Are Security Identifier (SID) in Windows or active directory domains considered as sensitive information? Is it possible for a hacker to use that information for malicious purposes?
I would not consider the SID anymore sensitive than a GUID. The SID is used to identify objects in ACLs. However, there are well known SIDs for builtin groups and accounts that make certain objects easily discoverable.
For instance, if you were to rename the built in Administrator account in AD in an attempt to hide or obscure it, someone could still locate it simply based off its SID. So a hacker or someone with malicious intent could leverage the data. But if they are already able to get a hold of the data, you probably have bigger things to worry about. Anyone with access to AD can query to obtain SID information for other users/groups/etc.