A Nettle security update has been released for Ubuntu Linux 16.04 LTS, 18.04 LTS, 20.04 LTS, and 20.10.

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Ubuntu Security Notice USN-4906-1
April 13, 2021

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

- Ubuntu 20.10
- Ubuntu 20.04 LTS
- Ubuntu 18.04 LTS
- Ubuntu 16.04 LTS

Summary:

Nettle could be made to crash or bypass signature verification if it
opened a specially crafted certificate.

Software Description:
- nettle: low level cryptographic library

Details:

It was discovered that Nettle incorrectly handled signature verification.
A remote attacker could use this issue to cause Nettle to crash, resulting
in a denial of service, or possibly force invalid signatures.

Update instructions:

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

Ubuntu 20.10:
libnettle8 3.6-2ubuntu0.1

Ubuntu 20.04 LTS:
libnettle7 3.5.1+really3.5.1-2ubuntu0.1

Ubuntu 18.04 LTS:
libnettle6 3.4-1ubuntu0.1

Ubuntu 16.04 LTS:
libnettle6 3.2-1ubuntu0.16.04.2

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

References:
  https://ubuntu.com/security/notices/USN-4906-1
CVE-2021-20305

Package Information:
  https://launchpad.net/ubuntu/+source/nettle/3.6-2ubuntu0.1
  https://launchpad.net/ubuntu/+source/nettle/3.5.1+really3.5.1-2ubuntu0.1
  https://launchpad.net/ubuntu/+source/nettle/3.4-1ubuntu0.1
  https://launchpad.net/ubuntu/+source/nettle/3.2-1ubuntu0.16.04.2