USN-8447-2: LXD vulnerabilities

USN-8447-1 fixed vulnerabilities in Go Cryptography. This update provides
the corresponding updates for Go Cryptography code embedded in LXD for
CVE-2026-39830, CVE-2026-39833, CVE-2026-39834, and CVE-2026-42508.

Original advisory details:

It was discovered that Go Cryptography did not properly handle SSH global
request responses. A remote attacker could possibly use this issue to cause
a denial of service. (CVE-2026-39830)

It was discovered that Go Cryptography did not properly verify user
presence when using FIDO/U2F security keys. An attacker could possibly use
this issue to bypass user presence verification for hardware security keys.
This issue only affected Ubuntu 20.04 LTS, Ubuntu 22.04 LTS, Ubuntu 24.04
LTS, and Ubuntu 26.04 LTS. (CVE-2026-39831)

It was discovered that Go Cryptography did not properly serialize SSH agent
key constraint extensions. An attacker could possibly use this issue to
bypass intended key usage restrictions. This issue only affected Ubuntu
20.04 LTS, Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 26.04 LTS.
(CVE-2026-39832)

It was discovered that Go Cryptography did not properly enforce the
confirm-before-use constraint in the SSH agent keyring. An attacker could
possibly use this issue to use SSH keys without the required user
confirmation. (CVE-2026-39833)

It was discovered that Go Cryptography had an integer overflow when
handling large SSH channel writes. A remote attacker could possibly use
this issue to cause a denial of service. (CVE-2026-39834)

It was discovered that Go Cryptography did not properly check certificate
authority key revocation. An attacker could possibly use this issue to
bypass certificate authority revocation checks. This issue only affected
Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and
Ubuntu 26.04 LTS. (CVE-2026-42508)

It was discovered that Go Cryptography did not properly enforce the source-
address critical option for all SSH server callback types. An attacker
could possibly use this issue to bypass source address authorization
restrictions. This issue only affected Ubuntu 26.04 LTS. (CVE-2026-46595)