A libssh security update has been released for Ubuntu Linux 20.04 LTS and 21.04.

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Ubuntu Security Notice USN-5053-1
August 26, 2021

libssh vulnerability
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 21.04
- Ubuntu 20.04 LTS

Summary:

libssh could be made to crash or run programs if it received specially
crafted network traffic.

Software Description:
- libssh: A tiny C SSH library

Details:

It was discovered that libssh incorrectly handled rekeying. A remote
attacker could use this issue to cause libssh to crash, resulting in a
denial of service, or possibly execute arbitrary code.

Update instructions:

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

Ubuntu 21.04:
libssh-4 0.9.5-1ubuntu0.1

Ubuntu 20.04 LTS:
libssh-4 0.9.3-2ubuntu2.2

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

References:
  https://ubuntu.com/security/notices/USN-5053-1
CVE-2021-3634

Package Information:
  https://launchpad.net/ubuntu/+source/libssh/0.9.5-1ubuntu0.1
  https://launchpad.net/ubuntu/+source/libssh/0.9.3-2ubuntu2.2