chmod 775: Owner and Group Write, Everyone Else Reads

775 is what you want when a set of people need to collaborate in one directory. Members of the owning group can create, edit and delete entries; everyone else can look but not touch. The mode moves the decision about who may write from the file system to group membership, which is where it belongs, because groups are managed and audited and a mode digit is not.

On its own it has a well known gap. A file created inside a 775 directory takes the primary group of whoever created it, not the group of the directory, so one careless member creates a file the rest of the team cannot edit. The fix is the setgid bit, giving 2775, which makes new entries inherit the directory's group automatically.

For files, 775 is usually wrong rather than dangerous. It marks a file executable for the owner, the group and everyone else, which is meaningless on a configuration file or a document and confusing in a listing. Data files in a shared directory want 664, and the directory itself wants 775 or 2775.

The security question to ask is not about the mode but about the group. If the owning group is a small team, 775 is fine. If it is a group that half the machine belongs to, such as users or staff on some systems, then 775 is functionally close to 777 and should be tightened to 750 with a purpose built group.

Note that others still get execute on a directory here, meaning any account can enter it and open a file inside if it knows the name. That is usually what you want on a path leading to public content, and clearly not what you want on a directory holding anything private, where 750 is the right neighbour.