CVE-2021-41732Known exploitationPublic exploitNetwork attack vector

HTTP request splitting via duplicate Content-Length

Published Sep 29, 2021 · Updated Aug 4, 2024

HTTP request splitting in Zeek 4.1.0 allows remote attackers to invalidate HTTP-based security analysis via crafted requests. The HTTP analyzer treats repeated Content-Length headers as separate message boundaries, splitting one input into multiple records with misplaced methods, hosts, and URIs. The attack requires network access to traffic monitored by Zeek and can corrupt analysis and logs; the vendor disputes that this behavior is a vulnerability and considers it intended.

CVSS severity7.5
High
EPSS probability0.88%
Next 30 days · Sep 16, 2026
Known exploitationReported
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 splitting in Zeek 4.1.0 allows remote attackers to invalidate HTTP-based security analysis via crafted requests. The HTTP analyzer treats repeated Content-Length headers as separate message boundaries, splitting one input into multiple records with misplaced methods, hosts, and URIs. The attack requires network access to traffic monitored by Zeek and can corrupt analysis and logs; the vendor disputes that this behavior is a vulnerability and considers it intended.

The record

CVE
CVE-2021-41732
Published
Sep 29, 2021
Updated
Aug 4, 2024
Vendor
Zeek Project
Product
Zeek
Classifications
CWE-444, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 29, 2021CVE publishedPublication date reported by the CVE source.
  2. May 1, 2024Exploitation reportedCISA SSVC proof-of-concept assessment
  3. Aug 4, 2024Record 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=4.1.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Zeek Project · Zeekversion=4.1.0

Published CVSS scores

7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
5.0NVDAV:N/AC:L/Au:N/C:N/I:P/A:N
8.2CISA ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/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 exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1557Adversary-in-the-Middle

Public exploit references

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

Reported exploitation

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