CVE-2026-33523Network attack vector

Header injection via unsanitized backend status line

Published May 4, 2026 · Updated May 4, 2026

HTTP response splitting in Apache HTTP Server 2.4.0 through 2.4.66 allows remote attackers to inject headers through a malicious backend. The response filter forwards a backend-supplied status line without rejecting embedded newlines or control characters, allowing downstream HTTP framing to be altered. Exploitation requires a reverse or forward proxy connected to an untrusted or compromised backend; the bounded impact is injected response headers and altered downstream responses.

CVSS severity6.5
Medium
EPSS probability0.44%
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 response splitting in Apache HTTP Server 2.4.0 through 2.4.66 allows remote attackers to inject headers through a malicious backend. The response filter forwards a backend-supplied status line without rejecting embedded newlines or control characters, allowing downstream HTTP framing to be altered. Exploitation requires a reverse or forward proxy connected to an untrusted or compromised backend; the bounded impact is injected response headers and altered downstream responses.

The record

CVE
CVE-2026-33523
Published
May 4, 2026
Updated
May 4, 2026
Vendor
The Apache Software Foundation
Product
Apache HTTP Server
Classifications
CWE-443, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 4, 2026CVE publishedPublication date reported by the CVE source.
  2. May 4, 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=2.4.0 <=2.4.66

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache HTTP Serverversion=2.4.0 <=2.4.66

Published CVSS scores

6.5CISA-ADPCVSS: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-443DEPRECATED: HTTP response splitting
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