The perl Module::Signature module adds signing capabilities to CPAN modules.  The 'cpansign verify' command will automatically download keys and use them to check the signature of CPAN packages via the SIGNATURE file.

The format of the SIGNATURE file includes the cipher to use to match the provided hash; for instance:

SHA1 955ba924e9cd1bafccb4d6d7bd3be25c3ce8bf75 README

If an attacker were to replace this (SHA1) with a special unknown cipher (e.g. 'Special') and were to include in the distribution a 'Digest/Special.pm', the code in this perl module would be executed when 'cpansign -verify' is run.  This will execute arbitrary code with the privileges of the user running cpansign.

Because cpansign will download public keys from a public key repository, the GPG key used to sign the SIGNATURE file may also be suspect; an attacker able to modify a CPAN module distribution file and sign the SIGNATURE file with their own key only has to make their key public.  cpansign will download the attacker's key, validate the SIGNATURE file as being correctly signed, but will then execute code as noted above, if the SIGNATURE file is crafted in this way.

Module::Signature version 0.72 corrects [1],[2] this issue by refusing to load Digest::* modules from relative paths in @INC.

[1] https://github.com/audreyt/module-signature/commit/575f7bd6ba4cc7c92f841e8758f88a131674ebf2
[2] https://github.com/audreyt/module-signature/commit/cbd06b392a73c63159dc5c20ff5b3c8fc88c4896


Acknowledgements:

This issue was discovered by Florian Weimer of the Red Hat Product Security Team.