A flaw was found in the way Bundler determined the source repository when installing dependencies of source-restricted gem packages. In configurations that use multiple gem repositories and explicitly define from which source repository certain gems are to be installed, a dependency of a source-restricted gem could be installed form a different source if that repository provided higher version of the package. This could lead to installation of a malicious gem version and arbitrary code execution.

Bundler prior to 2.1.0 uses a predictable path in /tmp/, created with insecure permissions as a storage location for gems, if locations under the user's home directory are not available. If Bundler is used in a scenario where the user does not have a writable home directory, an attacker could place malicious code in this directory that would be later loaded and executed.