weakest link

• weak link

The figurative sense comes by metaphor from the literal sense: the notion that a chain is only as strong as its weakest link. (See historical details there.)

weakest link (plural weakest links)

1. The part of a system that is most likely to fail or cause problems.

• Achilles heel
• weak point

• a chain is only as strong as its weakest link

•   Weakest Link (disambiguation) on Wikipedia.Wikipedia
• “weakest link”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.
• “the weakest link”, in Collins English Dictionary.
• “the weak weakest link” in Longman Dictionary of Contemporary English, Longman.