CVE-2026-32240Network attack vector

HTTP smuggling via truncated chunk sizes

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 messages via oversized chunks. The hexadecimal chunk-size parser in c++/src/kj/compat/http.c++ multiplies into a uint64_t without a pre-overflow bound check, truncating values of 2^64 or larger. Exploitation requires direct KJ-HTTP use behind a non-KJ proxy that forwards the oversized chunk unchanged; Cap'n Proto alone is unaffected.

CVSS severity6.5
Medium
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in Cap'n Proto KJ-HTTP before 1.4.0 allows remote attackers to desynchronize messages via oversized chunks. The hexadecimal chunk-size parser in c++/src/kj/compat/http.c++ multiplies into a uint64_t without a pre-overflow bound check, truncating values of 2^64 or larger. Exploitation requires direct KJ-HTTP use behind a non-KJ proxy that forwards the oversized chunk unchanged; Cap'n Proto alone is unaffected.

The record

CVE
CVE-2026-32240
Published
Mar 12, 2026
Updated
Mar 12, 2026
Vendor
Cap'n Proto
Product
Cap'n Proto
Classifications
CWE-444, CWE-197, 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

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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

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
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

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-197Numeric Truncation Error
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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