# CVE-2023-48230: capnproto: Cap'n Proto is a data interchange format and capability-based RPC system

CVE-2023-48230 is fixed in capnproto on 1 platform, first by DEBIAN-CVE-2023-48230, released November 21, 2023. It is not reported as exploited. Patch Radar priority: Urgent.

- Web page: https://patchblox.com/security/cve/CVE-2023-48230
- Priority: Urgent
- CVSS: 9.8 (critical)
- EPSS: 0.0189
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Nov 21, 2023

## Description

Cap'n Proto is a data interchange format and capability-based RPC system. In versions 1.0 and 1.0.1, when using the KJ HTTP library with WebSocket compression enabled, a buffer underrun can be caused by a remote peer. The underrun always writes a constant value that is not attacker-controlled, likely resulting in a crash, enabling a remote denial-of-service attack. Most Cap'n Proto and KJ users are unlikely to have this functionality enabled and so unlikely to be affected. Maintainers suspect only the Cloudflare Workers Runtime is affected.  If KJ HTTP is used with WebSocket compression enabled, a malicious peer may be able to cause a buffer underrun on a heap-allocated buffer. KJ HTTP is an optional library bundled with Cap'n Proto, but is not directly used by Cap'n Proto. WebSocket compression is disabled by default. It must be enabled via a setting passed to the KJ HTTP library via `HttpClientSettings` or `HttpServerSettings`. The bytes written out-of-bounds are always a specific constant 4-byte string `{ 0x00, 0x00, 0xFF, 0xFF }`. Because this string is not controlled by the attacker, maintainers believe it is unlikely that remote code execution is possible. However, it cannot be ruled out. This functionality first appeared in Cap'n Proto 1.0. Previous versions are not affected.  This issue is fixed in Cap'n Proto 1.0.1.1.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | capnproto Debian 13.5 (trixie) | 1.0.1-3 | [DEBIAN-CVE-2023-48230](https://patchblox.com/security/data/fix/linux/debian-cve-2023-48230.md) | Nov 21, 2023 |
| Linux | capnproto Debian 14 | 1.0.1-3 | [DEBIAN-CVE-2023-48230](https://patchblox.com/security/data/fix/linux/debian-cve-2023-48230.md) | Nov 21, 2023 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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