# CVE-2021-40438: Apache HTTP Server-Side Request Forgery (SSRF)

- Exploited (CISA KEV): yes, added 2021-12-01, known ransomware use
- Published: 2021-09-16
- CVSS: 9 CRITICAL (v3.1)
- EPSS: 100.0% (100th percentile)
- CWE: CWE-918

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.

## Edge WAF coverage (claimed by vendors, not measured)

| Vendor | Status | First rule | Days after publication |
|---|---|---|---|
| Cloudflare | Blocks by default | 2025-03-17 | 1278 |
| AWS | No public claim | — | — |
| Microsoft Azure | No public claim | — | — |
| Google Cloud | Before feed history | — | — |
| Fastly | Before feed history | — | — |
| F5 | Before feed history | — | — |
| Akamai | No public source | — | — |
| OWASP CRS | Rule published | 2024-02-14 | 881 |

### Cloudflare

- 2025-03-17 · new · rule 2ce80fe815254f25b3c8f47569fe1e0d · default Block
  > Apache SSRF vulnerability CVE-2021-40438. Previous action: N/A. New action: Block.
  Source: https://developers.cloudflare.com/changelog/post/2025-03-17-waf-release/

### OWASP CRS

- 2024-02-14 · new · rule 921240
  > feat: new rule to detect mod_proxy attack (CVE-2021-40438) (921240 PL1) (Franziska Bühler) [#2818]
  Source: https://github.com/coreruleset/coreruleset/blob/main/CHANGES.md

## Our test (measured)

- OWASP Core Rule Set 4.25.0, paranoia level 1, anomaly threshold 5, default CRS settings: blocked (1 of 1 requests blocked), template CVE-2021-40438, 2026-11-02
- OWASP Core Rule Set 4.25.0, paranoia level 2, anomaly threshold 5, default CRS settings: blocked (1 of 1 requests blocked), template CVE-2021-40438, 2026-11-02

"No public claim" means no vendor statement names this CVE; "Before feed history" means the CVE is older than the vendor feed. In both cases generic rules may still block it.

HTML: https://cve.vigilbase.com/cve/CVE-2021-40438 · JSON: https://cve.vigilbase.com/api/v1/cves/CVE-2021-40438