Package security in stack.

Posted by Michael Snoyman - 20 July, 2015

As readers of this blog or the mailing lists are likely already aware: package security is important to both FP Complete and other members of the Commercial Haskell community. While there was quite a bit of public discussion around this during the planning phase, I was reminded in a conversation on Friday that we never announced the outcome of these plans.

tl;dr: Secure package distribution is fully implemented in stack, with some options to harden the default. We're still implementing an easy author signing story, and that will be announced soon.

The implementation we have in stack follows the plan in the above-linked proposal pretty directly. Let me just flesh it out fully here:

  • The all-cabal-hashes repository is used by default by stack for getting the collection of cabal files (known as the package index). This is downloaded over https. In addition to the raw .cabal files, this repository also contains hashes and download sizes for all tarballs available.
  • When downloading tarballs, the file size and content hash will be verified against the information provided in the index, if available. If more bytes are provided than indicated, the download is aborted. Only after verification is complete is the file moved into its final destination and available for future operations.
  • For added security (which I'd recommend), you can also turn on GPG verification and requiring hashes for this index (see the stack.yaml configuration settings).
    • GPG verification will use Git's built-in GPG support to verify the signature on the all-cabal-hashes tag before accepting the new content, and will refuse to update the index if the GPG verification fails. (You'll need to add our GPG key to your keychain.)
    • Requiring hashes means that the package index will not be accepted unless every package listed also has package hash/download size information. This is disabled by default for those who download the package index without Git support.

The story still isn't complete: we have no way to verify that the package author really is the person who uploaded the package. Stay tuned to the upload/signature author work we're doing, which will hopefully be available Real Soon Now(tm).

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