Security readout for executives and security teams
Plain-English summary
CVE-2021-40438 lets a remote attacker abuse Apache HTTP Server's proxy handling so the server may fetch an origin chosen by the attacker. For a business, the concern is server-side request forgery: exposed web servers could be turned into a path toward internal services or sensitive data.
Executive priority
Treat as urgent for exposed Apache reverse proxy infrastructure because it is critical severity and in CISA KEV. Focus first on externally reachable systems and vendor-managed products that include Apache HTTP Server.
Technical view
Apache HTTP Server 2.4.48 and earlier are affected when mod_proxy handles a crafted URI path. The CVE is CWE-918 SSRF with CVSS 3.1 score 9.0. Impact spans confidentiality, integrity, and availability, though the vector notes high attack complexity.
Likely exposure
Most relevant exposure is internet-facing or partner-facing Apache HTTP Server 2.4 deployments using proxy functionality, especially reverse proxies. Products embedding Apache may also be affected through vendor advisories, but the bundle only directly names Apache HTTP Server 2.4.
Exploitation context
CISA KEV lists CVE-2021-40438, so active exploitation is supported. The source bundle does not include exploit details, observed campaigns, or attacker tooling, so exploitation scale and methods should be treated as unknown from these sources.
Researcher notes
The evidence supports SSRF in mod_proxy via crafted URI path and affected Apache HTTP Server 2.4.48 and earlier. The bundle does not provide safe configuration conditions, exploit prerequisites beyond the CVSS vector, or detailed fixed-version text beyond linked vendor advisories.
Mitigation direction
- Apply Apache HTTP Server or OS vendor security updates for CVE-2021-40438.
- Prioritize Apache HTTP Server 2.4.48 and earlier systems with mod_proxy enabled.
- Use vendor guidance for any temporary workaround if immediate patching is impossible.
- Review downstream vendor advisories for appliances or products bundling Apache HTTP Server.
- Increase monitoring for unusual proxy requests and unexpected outbound web server traffic.
Validation and detection
- Inventory Apache HTTP Server versions and identify 2.4.48 or earlier.
- Confirm whether mod_proxy or reverse proxy functionality is enabled on exposed servers.
- Check package manager or vendor advisory status for the fixed apache/httpd package.
- Verify internet-facing Apache instances are updated before lower-exposure internal systems.
- Review logs for requests that caused unexpected proxying or outbound destinations.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-918: Information exposure and cloud metadata lookup
Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCloud metadata behavior lookup
The CVE wording references SSRF or metadata access, so cloud discovery and credential material review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-40438 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9 (3.1)
- Known Exploited
- Yes
- Published
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H2.26Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
9CriticalVector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://httpd.apache.org/security/vulnerabilities_24.htmlCVE reference · x_refsource_MISC
- FEDORA-2021-dce7e7738eCVE reference · vendor-advisory, x_refsource_FEDORA
- [httpd-users] 20210923 [users@httpd] 2.4.49 security fixes: more infoCVE reference · mailing-list, x_refsource_MLIST
- [httpd-users] 20210923 Re: [users@httpd] 2.4.49 security fixes: more infoCVE reference · mailing-list, x_refsource_MLIST
- [httpd-users] 20210923 [users@httpd] Re: [External] : [users@httpd] 2.4.49 security fixes: more infoCVE reference · mailing-list, x_refsource_MLIST
- [httpd-users] 20210923 Re: [users@httpd] Re: [External] : [users@httpd] 2.4.49 security fixes: more infoCVE reference · mailing-list, x_refsource_MLIST
- FEDORA-2021-e3f6dd670dCVE reference · vendor-advisory, x_refsource_FEDORA
- [debian-lts-announce] 20211002 [SECURITY] [DLA 2776-1] apache2 security updateCVE reference · mailing-list, x_refsource_MLIST
- [httpd-bugs] 20211008 [Bug 65616] CVE-2021-36160 regressionCVE reference · mailing-list, x_refsource_MLIST
- DSA-4982CVE reference · vendor-advisory, x_refsource_DEBIAN
- [httpd-users] 20211019 [users@httpd] Regarding CVE-2021-40438CVE reference · mailing-list, x_refsource_MLIST
- [httpd-users] 20211019 Re: [users@httpd] Regarding CVE-2021-40438CVE reference · mailing-list, x_refsource_MLIST
- 20211124 Multiple Vulnerabilities in Apache HTTP Server Affecting Cisco Products: November 2021CVE reference · vendor-advisory, x_refsource_CISCO
- https://www.oracle.com/security-alerts/cpujan2022.htmlCVE reference · x_refsource_MISC
- https://www.tenable.com/security/tns-2021-17CVE reference · x_refsource_CONFIRM
- https://security.netapp.com/advisory/ntap-20211008-0004/CVE reference · x_refsource_CONFIRM
- https://www.oracle.com/security-alerts/cpuapr2022.htmlCVE reference · x_refsource_MISC
- https://cert-portal.siemens.com/productcert/pdf/ssa-685781.pdfCVE reference · x_refsource_CONFIRM
- GLSA-202208-20CVE reference · vendor-advisory, x_refsource_GENTOO
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40438CVE reference · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Server-Side Request Forgery (SSRF)
Server-Side Request Forgery (SSRF) represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.