Windows 7 and Windows Server 2008 users will imminently have to deploy a mandatory patch if they want to continue updating their systems, as spotted by Mary Jo Foley.
Currently, Microsoft's Windows updates use two different hashing algorithms to enable Windows to detect tampering or modification of the update files: SHA-1 and SHA-2. Windows 7 and Server 2008 verify the SHA-1 patches; Windows 8 and newer use the SHA-2 hashes instead. March's Patch Tuesday will include a standalone update for Windows 7, Windows Server 2008 R2, and WSUS to provide support for patches hashed with SHA-2. April's Patch Tuesday will include an equivalent update for Windows Server 2008.
The SHA-1 algorithm, first published in 1995, takes some input and produces a value known as a hash or a digest that's 20 bytes long. By design, any small change to the input should produce, with high probability, a wildly different hash value. SHA-1 is no longer considered to be secure, as well-funded organizations have managed to generate hash collisions—two different files that nonetheless have the same SHA-1 hash. If a collision could be generated for a Windows update, it would be possible for an attacker to produce a malicious update that nonetheless appeared to the system to have been produced by Microsoft and not subsequently altered.