simplestreams regression

USN-2746-1 fixed a vulnerability in Simple Streams. The update caused a
regression preventing MAAS from downloading PXE images. This update fixes
the problem.

We apologize for the inconvenience.

Original advisory details:

It was discovered that Simple Streams did not properly perform gpg
verification in some situations. A remote attacker could use this to
perform a machine-in-the-middle attack and inject malicious content into
the stream.