Gemfile.lock is Bundler’s lockfile for a Ruby project. It records the exact versions of gems, and often their resolved dependency tree, that were installed when the application was last updated. This makes builds reproducible: developers, CI systems, and production hosts can install the same dependency set instead of re-resolving newer versions each time.
In cyber security, that stability is a defense against supply-chain drift. If an attacker publishes a malicious gem or compromises a dependency, a pinned lockfile can prevent an automatic upgrade to the bad version. Security teams also use it to audit changes, compare dependency updates, and detect unexpected package churn. However, a lockfile is not a guarantee of safety: it only freezes what was chosen, so teams still need code review, dependency scanning, and trusted sources. In practice, Gemfile.lock is one of the simplest ways to reduce surprise in Ruby builds and make package-related attacks harder to slip in unnoticed.