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Vulnerability record · CVE-2023-44487 · published 10 October 2023

CVE-2023-44487: HTTP/2 Rapid Reset stream cancellation denial of service

Siemens · Simatic S7 1500 Cpu 1518f 4 Pn\/Dp Mfp Firmware

The HTTP/2 protocol permits a client to cancel many streams quickly, and the server's handling of those resets consumes disproportionate resources. This enables a denial of service against HTTP/2 servers and was exploited in the wild from August through October 2023.

7.5 CVSS 3.1 High CISA KEV since 10 Oct 2023 EPSS 100% · top 0.1% CWE-400 · Uncontrolled resource consumption
7.5CVSS 3.1 base score
100%EPSS exploitation probability, 30 days
YesIn CISA KEV, fix deadline passed
150Affected product versions listed by NVD
292References, 4 tagged exploit
11 Aug 2026Last modified by NVD

Description

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 16 September 2026. Confidence: high.

critical priorityIt is in CISA KEV with confirmed in-the-wild exploitation, an EPSS near 1.0, and a network-reachable unauthenticated denial of service affecting a broad set of HTTP/2 products.

What it is

The HTTP/2 protocol permits a client to cancel many streams quickly, and the server's handling of those resets consumes disproportionate resources. This enables a denial of service against HTTP/2 servers and was exploited in the wild from August through October 2023.

Impact

An unauthenticated remote attacker can exhaust server CPU and memory, degrading or taking down HTTP/2 services. No data confidentiality or integrity impact is described; the effect is availability loss.

Attack surface

Reachable over the network via HTTP/2 connections; the CVSS vector shows no privileges or user interaction required. Any exposed HTTP/2 endpoint, including proxies, load balancers and application servers, is in scope.

Exploitation

CISA added it to KEV on 2023-10-10 with a 2023-10-31 due date, and EPSS is 0.99999 (99.999th percentile); the description states it was exploited in the wild. No ransomware campaign use is recorded.

What to do

  • Apply vendor patches or configuration mitigations for every HTTP/2 implementation in the environment (servers, proxies, load balancers, service meshes).
  • If a patch is unavailable, follow the vendor's interim guidance, which may include limiting concurrent streams or disabling HTTP/2.
  • Inventory all internet-facing HTTP/2 endpoints and confirm each has been remediated.
  • Apply rate limiting and connection/stream caps at the edge to blunt rapid-reset floods.
  • Track the CISA KEV due date and BOD 22-01 requirements for cloud services.

Detection

  • Monitor for spikes in HTTP/2 RST_STREAM frames or stream cancellations per connection.
  • Alert on abnormal concurrent stream counts and rapid connect/reset patterns from single clients.
  • Watch server CPU and memory saturation correlated with HTTP/2 traffic surges.
  • Review edge and load balancer logs for high-volume short-lived HTTP/2 streams.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.

Exploitation in the wild

CISA added CVE-2023-44487 to the Known Exploited Vulnerabilities catalog on 10 October 2023 as "HTTP/2 Rapid Reset Attack Vulnerability". Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Federal deadline 31 October 2023.

Affected products

150 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
http://www.openwall.com/lists/oss-security/2023/10/10/6 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/10/7 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/13/4 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/13/9 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/18/4 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/18/8 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/19/6 Mailing ListThird Party Advisory
http://www.openwall.com/lists/oss-security/2023/10/20/8 Mailing ListThird Party Advisory
https://access.redhat.com/security/cve/cve-2023-44487 Vendor Advisory
https://arstechnica.com/security/2023/10/how-ddosers-used-the-http-2-protocol-to-deliver-attacks-of-unprecedented-size/ Press/Media CoverageThird Party Advisory
https://aws.amazon.com/security/security-bulletins/AWS-2023-011/ Third Party Advisory
https://blog.cloudflare.com/technical-breakdown-http2-rapid-reset-ddos-attack/ Technical DescriptionVendor Advisory
https://blog.cloudflare.com/zero-day-rapid-reset-http2-record-breaking-ddos-attack/ Third Party AdvisoryVendor Advisory
https://blog.litespeedtech.com/2023/10/11/rapid-reset-http-2-vulnerablilty/ Vendor Advisory
https://blog.qualys.com/vulnerabilities-threat-research/2023/10/10/cve-2023-44487-http-2-rapid-reset-attack Press/Media CoverageThird Party Advisory
https://blog.vespa.ai/cve-2023-44487/ Vendor Advisory
https://bugzilla.proxmox.com/show_bug.cgi?id=4988 Issue TrackingThird Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=2242803 Issue TrackingVendor Advisory
https://bugzilla.suse.com/show_bug.cgi?id=1216123 Issue TrackingVendor Advisory
https://cgit.freebsd.org/ports/commit/?id=c64c329c2c1752f46b73e3e6ce9f4329be6629f9 Mailing ListPatchVendor Advisory
https://cloud.google.com/blog/products/identity-security/google-cloud-mitigated-largest-ddos-attack-peaking-above-398-mi Technical DescriptionVendor Advisory
https://cloud.google.com/blog/products/identity-security/how-it-works-the-novel-http2-rapid-reset-ddos-attack Technical DescriptionVendor Advisory
https://community.traefik.io/t/is-traefik-vulnerable-to-cve-2023-44487/20125 Vendor Advisory
https://discuss.hashicorp.com/t/hcsec-2023-32-vault-consul-and-boundary-affected-by-http-2-rapid-reset-denial-of-service Third Party Advisory
https://edg.io/lp/blog/resets-leaks-ddos-and-the-tale-of-a-hidden-cve Broken Link
https://forums.swift.org/t/swift-nio-http2-security-update-cve-2023-44487-http-2-dos/67764 Vendor Advisory
https://gist.github.com/adulau/7c2bfb8e9cdbe4b35a5e131c66a0c088 Issue TrackingPatch
https://github.com/Azure/AKS/issues/3947 Issue Tracking
https://github.com/Kong/kong/discussions/11741 Issue Tracking
https://github.com/advisories/GHSA-qppj-fm5r-hxr3 Vendor Advisory
https://github.com/advisories/GHSA-vx74-f528-fxqg MitigationPatchVendor Advisory
https://github.com/advisories/GHSA-xpw8-rcwv-8f8p PatchVendor Advisory
https://github.com/akka/akka-http/issues/4323 Issue Tracking
https://github.com/alibaba/tengine/issues/1872 Issue Tracking
https://github.com/apache/apisix/issues/10320 Issue Tracking
https://github.com/apache/httpd-site/pull/10 Issue Tracking
https://github.com/apache/httpd/blob/afcdbeebbff4b0c50ea26cdd16e178c0d1f24152/modules/http2/h2_mplx.c#L1101-L1113 Product
https://github.com/apache/tomcat/tree/main/java/org/apache/coyote/http2 ProductThird Party Advisory
https://github.com/apache/trafficserver/pull/10564 Issue TrackingPatch
https://github.com/arkrwn/PoC/tree/main/CVE-2023-44487 Vendor Advisory

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Source: NIST National Vulnerability Database (record CVE-2023-44487), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.