Vulnerability · Published Mar 12, 2026 · Assigned by GitHub_M

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

Medium 6.3

In plain English

A medium-severity security flaw in capnproto from capnproto. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 13% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)6.3Medium

Exploit odds (30 days)0.2%top 87.4% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	capnproto	no fix listed	DEBIAN-CVE-2026-32240
Debian:13	capnproto	no fix listed	DEBIAN-CVE-2026-32240
Debian:14	capnproto	1.4.0-2	DEBIAN-CVE-2026-32240
Ubuntu:25.10	capnproto	no fix listed	UBUNTU-CVE-2026-32240
Ubuntu:26.10	capnproto	1.4.0-3build1	UBUNTU-CVE-2026-32240

Affected products (as reported)

Vendor	Product	Versions
capnproto	capnproto	< 1.4.0

Weakness: CWE-197, CWE-444 · Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

References

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