Vulnerability report

Exploited in the wild Confirmed confidence In CISA KEV

CVE-2021-40438

mod_proxy SSRF

Apache HTTP Server · Apache HTTP Server 2.4 to <= 2.4.48

Watch CVE-2021-40438 Browse KEV feed
Severity
CVSS 9.0 · Critical
Confidence
Confirmed
Exploit status
Exploited in the wild
EPSS
100.0%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-40438 is an unauthenticated mod_proxy SSRF affecting Apache HTTP Server. A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue...

Is it exploited?

Yes. Previdian tracks this CVE as a known exploited vulnerability. Confidence is confirmed. Listed in CISA KEV. Also confirmed by third-party sources.

Who is affected?

Apache HTTP Server apache http server 2.4 to <= 2.4.48.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user

A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.

Apache Software Foundation Affected
Apache HTTP Server
Through 2.4.48
View vendor advisory (opens in new tab)
Published
16 Sep 2021
Exploitation Reported
01 Dec 2021
Attack vector
Remote
Complexity
High
Privileges
None
User interaction
None

Tags

apache nuclei_scanner cisa

CVE References

Exploitation evidence

Why Previdian marks this CVE as exploited

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploited in the wild

CISA

Recorded 01 Dec 2021

A trusted third party reported exploitation.

Proof of concept available

GitHub

Recorded 18 Oct 2021

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

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Source Added
CISA First 2021-12-01 00:00 UTC
The Shadowserver 2026-05-30 00:00 UTC
CVE 2026-08-01 04:50 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Observed signals

Request targets
User-Agents
Callback hosts
0
0
0

Request targets and User-Agents available in Pro. Callback host details available in Enterprise.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Nuclei https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-40438.yaml 25 Apr 2025

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.0 Critical

Potential damage if exploited. Separate from whether attackers are using it.

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

EPSS

100.0%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

Server-Side Request Forgery (SSRF) CWE-918

All CVSS Scores

CVSS v3.1 9.0 Critical

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

CVSS v2.0 6.8 Medium

AV:N/AC:M/Au:N/C:P/I:P/A:P

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

Apache <= 2.4.48 Mod_Proxy - Server-Side Request Forgery

nuclei · Created Unknown

Timeline

From disclosure to observed exploitation

  1. 04:50 UTC

    KEV confirmed by CVE

    Exploitation attested by an external source

  2. 14:20 UTC

    Public PoC available

    Public proof-of-concept code published

  3. 00:00 UTC

    KEV confirmed by The Shadowserver

    Exploitation attested by an external source

  4. 00:00 UTC

    Nuclei template available

    Scanner coverage available

  5. 00:00 UTC

    Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  6. 14:40 UTC

    CVE published

    Vulnerability disclosed publicly

  7. 00:00 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-40438

Free JSON includes basic KEV fields 
{
  "cve_id": "CVE-2021-40438",
  "confidence": "Confirmed",
  "cvss_score": 9.0,
  "cvss_estimated": false,
  "epss_score": 0.99999,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}