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Apache HTTP Server-Side Request Forgery (SSRF)

Exploited · KEV 2021-12-01RansomwareCVE-2021-40438CWE-918

Exploited since 2021-12-01. 2 of 8 rule sources name it; Cloudflare blocks by default. Our CRS test: blocked.

Edge rules
2/8
Our test
Blocked
CVSS
9.0
EPSS
100.0% Top 0.1%
Posts, 14d
02026-10-21: 0 posts2026-10-22: 0 posts2026-10-23: 0 posts2026-10-24: 0 posts2026-10-25: 0 posts2026-10-26: 0 posts2026-10-27: 0 posts2026-10-28: 0 posts2026-10-29: 0 posts2026-10-30: 0 posts2026-10-31: 0 posts2026-11-01: 0 posts2026-11-02: 0 posts2026-11-03: 0 posts
Published
2021-09-16

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.

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Ask about CVE-2021-40438: who's covered, what to patch first, how vendors compare. I cite sources and won't help with attacks.

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Severity, decoded

CVSS 3.1
Critical · 9.0 / 10

Exploitable over the network under specific conditions, with no privileges and no user interaction, leading to full loss of confidentiality, integrity and availability, and the impact can reach components beyond the vulnerable one.

Over the internetNo login neededNo user actionHard to pull offSpreads beyond the appFull compromise
How the score is built8 metrics
  • Attack vectorAn attacker can reach it over the internet; no access to your network needed.
    Network
  • Attack complexityThe attack only works when conditions outside the attacker's control line up.
    High
  • Privileges requiredNo account or login is needed.
    None
  • User interactionNo one has to click or open anything; the attack works on its own.
    None
  • ScopeThe attack can spread beyond the vulnerable component to other parts of the system.
    Changed
  • Confidentiality impactAn attacker can read all data the affected component holds, or the most sensitive parts of it.
    High
  • Integrity impactAn attacker can change any data or files the affected component protects.
    High
  • Availability impactAn attacker can take the component down completely or keep it from serving anyone.
    High
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
CISA's assessmentSSVC, from the CVE record
ExploitationActive

Attackers are exploiting this in the wild.

AutomatableYes

Attackers can script the attack end to end and hit many targets at once.

Technical impactTotal

A successful attack gives full control over the affected software or all of its data.

Public exploits

1 found
0 Metasploit0 Exploit-DB1 Scanner template0 GitHub PoCs

Links are to third-party code: never run it outside an isolated lab.

Timeline

  1. 2021-09-16
    CVE published
    CVE List
  2. 2021-12-01
    Added to CISA KEV
    Confirmed exploitation in the wild
  3. 2024-02-14
    OWASP CRS ships a rule
    feat: new rule to detect mod_proxy attack (CVE-2021-40438) (921240 PL1) (Franziska Bühler) [#2818]
  4. 2025-03-17
    Cloudflare ships a rule
    Apache SSRF vulnerability CVE-2021-40438
  5. 2026-11-02
    Our test: OWASP Core Rule Set blocked it
    paranoia level 1, anomaly threshold 5, default CRS settings
  6. 2026-11-02
    Our test: OWASP Core Rule Set blocked it
    paranoia level 2, anomaly threshold 5, default CRS settings

Social

2026-10-21: 0 posts2026-10-22: 0 posts2026-10-23: 0 posts2026-10-24: 0 posts2026-10-25: 0 posts2026-10-26: 0 posts2026-10-27: 0 posts2026-10-28: 0 posts2026-10-29: 0 posts2026-10-30: 0 posts2026-10-31: 0 posts2026-11-01: 0 posts2026-11-02: 0 posts2026-11-03: 0 posts

No posts seen yet on X, Bluesky, Mastodon, Hacker News or GitHub.

Vendor statements

Quoted
CloudflareWAF Managed Rules1 quoteBlocks by default

First rule 2025-03-17, 1278 days after the CVE was published.

2025-03-17New rule…69fe1e0dDefault: BlockSource
Apache SSRF vulnerability CVE-2021-40438. Previous action: N/A. New action: Block.
AWSAWS WAF Managed RulesNo public claim

We found no public statement from this vendor naming this CVE. That is not proof of no protection: generic rules may still catch it. AWS often describes a vulnerability without naming its CVE (for example "Log4j"). We count only entries that name the CVE, so AWS coverage is understated.

Microsoft AzureFront Door and Application Gateway WAF (DRS)No public claim

We found no public statement from this vendor naming this CVE. That is not proof of no protection: generic rules may still catch it. The rule tables name CVEs but carry no dates, so Azure is left out of every time-to-rule figure.

Google CloudCloud Armor preconfigured WAF rulesBefore feed history

The CVE predates the oldest entry in this vendor's public feed, so a rule may exist that we cannot see.

FastlyNext-Gen WAF virtual patchesBefore feed history

The CVE predates the oldest entry in this vendor's public feed, so a rule may exist that we cannot see.

F5Attack signatures (Distributed Cloud WAF)Before feed history

The CVE predates the oldest entry in this vendor's public feed, so a rule may exist that we cannot see.

AkamaiApp & API Protector (Adaptive Security Engine)No public source

This vendor publishes no machine-readable rule changelog we can check.

OWASP CRSCore Rule Set (open source, used inside many WAFs)1 quoteRule published

First rule 2024-02-14, 881 days after the CVE was published.

2024-02-14New rule921240Source
feat: new rule to detect mod_proxy attack (CVE-2021-40438) (921240 PL1) (Franziska Bühler) [#2818]