critical Known exploitedREV. 01
CVE-2021-40438Apache httpd mod_proxy — Request Forgery via Crafted URI Path
A crafted request path makes the Apache web server's proxy module forward the request to an origin server the caller picks, turning the front-end into a relay into networks the attacker cannot otherwise reach. Versions 2.4.48 and earlier are affected.
Read the risk
THE VITAL STATS- CVSS score
- 9.0/ 10 CVSS v3.1
- EPSS probability
- 100% Likelihood of exploitation
- Attack complexity
- High Conditions needed to exploit
- Known exploited
- Yes — CISA KEV
- CVE published
- 2021-09-16
- Added to KEV
- 2021-12-01
- Exploit published
- Not recorded
- Confidence
- high
- Kill chain
- initial access
- MITRE ATT&CK
- T1190
- Severity
- critical
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Attacker needsAn Apache server 2.4.48 or earlier that has the proxy module enabled, reachable over the network. No credentials needed.
Behind the card
4 REFERENCES- 01CVE recordNVD recordnvd.nist.gov
- 02Vendor advisoryApache httpd 2.4 vulnerabilities listhttpd.apache.org
- 03ReferenceDebian security advisory DSA-4982debian.org
- 04CISACISA Known Exploited Vulnerabilities entrycisa.gov
Field-level provenanceTRACE THE SOURCES +
Which source supports each field, when it was retrieved, and who extracted it.
| Fields | Source | Retrieved | Extractor | Confidence |
|---|---|---|---|---|
| title, summary, cvss, severity, exploitation.attack_complexity, exploitation.prerequisites, exploitation.exploit_available, remediation.patch_available, references | nvd.nist.gov | 2026-08-07 | claude-code | high |
| exploitation.known_exploited, recommended_action | cisa.gov | 2026-08-07 | claude-code | high |
| epss | api.first.org | 2026-08-07 | claude-code | high |
Printing history
THE RECORD- r1Initial base card
First printing. This card has not been reprinted since publication.