John Lightsey and Todd Rinaldo reported that the opportunistic loading of optional modules can make many programs unintentionally load code from the current working directory (which might be changed to another directory without the user realising) and potentially leading to privilege escalation, as demonstrated in Debian with certain combinations of installed packages.

The problem relates to Perl loading modules from the includes directory array ("@INC") in which the last element is the current directory ("."). That means that, when perl wants to load a module (during first compilation or during lazy loading of a module in run time), perl will look for the module in the current directory at the end, since '.' is the last include directory in its array of include directories to seek. The issue is with requiring libraries that are in "." but are not otherwise installed.

With this update several modules which are known to be vulnerable are updated to not load modules from current directory.

Additionally the update allows configurable removal of "." from @INC in /etc/perl/sitecustomize.pl for a transitional period. It is recommended to enable this setting if the possible breakage for a specific site has been evaluated. Problems in packages provided in Debian resulting from the switch to the removal of '.' from @INC should be reported to the Perl maintainers at perl@packages.debian.org .

It is planned to switch to the default removal of '.' in @INC in a subsequent update to perl via a point release if possible, and in any case for the upcoming stable release Debian 9 (stretch).