CVE-2021-40438Known exploitationPublic exploitNetwork attack vector

Unauthenticated origin proxying via crafted URI

Published Sep 16, 2021 · Updated Aug 6, 2026

Server-Side Request Forgery in Apache HTTP Server 2.4.48 and earlier allows remote attackers to redirect mod_proxy requests to attacker-chosen origins. The mod_proxy handler parses a crafted request URI path and uses attacker-controlled origin data when constructing the proxied destination. Exploitation requires a reachable server with proxying enabled; successful requests can access internal services and affect confidentiality, integrity, and availability.

CVSS severity9.0
Critical
EPSS probability100.00%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Server-Side Request Forgery in Apache HTTP Server 2.4.48 and earlier allows remote attackers to redirect mod_proxy requests to attacker-chosen origins. The mod_proxy handler parses a crafted request URI path and uses attacker-controlled origin data when constructing the proxied destination. Exploitation requires a reachable server with proxying enabled; successful requests can access internal services and affect confidentiality, integrity, and availability.

The record

CVE
CVE-2021-40438
Published
Sep 16, 2021
Updated
Aug 6, 2026
Vendor
The Apache Software Foundation
Product
Apache HTTP Server
Classifications
CWE-918, T1090
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 16, 2021CVE publishedPublication date reported by the CVE source.
  2. Dec 1, 2021Exploitation reportedCISA Known Exploited Vulnerabilities listing
  3. Aug 6, 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=Apache HTTP Server 2.4 <=2.4.48

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache HTTP Serverversion=Apache HTTP Server 2.4 <=2.4.48

Published CVSS scores

9.0NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

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 IPsShadowserver observations through Sep 16, 2026. Gaps indicate unavailable data.
Sep 10, 2026Sep 16, 2026
View daily counts
Reporting dayUnique IPs
Sep 10, 2026Unavailable
Sep 11, 2026Unavailable
Sep 12, 2026Unavailable
Sep 13, 2026Unavailable
Sep 14, 20266
Sep 15, 2026Unavailable
Sep 16, 20262
Latest reporting daySep 16, 2026
Latest daily unique IPs2
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-918Server-Side Request Forgery
T1090Proxy

Public exploit references

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

Reported exploitation

Technologies

Your stack

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Your stack

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