CVE-2021-43799: Set a secure Erlang cookie.

The RabbitMQ docs state ([1]):

    RabbitMQ nodes and CLI tools (e.g. rabbitmqctl) use a cookie to
    determine whether they are allowed to communicate with each
    other. [...] The cookie is just a string of alphanumeric
    characters up to 255 characters in size. It is usually stored in a
    local file.

...and goes on to state (emphasis ours):

    If the file does not exist, Erlang VM will try to create one with
    a randomly generated value when the RabbitMQ server starts
    up. Using such generated cookie files are **appropriate in
    development environments only.**

The auto-generated cookie does not use cryptographic sources of
randomness, and generates 20 characters of `[A-Z]`.  Because of a
semi-predictable seed, the entropy of this password is thus less than
the idealized 26^20 = 94 bits of entropy; in actuality, it is 36 bits
of entropy, or potentially as low as 20 if the performance of the
server is known.

These sizes are well within the scope of remote brute-force attacks.

On provision, install, and upgrade, replace the default insecure
20-character Erlang cookie with a cryptographically secure
255-character string (the max length allowed).

[1] https://www.rabbitmq.com/clustering.html#erlang-cookie

scripts/lib/upgrade-zulip-stage-2

@@ -13,7 +13,7 @@ import re
 import subprocess
 import sys
 import time
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 if TYPE_CHECKING:
     from typing import NoReturn
@@ -116,15 +116,31 @@ IS_SERVER_UP = True
 
 # Check if rabbitmq port 25672 is listening on anything except 127.0.0.1
 rabbitmq_dist_listen = listening_publicly(25672)
+# Check the erlang magic cookie size
+cookie_size: Optional[int] = None
+if os.path.exists("/var/lib/rabbitmq/.erlang.cookie"):
+    with open("/var/lib/rabbitmq/.erlang.cookie", "r") as cookie_fh:
+        cookie_size = len(cookie_fh.readline())
+else:
+    logging.info("No RabbitMQ erlang cookie found, not auditing RabbitMQ security.")
 if args.skip_puppet and rabbitmq_dist_listen:
     logging.error(
         "RabbitMQ is publicly-accessible on %s; this is a security vulnerability!",
         ", ".join(rabbitmq_dist_listen),
     )
+    issue = "issue"
+    if cookie_size is not None and cookie_size == 20:
+        # See the below comment -- this is used as a lightweight
+        # signal for a cookie made with Erlang's bad randomizer.
+        logging.error(
+            "RabbitMQ erlang cookie is insecure; this is a critical security vulnerability!"
+        )
+        issue = "issues"
     logging.error(
-        "To fix the above security issue, re-run the upgrade without --skip-puppet "
+        "To fix the above security %s, re-run the upgrade without --skip-puppet "
         "(which may be set in /etc/zulip/zulip.conf), in order to restart the "
-        "necessary services.  Running zulip-puppet-apply by itself is not sufficient."
+        "necessary services.  Running zulip-puppet-apply by itself is not sufficient.",
+        issue,
     )
     sys.exit(1)
 
@@ -304,6 +320,16 @@ if rabbitmq_dist_listen:
     logging.info("Shutting down rabbitmq to adjust its ports...")
     subprocess.check_call(["/usr/sbin/service", "rabbitmq-server", "stop"])
 
+if cookie_size is not None and cookie_size == 20:
+    # Checking for a 20-character cookie is used as a signal that it
+    # was generated by Erlang's insecure randomizer, which only
+    # provides between 20 and 36 bits of entropy; were it 20
+    # characters long by a good randomizer, it would be 96 bits and
+    # more than sufficient.  We generate, using good randomness, a
+    # 255-character cookie, the max allowed length.
+    logging.info("Generating a secure erlang cookie...")
+    subprocess.check_call(["./scripts/setup/generate-rabbitmq-cookie"])
+
 # Adjust Puppet class names for the manifest renames in the 4.0 release
 class_renames = {
     "zulip::app_frontend": "zulip::profile::app_frontend",