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Ubuntu Security Notice USN-5090-4
September 28, 2021

apache2 regression
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 16.04 ESM

Summary:

USN-5090-1 introduced a regression in Apache HTTP Server.

Software Description:
- apache2: Apache HTTP server

Details:

USN-5090-1 fixed vulnerabilities in Apache HTTP Server. One of the upstream
fixes introduced a regression in UDS URIs. This update fixes the problem.

Original advisory details:

 James Kettle discovered that the Apache HTTP Server HTTP/2 module
 incorrectly handled certain crafted methods. A remote attacker could
 possibly use this issue to perform request splitting or cache poisoning
 attacks. (CVE-2021-33193)
 
 It was discovered that the Apache HTTP Server incorrectly handled certain
 malformed requests. A remote attacker could possibly use this issue to
 cause the server to crash, resulting in a denial of service.
 (CVE-2021-34798)
 
 Li Zhi Xin discovered that the Apache mod_proxy_uwsgi module incorrectly
 handled certain request uri-paths. A remote attacker could possibly use
 this issue to cause the server to crash, resulting in a denial of service.
 This issue only affected Ubuntu 20.04 LTS and Ubuntu 21.04.
 (CVE-2021-36160)
 
 It was discovered that the Apache HTTP Server incorrectly handled escaping
 quotes. If the server was configured with third-party modules, a remote
 attacker could use this issue to cause the server to crash, resulting in a
 denial of service, or possibly execute arbitrary code. (CVE-2021-39275)
 
 It was discovered that the Apache mod_proxy module incorrectly handled
 certain request uri-paths. A remote attacker could possibly use this issue
 to cause the server to forward requests to arbitrary origin servers.
 (CVE-2021-40438)

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 16.04 ESM:
  apache2                         2.4.18-2ubuntu3.17+esm3
  apache2-bin                     2.4.18-2ubuntu3.17+esm3

In general, a standard system update will make all the necessary changes.

References:
  https://ubuntu.com/security/notices/USN-5090-4
  https://ubuntu.com/security/notices/USN-5090-1
  https://launchpad.net/bugs/XXXXXX