Threats / Apache / CVE-2021-40438
CVE-2021-40438
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Apache vulnerability
Apache HTTP Server mod_proxy can be manipulated via crafted request URI paths to forward requests to attacker-chosen origin servers, enabling request smuggling and server-side request forgery attacks.
Verdict
Today item, not a backlog item.
Remote attackers can exploit mod_proxy request routing to redirect traffic to arbitrary servers, potentially accessing internal resources, bypassing security controls, or launching attacks against third-party systems. Active exploitation observed in the wild.
CISA KEV Yes · 2021-12-015Ransomware use Flagged5EPSS 0.99999 (verify live)6Exploit Weaponized · public PoC7CVSS 9 (CISA)4CISA SSVC Exploitation: active · Automatable: rapid · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Fully weaponized — public exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
541 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-12-01), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.99999 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 9 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H) · Affected: Apache, Apache. Confirm exact fixed builds in the vendor advisory.CVSS — CISA-assigned via the ADP container (CVE.org), version 3.1; the CNA left no metrics, so CISA's own score is used ahead of NVD.
Weakness (CWE)
Mapped to CWE-918 Server-Side Request Forgery (SSRF).CWE — CNA-assigned (CVE.org); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: rapid · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 76 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a public proof-of-concept / detection template). Catalog entries: nuclei template. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious URI path that exploits mod_proxy's request forwarding logic to redirect the proxy to an origin server under my control.
Business
The organization's proxy infrastructure becomes an unwitting relay for attacker-controlled traffic, compromising request integrity and enabling lateral movement.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I use the compromised proxy to access internal backend services or resources that should be isolated from external networks.
Business
Internal systems and sensitive data become exposed through the proxy, violating network segmentation and data protection policies.
3
Lateral reach — past segmentation narrative 3
Attacker
I leverage the proxy to launch attacks against third-party systems, masking my identity behind the organization's infrastructure.
Business
The organization faces legal liability, reputation damage, and potential sanctions for hosting attack infrastructure targeting external victims.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05