Add instructions for protection against vulnerabilities

[maarten-litmaath]

Hi,
I think we need implementations to take steps to protect users and others
against accidental or deliberate abuses via world-writable directories,
to the extent possible. If someone else e.g. creates /tmp/bt_u$MY_UID,
then the code must not consider that file, but rather fail with a warning...

[DrDaveD]

It's easy to understand why one needs to make sure that writing doesn't follow symlinks created by someone else, but what is the vulnerability if someone creates a token for someone else? What kind of harm could be done, for example?

[maarten-litmaath]

Hi Dave,
at the very least we should try to avoid having to debug perplexities like:
"how can John Doe's token end up being used for my request?!"
Better fail much earlier in the chain. I do not think there is a valid
use case for someone preparing a token for another (in such a way).
I suspect that in principle worse abuses would be possible,
but I do not have something realistic beyond a DoS yet.

[DrDaveD]

In that case maybe the MUSTs should be changed to SHOULDs.

[msalle]

Note that if the file isn't owned by the user it typically also means it needs to be group or even world-readable. I think something like that would only be acceptable on servers where users don't have access. For any shared system (UIs, WNs), I think a MUST is the correct one. Note that we have the same for proxy files: they need to be readable only for the owner.
As an example of an attack scenario: if a user would upload private data with an attackers credentials, the attacker then has access to that private data.
In short I think a proper ownership check + file permissions (incl. the standard paths checks to prevent symlink attacks) is necessary on any shared system.

[DrDaveD]

Well then if we're getting into requiring security checks in every token reader, it ought to be complete and require that the file is only accessible by the current user.