CVE-2023-48230: 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…

PriorityP262critical9.8CVSS 3.1

AVNACLPRNUINSUCHIHAH

EPSS

1.89%

76.9th percentile

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.

Affected

6 ranges

Vendor	Product	Version range	Fixed in
capnproto	capnproto	—	—
capnproto	capnproto	—	—
capnproto	capnproto	—	—
capnproto	capnproto	>= 0 < 1.0.1-3	1.0.1-3
capnproto	capnproto	>= 0 < 1.0.1-3	1.0.1-3
debian	capnproto	< capnproto 1.0.1-3 (forky)	capnproto 1.0.1-3 (forky)

Detection & IOCsextracted from sources · hover to see the quote

bytes↗

{ 0x00, 0x00, 0xFF, 0xFF }

→Trigger condition: KJ HTTP library must have WebSocket compression explicitly enabled via HttpClientSettings or HttpServerSettings; default installs are NOT vulnerable. ↗
→Exploitation results in a heap-buffer underrun writing the constant 4-byte sequence 0x00 0x00 0xFF 0xFF out-of-bounds; monitor for unexpected crashes or heap corruption in processes using KJ HTTP with WebSocket compression. ↗
→Scope is network-reachable (remote peer); the attack surface is any Cap'n Proto 1.0 or 1.0.1 service exposing a WebSocket endpoint with compression enabled. ↗

·Only Cap'n Proto versions 1.0 and 1.0.1 are affected; versions prior to 1.0 are not vulnerable. ↗
·The vulnerability is fixed in Cap'n Proto 1.0.1.1 (upstream) and package version 1.0.1-3 in Debian bookworm/bullseye/forky/sid/trixie. ↗
·Most deployments are unaffected; maintainers believe only the Cloudflare Workers Runtime is a likely real-world target. ↗

CVSS provenance

nvdv3.19.8CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

osv9.8CRITICAL

vendor_debian5.9LOW

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