CVE-2026-32239Network attack vector

HTTP request smuggling via negative body length

Published Mar 12, 2026 · Updated Mar 12, 2026

HTTP request smuggling in Cap'n Proto KJ-HTTP before 1.4.0 allows remote attackers to desynchronize proxied HTTP message boundaries. HttpInputStreamImpl::getEntityBody passes a negative Content-Length string to strtoull(), which silently converts it to a large uint64_t body length. Exposure requires direct use of KJ-HTTP behind a proxy that accepts and forwards the negative header unchanged; Cap'n Proto without KJ-HTTP is unaffected.

CVSS severity6.3
Medium
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

HTTP request smuggling in Cap'n Proto KJ-HTTP before 1.4.0 allows remote attackers to desynchronize proxied HTTP message boundaries. HttpInputStreamImpl::getEntityBody passes a negative Content-Length string to strtoull(), which silently converts it to a large uint64_t body length. Exposure requires direct use of KJ-HTTP behind a proxy that accepts and forwards the negative header unchanged; Cap'n Proto without KJ-HTTP is unaffected.

The record

CVE
CVE-2026-32239
Published
Mar 12, 2026
Updated
Mar 12, 2026
Vendor
Cap'n Proto
Product
Cap'n Proto
Classifications
CWE-444, CWE-190, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 12, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 12, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.4.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Cap'n Proto · Cap'n Protoversion=< 1.4.0

Published CVSS scores

6.3GitHub, Inc.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
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CWE-190Integer Overflow or Wraparound
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo