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Actively exploited

CVE-2023-44487 HTTP/2 Rapid Reset Attack Vulnerability

HTTP/2 contains a rapid reset vulnerability that allows for a distributed denial-of-service attack (DDoS).

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch urgentlyCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 10 Oct 2023.
  • Rated High severity (CVSS 7.5, scored by CISA).
  • It has been in the catalogue for 1,123 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

The HTTP/2 protocol allows a denial of service (server resource consumption) because request cancellation can reset many streams quickly, as exploited in the wild in August through October 2023.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for IETF HTTP/2. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 31 Oct 2023 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited IETF Flaws

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CVE-2023-29552Service Location Protocol (SLP) Denial-of-Service VulnerabilityAdded 8 Nov 2023

All 2 exploited IETF vulnerabilities

Not Sure If You Are Exposed?

Our attack surface and vulnerability teams can check your internet-facing estate for CVE-2023-44487 and every other exploited flaw on the list.

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2023-44487
  3. CVE record: CVE-2023-44487
  4. FIRST EPSS
  5. blog.cloudflare.com/technical-breakdown-http2-rapid-reset-ddos-attack/
  6. github.com/dotnet/core/blob/e4613450ea0da7fd2fc6b61dfb2c1c1dec1ce9ec/release-notes/6.0/6.0.23/6.0.23
  7. aws.amazon.com/security/security-bulletins/AWS-2023-011/
  8. cloud.google.com/blog/products/identity-security/how-it-works-the-novel-http2-rapid-reset-ddos-attac
  9. nginx.com/blog/http-2-rapid-reset-attack-impacting-f5-nginx-products/
  10. cloud.google.com/blog/products/identity-security/google-cloud-mitigated-largest-ddos-attack-peaking-
  11. news.ycombinator.com/item?id=37831062
  12. blog.cloudflare.com/zero-day-rapid-reset-http2-record-breaking-ddos-attack/