CVE-2026-32239: Cap'n Proto is a data interchange format and capability-based RPC system

Q: How do I fix CVE-2026-32239?

capnproto: upgrade to 1.4.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-32239 being exploited?

CVE-2026-32239 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-32239 affect?

CVE-2026-32239 affects capnproto. Severity is medium (CVSS 6.5). Cap'n Proto is a data interchange format and capability-based RPC system. Prior to 1.4.0, a negative Content-Length value was converted to unsigned, treating it as an impossibly large length instead. In theory, this bug could enable HTTP re

Q: How urgent is CVE-2026-32239?

Priority 31/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 6.5REMOTEUPGRADE

CVE-2026-32239 is a medium-severity flaw (CVSS 6.5) affecting capnproto. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Cap'n Proto is a data interchange format and capability-based RPC system. Prior to 1.4.0, a negative Content-Length value was converted to unsigned, treating it as an impossibly large length instead. In theory, this bug could enable HTTP request/response smuggling. This vulnerability is fixed in 1.4.0.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 88%)
CISA exploited list	No
Weakness	CWE-190, CWE-444
Published	2026-03-12
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: capnproto

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-12 20:16 UTCPublished CVE record published
2026-06-17 10:35 UTCUpdated Record last modified

capnproto capnproto UPGRADE

Affected: < 1.4.0

Fixed in: 1.4.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy capnproto || rpm -q capnproto || capnproto --version

2. UPGRADE to 1.4.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade capnproto

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade capnproto

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-32239?

capnproto: upgrade to 1.4.0. Exact commands for each OS are on this page.

Is CVE-2026-32239 being exploited?

CVE-2026-32239 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-32239 affect?

CVE-2026-32239 affects capnproto. Severity is medium (CVSS 6.5). Cap'n Proto is a data interchange format and capability-based RPC system. Prior to 1.4.0, a negative Content-Length value was converted to unsigned, treating it as an impossibly large length instead. In theory, this bug could enable HTTP re

How urgent is CVE-2026-32239?

Priority 31/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-32240 MEDIUM 6.5 UPGRADE Cap'n Proto is a data interchange format and capability-based RPC system

References

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