What’s Vulnerable › Vulnerabilities

CVE-2023-43622: Apache HTTP Server: DoS in HTTP/2 with initial windows size 0

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 68.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2023-10-23 · updated 2026-06-17
• Weakness: CWE-400

Description

An attacker, opening a HTTP/2 connection with an initial window size of 0, was able to block handling of that connection indefinitely in Apache HTTP Server. This could be used to exhaust worker resources in the server, similar to the well known "slow loris" attack pattern. This has been fixed in version 2.4.58, so that such connection are terminated properly after the configured connection timeout. This issue affects Apache HTTP Server: from 2.4.55 through 2.4.57. Users are recommended to upgrade to version 2.4.58, which fixes the issue.

Affected products

• Apache HTTP Server: from 2.4.55 before 2.4.58 — fixed in 2.4.58

How to fix

Upgrade to a fixed version: Apache HTTP Server 2.4.58. Check the vendor advisory for mitigations.

References

• https://httpd.apache.org/security/vulnerabilities_24.html
• https://security.netapp.com/advisory/ntap-20231027-0011/
• https://httpd.apache.org/security/vulnerabilities_24.html
• https://security.netapp.com/advisory/ntap-20231027-0011/

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