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    transport: add a protocol-whitelist environment variable · a5adaced
    Jeff King authored and Junio C Hamano's avatar Junio C Hamano committed
    
    
    If we are cloning an untrusted remote repository into a
    sandbox, we may also want to fetch remote submodules in
    order to get the complete view as intended by the other
    side. However, that opens us up to attacks where a malicious
    user gets us to clone something they would not otherwise
    have access to (this is not necessarily a problem by itself,
    but we may then act on the cloned contents in a way that
    exposes them to the attacker).
    
    Ideally such a setup would sandbox git entirely away from
    high-value items, but this is not always practical or easy
    to set up (e.g., OS network controls may block multiple
    protocols, and we would want to enable some but not others).
    
    We can help this case by providing a way to restrict
    particular protocols. We use a whitelist in the environment.
    This is more annoying to set up than a blacklist, but
    defaults to safety if the set of protocols git supports
    grows). If no whitelist is specified, we continue to default
    to allowing all protocols (this is an "unsafe" default, but
    since the minority of users will want this sandboxing
    effect, it is the only sensible one).
    
    A note on the tests: ideally these would all be in a single
    test file, but the git-daemon and httpd test infrastructure
    is an all-or-nothing proposition rather than a test-by-test
    prerequisite. By putting them all together, we would be
    unable to test the file-local code on machines without
    apache.
    
    Signed-off-by: default avatarJeff King <peff@peff.net>
    Signed-off-by: default avatarJunio C Hamano <gitster@pobox.com>
    a5adaced