get`\n\n```rust\npub fn get(\u0026self) -\u003e Result\u003c\u003cT as Owned\u003e::Reader\u003c\u0027static\u003e\u003e {\n    // ...\n    // UNSAFE: access `words` without validation\n}\n```\n\n## `StructSchema::new`\n\n```rust\npub fn new(builder: RawBrandedStructSchema) -\u003e StructSchema {\n    // ...\n    // UNSAFE: access encoded nodes without validation\n}\n```\n\nThis vulnerability allows safe Rust code to trigger UB, which violates Rust\u0027s safety guarantees.\n\nThe 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.",
  "id": "GHSA-5w5r-mf82-595p",
  "modified": "2026-01-28T16:06:09Z",
  "published": "2026-01-28T16:06:09Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/capnproto/capnproto-rust/issues/605"
    },
    {
      "type": "WEB",
      "url": "https://github.com/capnproto/capnproto-rust/commit/7b981f4c75a975c80444cd38729bcdf12bf3eabf"
    },
    {
      "type": "WEB",
      "url": "https://github.com/capnproto/capnproto-rust/commit/e3aeec213e6d1b30a182bf61682a370f20d8a02c"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/capnproto/capnproto-rust"
    },
    {
      "type": "WEB",
      "url": "https://rustsec.org/advisories/RUSTSEC-2025-0143.html"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Cap\u0027n Proto has Undefined Behavior in constant::Reader and StructSchema"
}



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