CVE-2021-40438 KEVRansomwarePublic exploit Critical · 9.0 EPSS >99.9%
Apache HTTP Server-Side Request Forgery (SSRF)
Apache · Apache
Summary
A flaw in Apache HTTP Server's mod_proxy component allows a crafted request URI path to cause the server to forward requests to an origin server chosen by the remote user, affecting Apache HTTP Server 2.4.48 and earlier. An unauthenticated remote attacker can exploit this over the network by sending a specially crafted request, with no user interaction required, though exploitation is noted as having high complexity.
Official description · NVD
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.
Priority scorePatchfirst’s 0 to 100 ranking. It adds points for CISA KEV listing, ransomware use, EPSS, threat reports, public exploit code and CVSS. Every CVE page shows the breakdown. Breakdown
79.3 / 100
- Exploited
- Yes, in CISA KEV
- Reports
- 0
- Tactics
- 2/15
How an attack unfolds
Based on MITRE ATT&CKA public catalogue of the techniques attackers use, maintained by MITRE. Patchfirst uses it to describe what attackers do with a vulnerability. mappings. Select a step to see the evidence.How attackers get in
What they do
Exploitation timelineDated milestones for a CVE: when it was disclosed, when exploit code became public, when exploitation was confirmed, and when threat reports named it.
First seen exploited 3 months after disclosure. Public exploit code (PoC-in-GitHub) appeared 32 days after disclosure.
- Disclosedday 0
- Public exploit codePoC-in-GitHub+32 days
- First seen exploitedVulnCheck KEV evidenceAdded to CISA KEVknown ransomware use; EU database agrees+3 mo
CISA KEV listing is when exploitation was confirmed, not necessarily the first attack.
MITRE ATT&CKA public catalogue of the techniques attackers use, maintained by MITRE. Patchfirst uses it to describe what attackers do with a vulnerability. techniques
2 TechniqueOne specific attacker behaviour in MITRE ATT&CK, such as T1190 “Exploit Public-Facing Application”. IDs with a dot, like T1059.004, are sub-techniques., grouped by TacticThe attacker’s goal at a stage of an attack in MITRE ATT&CK, such as Initial Access or Persistence. Each technique belongs to one or more tactics. in kill-chain order, 2 cited passages 2Initial AccessTA0001
Exploit Public-Facing Application T1190 Medium Confidence (technique links)How sure Patchfirst is that an ATT&CK technique applies to this CVE. High and medium links are shown by default; low ones come from early or generic mappings, text matching, or contradict the CVE’s details.
AI mapping from the description · how it is exploited
Crafted HTTP request exploits public-facing Apache server
A crafted request uri-path can cause mod_proxy to forward the request
Command and ControlTA0011
Proxy T1090 Medium Confidence (technique links)How sure Patchfirst is that an ATT&CK technique applies to this CVE. High and medium links are shown by default; low ones come from early or generic mappings, text matching, or contradict the CVE’s details.
AI mapping from the description · primary impact
Proxy redirection to attacker-chosen origin server
forward the request to an origin server choosen by the remote user
Detection
0 rules for this CVE · 171 general rules for its techniquesRules for this CVE
0Sigma ruleAn open, vendor-neutral detection rule for log data. Security teams convert Sigma rules into searches for their own SIEM. that reference CVE-2021-40438 directly.CVE-specific rules are not available in this build. Search SigmaHQ ↗
General rules for this CVE’s techniques
These detect the technique in general, not this specific vulnerability.
Exploit Public-Facing ApplicationT1190149 general rules
- Arcadyan Router Exploitationscriticaltest
- Citrix ADS Exploitation CVE-2020-8193 CVE-2020-8195criticaltest
- Citrix Netscaler Attack CVE-2019-19781criticaltest
- Confluence Exploitation CVE-2019-3398criticaltest
- CVE-2010-5278 Exploitation Attemptcriticaltest
- CVE-2020-0688 Exchange Exploitation via Web Logcriticaltest
- CVE-2020-10148 SolarWinds Orion API Auth Bypasscriticaltest
- CVE-2020-5902 F5 BIG-IP Exploitation Attemptcriticaltest
ProxyT109022 general rules
- Communication To LocaltoNet Tunneling Service Initiatedhightest
- Communication To LocaltoNet Tunneling Service Initiated - Linuxhightest
- Communication To Ngrok Tunneling Service - Linuxhightest
- Communication To Ngrok Tunneling Service Initiatedhightest
- HackTool - Htran/NATBypass Executionhightest
- Malicious IP Address Sign-In Failure Ratehightest
- Malicious IP Address Sign-In Suspicioushightest
- Ngrok Usage with Remote Desktop Servicehightest
Sigma rules © SigmaHQ, Detection Rule License 1.1; linked, not reproduced.
What security researchers reported
Not named in any threat report yetAlso mentioned in 50 community threat pulses (AlienVault OTX)
Weaker evidencePulses are indicator collections that anyone can publish. A mention is not confirmation of exploitation.
- Actively_Exploited_Kev | Oct 29, 202628 Oct 2026
- Actively_Exploited_Kev | Oct 28, 202627 Oct 2026
- Actively_Exploited_Kev | Oct 27, 202626 Oct 2026
- Actively_Exploited_Kev | Oct 24, 202623 Oct 2026
- Actively_Exploited_Kev | Oct 23, 202622 Oct 2026
- Actively_Exploited_Kev | Oct 22, 202621 Oct 2026
- 44 more on OTX