CVE-2026-32240: Cap'n Proto: Integer overflow in KJ-HTTP chunk size

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-12 · updated 2026-06-17
Weakness: CWE-197

Description

Cap'n Proto is a data interchange format and capability-based RPC system. Prior to 1.4.0, when using Transfer-Encoding: chunked, if a chunk's size parsed to a value of 2^64 or larger, it would be truncated to a 64-bit integer. In theory, this bug could enable HTTP request/response smuggling. This vulnerability is fixed in 1.4.0.

Affected products

capnproto: before 1.4.0 — fixed in 1.4.0

How to fix

Upgrade to a fixed version: capnproto 1.4.0. Check the vendor advisory for mitigations.

References

https://capnproto.org/capnproto-c++-1.4.0.tar.gz
https://capnproto.org/capnproto-c++-win32-1.4.0.zip
https://github.com/capnproto/capnproto/commit/2744b3c012b4aa3c31cefb61ec656829fa5c0e36
https://github.com/capnproto/capnproto/commit/e929f0ba7901a6b8f4b5ba9a4db00af43288cbb0
https://github.com/capnproto/capnproto/security/advisories/GHSA-vpcq-mx5v-32wm

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