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Vulnerability record · CVE-2020-11993 · published 7 August 2020

CVE-2020-11993: Apache HTTP Server mod_http2 memory pool misuse under debug logging

Apache · Http Server

Apache HTTP Server 2.4.20 through 2.4.43, when trace/debug logging is enabled for the HTTP/2 module, can write log statements to the wrong connection under certain traffic edge patterns, causing concurrent use of memory pools. This is classified as CWE-444 (HTTP request smuggling) and can crash or destabilize the server. It only applies when mod_http2 LogLevel is set above "info".

7.5 CVSS 3.1 High EPSS 56% · top 1.0% CWE-444 · HTTP request smuggling
7.5CVSS 3.1 base score, v2 4.3
56%EPSS exploitation probability, 30 days
NoNot in CISA KEV
13Affected product versions listed by NVD
52References, 2 tagged exploit
17 Jun 2026Last modified by NVD

Description

Apache HTTP Server versions 2.4.20 to 2.4.43 When trace/debug was enabled for the HTTP/2 module and on certain traffic edge patterns, logging statements were made on the wrong connection, causing concurrent use of memory pools. Configuring the LogLevel of mod_http2 above "info" will mitigate this vulnerability for unpatched servers.

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 19 September 2026. Confidence: high.

high priorityHigh CVSS (7.5) and very high EPSS with public exploit material, but exploitation requires non-default debug logging, limiting real-world exposure.

What it is

Apache HTTP Server 2.4.20 through 2.4.43, when trace/debug logging is enabled for the HTTP/2 module, can write log statements to the wrong connection under certain traffic edge patterns, causing concurrent use of memory pools. This is classified as CWE-444 (HTTP request smuggling) and can crash or destabilize the server. It only applies when mod_http2 LogLevel is set above "info".

Impact

An unauthenticated remote attacker can trigger concurrent memory pool use, causing denial of service through server crashes or instability. No confidentiality or integrity impact is described.

Attack surface

Reached over the network via HTTP/2 traffic to an affected server; no authentication or user interaction is required per the CVSS vector (AV:N/AC:L/PR:N/UI:N). Exploitation depends on trace/debug logging being enabled for mod_http2 and specific traffic edge patterns.

Exploitation

Not listed in CISA KEV and no ransomware group usage is documented, but EPSS is high (0.56448, 99th percentile) and a Packet Storm exploit reference exists, indicating public exploit material is available.

What to do

  • Upgrade Apache HTTP Server past 2.4.43 to a fixed release
  • If patching is delayed, set mod_http2 LogLevel to "info" or lower to remove the vulnerable condition
  • Audit configurations for trace/debug logging on mod_http2 and revert to default levels
  • Apply vendor patches for bundled Apache HTTP Server in NetApp, Oracle, Canonical, Debian, Fedora and openSUSE products
  • Monitor for vendor advisories covering the affected product list

Detection

  • Search httpd configurations for LogLevel values above "info" on mod_http2
  • Monitor Apache error and crash logs for abnormal terminations or memory pool errors on HTTP/2 connections
  • Alert on unexpected HTTP/2 traffic patterns that coincide with debug logging being enabled
  • Track server restarts or availability drops on hosts running affected Apache versions

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

Affected products

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

References

LinkTags
http://lists.opensuse.org/opensuse-security-announce/2020-08/msg00068.html Mailing ListThird Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-08/msg00071.html Mailing ListThird Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00081.html Mailing ListThird Party Advisory
http://packetstormsecurity.com/files/160393/Apache-2-HTTP2-Module-Concurrent-Pool-Usage.html ExploitThird Party AdvisoryVDB Entry
https://httpd.apache.org/security/vulnerabilities_24.html#CVE-2020-11993 Vendor Advisory
https://lists.apache.org/thread.html/r06f0d87ebb6d59ed8379633f36f72f5b1f79cadfda72ede0830b42cf%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r09bb998baee74a2c316446bd1a41ae7f8d7049d09d9ff991471e8775%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r2c6083f6a2027914a0f5b54e2a1f4fa98c03f8693b58460911818255%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r3c5c3104813c1c5508b55564b66546933079250a46ce50eee90b2e36%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r5debe8f82728a00a4a68bc904dd6c35423bdfc8d601cfb4579f38bf1%40%3Cdev.httpd.apache.org Patch
https://lists.apache.org/thread.html/r623de9b2b2433a87f3f3a15900419fc9c00c77b26936dfea4060f672%40%3Cdev.httpd.apache.org Patch
https://lists.apache.org/thread.html/r76142b8c5119df2178be7c2dba88fde552eedeec37ea993dfce68d1d%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r9e9f1a7609760f0f80562eaaec2aa3c32d525c3e0fca98b475240c71%40%3Cdev.httpd.apache.org Patch
https://lists.apache.org/thread.html/r9f93cf6dde308d42a9c807784e8102600d0397f5f834890708bf6920%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/rc998b18880df98bafaade071346690c2bc1444adaa1a1ea464b93f0a%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/rdf3e5d0a5f5c3d90d6013bccc6c4d5af59cf1f8c8dea5d9a283d13ce%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/rf6449464fd8b7437704c55f88361b66f12d5b5f90bcce66af4be4ba9%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/rf71eb428714374a6f9ad68952e23611ec7807b029fd6a1b4f5f732d9%40%3Ccvs.httpd.apache.org Patch
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/4NKWG2EXAQQB6LMLATKZ7KL Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ITVFDBVM6E3JF3O7RYLRPRC Third Party Advisory
https://security.gentoo.org/glsa/202008-04 Third Party Advisory
https://security.netapp.com/advisory/ntap-20200814-0005/ Third Party Advisory
https://usn.ubuntu.com/4458-1/ Third Party Advisory
https://www.debian.org/security/2020/dsa-4757 Third Party Advisory
https://www.oracle.com/security-alerts/cpujan2021.html Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2020.html Vendor Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-08/msg00068.html Mailing ListThird Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-08/msg00071.html Mailing ListThird Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00081.html Mailing ListThird Party Advisory
http://packetstormsecurity.com/files/160393/Apache-2-HTTP2-Module-Concurrent-Pool-Usage.html ExploitThird Party AdvisoryVDB Entry
https://httpd.apache.org/security/vulnerabilities_24.html#CVE-2020-11993 Vendor Advisory
https://lists.apache.org/thread.html/r06f0d87ebb6d59ed8379633f36f72f5b1f79cadfda72ede0830b42cf%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r09bb998baee74a2c316446bd1a41ae7f8d7049d09d9ff991471e8775%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r2c6083f6a2027914a0f5b54e2a1f4fa98c03f8693b58460911818255%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r3c5c3104813c1c5508b55564b66546933079250a46ce50eee90b2e36%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r5debe8f82728a00a4a68bc904dd6c35423bdfc8d601cfb4579f38bf1%40%3Cdev.httpd.apache.org Patch
https://lists.apache.org/thread.html/r623de9b2b2433a87f3f3a15900419fc9c00c77b26936dfea4060f672%40%3Cdev.httpd.apache.org Patch
https://lists.apache.org/thread.html/r76142b8c5119df2178be7c2dba88fde552eedeec37ea993dfce68d1d%40%3Ccvs.httpd.apache.org Patch
https://lists.apache.org/thread.html/r9e9f1a7609760f0f80562eaaec2aa3c32d525c3e0fca98b475240c71%40%3Cdev.httpd.apache.org Patch
https://lists.apache.org/thread.html/r9f93cf6dde308d42a9c807784e8102600d0397f5f834890708bf6920%40%3Ccvs.httpd.apache.org Patch

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Source: NIST National Vulnerability Database (record CVE-2020-11993), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.