Word/phrase for a feature that could be both good and bad for something

Take this scenario:

a tool has a specific feature that could be regarded as an invaluable benefit for. However, this feature, if used in opposite direction may be counterproductive. In other words, the strength point may be the weakness point.

For example, feature X of a security mechanism could be leveraged to break that if used maliciously. So, what's the word/phrase to describe this feature? I think it should be something other than Achilles heel.

Solution 1:

You could call this a double-edged sword. It can be used both for good and bad.

Solution 2:

You can call such a thing a mixed blessing.

Definition from google:

mixed bless·ing

noun

a situation or thing that has disadvantages as well as advantages.

"having children so early in their marriage was a mixed blessing"

Solution 3:

You could describe this as an ambiguous feature.

http://dictionary.reference.com/browse/ambiguous