Every place a secret can leak from

The security review found four things wrong with the chat gateway, and none of them is a bug in the usual sense. The code does what it was told. The upstream API key is simply everywhere: in a world-readable file and in the unit's environment where systemctl show prints it to any account; and the clients' tokens are in the journal, because the gateway logs request headers. (The review feared the upstream key was logged too. On the lab machine it was not — until a careless check for leaks put it there, which is a story this journal tells below.) Meanwhile the gateway will serve anyone who can reach the port, and nothing stops one script in a loop from spending a month's budget in an afternoon.

An AI gateway is a particularly good place to learn this, because it concentrates the two things that make a leak expensive: a secret that costs money per use, and an endpoint whose whole purpose is to spend it. But the lesson is general. A secret is only as private as the most readable place it has ever been written — and "written" includes environment variables, command lines, and logs, not just files. Fixing the file permissions is the easy fifth of the job.

What you should be able to do after this

• List the places a process's secret can be read from — file, unit, command line, environment, log — and say which accounts can read each.
• Move a secret out of a systemd unit into a private file or a systemd credential, and verify that systemctl show no longer reveals it.
• Put bearer-token authentication in front of an internal service, and say why 401 and 403 are different answers.
• Rate-limit an expensive endpoint in nginx with limit_req, and explain rate, burst and nodelay in terms of the leaky bucket.
• Stop an application logging secrets, and deal with the secrets already in the journal.
• Explain why a secret that has leaked must be rotated even after every leak is closed.

The mechanism

Where a secret actually lives

A process's secret can be read from more places than its author usually thinks about. Take the gateway as the security review found it:

$ ls -l /etc/chat-gateway/upstream.key
-rw-r--r-- 1 root root 45 Sep 12 09:02 /etc/chat-gateway/upstream.key     # every account

$ systemctl show -p Environment chat-gateway          # run as an unprivileged user
Environment=UPSTREAM_KEY=sk-norboten-3f9a…

$ systemctl cat chat-gateway | grep KEY
Environment=UPSTREAM_KEY=sk-norboten-3f9a…

The last two need no privilege at all. Unit files under /etc/systemd/system are world-readable by convention, and systemd's D-Bus API exposes a unit's properties — including its Environment= — to any local user. So Environment= in a unit is not a place to keep a secret; it is a place to publish one to everyone with a shell.

The full map, for reference:

Two entries in that table surprise people most. Command-line arguments are public: a secret passed as a flag is visible to ps aux for as long as the process runs, which is why well-behaved tools take secrets from files or stdin. And environment variables are inherited: a secret in the environment is handed to every subprocess the service starts — every curl, every shell-out, every crash reporter — whether or not it needs it.

Three good homes for a secret, in increasing order of isolation

A private file the service reads by path. The gateway supports UPSTREAM_KEY_FILE, which is the most portable pattern: the secret is in one file, owned by the service account, readable by nobody else:

$ sudo chown chatgw:chatgw /etc/chat-gateway/upstream.key
$ sudo chmod 600 /etc/chat-gateway/upstream.key
$ sudo -u nobody cat /etc/chat-gateway/upstream.key
cat: /etc/chat-gateway/upstream.key: Permission denied

0600 owned by the service is correct. 0640 root:chatgw is also correct and slightly better on a machine where you do not want the service able to rewrite its own secret; either way, the o triad is empty.

EnvironmentFile= with a private file. Many applications only read their secrets from the environment. EnvironmentFile=/etc/app/env with that file at 0600 root:root keeps the value out of systemctl show and out of the world-readable unit — systemd reads the file as root at start. The value still ends up in the process environment, with the inheritance caveat above.

A systemd credential. This is the mechanism designed for exactly this problem:

[Service]
LoadCredential=upstream_key:/etc/chat-gateway/upstream.key

At start, systemd copies the file into a private, in-memory directory that only this unit's processes can read, and sets $CREDENTIALS_DIRECTORY to point at it. The application reads $CREDENTIALS_DIRECTORY/upstream_key. The source file can then be 0600 root:root — the service account never needs access to it at all — nothing appears in the environment, nothing is inherited, and the credential disappears when the unit stops. With systemd-creds encrypt, the stored file can even be encrypted to the machine's TPM or host key, which makes a copied /etc useless. The gateway in this lab checks for a credential first, then a key file, then the environment, which is the right order of preference.

Whichever you choose, verify from the outside as an unprivileged user — that is the view an attacker gets:

$ sudo -u nobody systemctl show chat-gateway | grep -c sk-norboten
0
$ sudo -u nobody systemctl cat chat-gateway | grep -c sk-norboten
0
$ ps -eo args | grep -c '[s]k-norboten'
0

Removing it from the unit, really

There is a detail here that catches careful people. The key is in the unit file itself:

[Service]
User=chatgw
EnvironmentFile=/etc/chat-gateway/gateway.env
Environment=UPSTREAM_KEY=sk-norboten-3f9a…

The drop-in reflex — override it in chat-gateway.service.d/ with an empty Environment= — does reset the variable for the running process. But systemctl cat still prints the original file, key and all, because a drop-in is layered on top of the unit rather than replacing it. The value has to be deleted from the file it is written in:

$ sudo sed -i '/^Environment=UPSTREAM_KEY=/d' /etc/systemd/system/chat-gateway.service
$ sudo systemctl daemon-reload
$ sudo systemctl restart chat-gateway

And check the rest of /etc while you are there. A secret that has been copied once has usually been copied twice — into a backup of the unit (chat-gateway.service.bak, .orig, ~), into an old env file, into a note:

$ sudo grep -rlF -f /etc/chat-gateway/upstream.key /etc 2>/dev/null | xargs -r sudo stat -c '%a %U %n'
600 chatgw /etc/chat-gateway/upstream.key
600 root /etc/model-api/key

Read the shape of that grep before you type anything like it, because the obvious version is itself a leak. sudo grep -rl "$(sudo cat /etc/chat-gateway/upstream.key)" /etc finds the same two files — and puts the key into the system journal, because sudo logs each command it runs with its full argument list:

sudo[1246]: northelks : PWD=/ ; USER=root ; COMMAND=/usr/bin/grep -rl sk-norboten-87a81936e2272b54d3475d9764629233 /etc

That line is real. On the lab machine, after every other leak had been closed, it was the only line in the journal containing the key — written by the check for leaks. -F -f FILE reads the pattern from the file, so the command line holds a path instead of the secret; searched again afterwards, the journal still had just that one old line. The same logic applies to any tool that takes a secret as an argument: prefer the form that reads it from a file or from stdin.

Two files, both now private. /etc/model-api/key is the upstream service's own copy, which belongs to a different service and is 0600 root:root already.

Authentication: who is asking

The gateway supports a shared bearer token, enabled by REQUIRE_TOKEN=1. The client sends:

Authorization: Bearer <token>

and the gateway compares it against the first line of /etc/chat-gateway/clients.token. Two response codes are involved and they are not interchangeable:

• 401 Unauthorized — "I do not know who you are": no credentials, or credentials that do not verify. Retry with credentials.
• 403 Forbidden — "I know who you are, and you may not do this". Retrying with the same credentials will not help.

An anonymous request to a protected endpoint is a 401.

A bearer token is a password with a different name, so everything true of passwords is true of it. It belongs in a file with a tight mode (clients.token is 0640 root:chatgw — readable by the service, by root, by nobody else). It should be compared in constant time — hmac.compare_digest in Python, not !=, which returns faster the earlier the first wrong byte is and in principle leaks the token a character at a time. And it must never be logged, which is the next section.

Where to enforce it is a design choice worth making consciously. In the application, as here, the check lives next to the code that needs it and cannot be bypassed by reaching the gateway's port directly — and the gateway binds 127.0.0.1:8100, so only the proxy can reach it anyway. At the proxy, nginx can do it with auth_request to a small auth endpoint, or with a map on $http_authorization; that protects several backends at once. Real deployments often do both. What matters is that a service that spends money never answers an unauthenticated request.

Rate limiting: the leaky bucket

nginx's limit_req implements a leaky bucket, and the three numbers only make sense in those terms:

# in the http context (conf.d/*.conf is included inside http{})
limit_req_zone $binary_remote_addr zone=chatgw:10m rate=2r/s;

server {
    listen 8090;
    location / {
        limit_req zone=chatgw burst=5 nodelay;
        limit_req_status 429;
        proxy_pass http://127.0.0.1:8100;
    }
}

• rate=2r/s — the bucket drains at two requests per second, per key. nginx tracks this in milliseconds, so 2r/s means "one every 500 ms", not "two in any given second".
• burst=5 — the bucket can hold five requests above the rate before it overflows. Without burst the default is zero, so the second request inside 500 ms is rejected — which is too strict for any real client.
• nodelay — serve burst requests immediately rather than delaying each one until it fits the rate. Without nodelay, nginx queues excess requests and releases them at 2 r/s, so a client sees slow responses rather than refusals — gentler, and it ties up connections.
• limit_req_status 429 — the default rejection status is 503, which tells clients and monitoring that the service is broken. 429 Too Many Requests tells them to slow down, and it is what well-behaved clients back off on.

The key is the other half of the design. $binary_remote_addr is the client IP in 4 or 16 bytes rather than a string, so a 10m zone holds roughly 160,000 addresses. But limiting by IP has two well-known failure modes: every client behind one NAT or corporate proxy shares a single budget, and if nginx itself sits behind a load balancer, every request has the balancer's address and one bucket throttles the whole company. For a token-authenticated API, limiting per token is often closer to the intent:

limit_req_zone $http_authorization zone=pertoken:10m rate=2r/s;

limit_conn is the companion for concurrency rather than rate — "at most two generations in flight per client" — which matters for model endpoints where a single request can hold a GPU for a minute. And limit_req_dry_run on; logs what would have been rejected without rejecting it, which is how you choose numbers on a live service.

Rate limiting belongs at the proxy rather than in the application for the same reason a firewall belongs in front of a service: excess requests are turned away before they cost anything, including the application's own threads. The briefing's phrasing — "rejected before it reaches the model" — is precise.

Test a limit with an actual burst, not a loop of sequential requests (which a 2 r/s limit may let through if each request takes more than half a second):

$ tok=$(sudo head -1 /etc/chat-gateway/clients.token)
$ seq 40 | xargs -P 10 -I{} curl -s -o /dev/null -w '%{http_code}\n' \
    -H "Authorization: Bearer $tok" -d '{"prompt":"ping"}' localhost:8090/ask \
  | sort | uniq -c
      6 200
     34 429

xargs -P 10 runs ten at once; sort | uniq -c is the frequency count from the linux-04 journal. Some 200s (the rate plus the burst) and many 429s is the shape of a working limit. All 200 means no limit; all 429 means a limit that refuses normal use.

Logs are a copy of every secret they print

if LOG_HEADERS:
    print(f"request headers: {dict(self.headers)}", flush=True)

Every request's Authorization header — a client token — goes to stdout, and systemd captures stdout into the journal. The journal is readable by root and by the adm and systemd-journal groups, it is retained for weeks, and on most real machines it is forwarded to a central log system where far more people can search it. A secret in a log has been copied to everywhere the log goes.

The fix is to stop logging headers (LOG_HEADERS=0), and the general rules are worth writing down:

• never log Authorization, Cookie, Set-Cookie, X-Api-Key, or request bodies of authentication endpoints — redact them by name if you log headers at all;
• log that a request was authenticated and as whom (a client id), not the credential;
• nginx's default access log format records none of these headers — keep it that way, and be careful with custom formats that add $http_authorization.

Two journal mechanics are useful here. Every start of a unit gets a fresh invocation ID, and each journal entry records it, so you can look at exactly one run:

$ id=$(systemctl show -p InvocationID --value chat-gateway)
$ sudo journalctl _SYSTEMD_INVOCATION_ID=$id --no-pager

That is how you verify the fix — the current run is clean — without the older, leaked entries getting in the way. And those older entries do not disappear because you fixed the logging. If they matter, remove them:

$ sudo journalctl --rotate                   # close the active journal files
$ sudo journalctl --vacuum-time=1s           # delete archived files older than one second

That is a blunt instrument — it deletes all history, not just the leaked lines, since journal files cannot be edited — and it does nothing about copies already forwarded elsewhere.

A leaked secret is a burned secret

Which leads to the part of the job no configuration change performs. Everything above closes the leaks. It does not un-leak anything. The key sat in a world-readable file and in systemctl show, and the journal has held client tokens for as long as LOG_HEADERS was on. Anyone who read them still has them.

So the professional finish is rotation: issue a new upstream key, revoke the old one at the provider, generate a new client token and hand it to the clients through a proper channel. The lab cannot do that for you — the stand-in model service owns its key — but on a real system, "we fixed the permissions" without "and we rotated the key" is a finding that stays open. Rotation is also why the secret should live in exactly one private file: replacing it is then one write and one restart, rather than an archaeology project.

A failure, walked through

The four findings are known. The job is to fix them without breaking the gateway, and to verify each from the outside.

1. See what an ordinary account sees. Do this before changing anything, so you can confirm the same commands come back clean afterwards:

$ sudo -u nobody systemctl show -p Environment chat-gateway
Environment=UPSTREAM_KEY=sk-norboten-3f9a…
$ ls -l /etc/chat-gateway/
-rw-r----- 1 root chatgw 33 Sep 12 09:02 clients.token
-rw-r--r-- 1 root root   79 Sep 12 09:02 gateway.env
-rw-r--r-- 1 root root   45 Sep 12 09:02 upstream.key
$ cat /etc/chat-gateway/gateway.env
REQUIRE_TOKEN=0
LOG_HEADERS=1
UPSTREAM_KEY_FILE=/etc/chat-gateway/upstream.key

The key file is world-readable, the unit publishes the key, authentication is off, header logging is on. gateway.env itself holds only switches and a path, so 0644 is fine for it — which is worth checking rather than assuming.

2. Make the key file private, and stop the unit carrying the key.

$ sudo chown chatgw:chatgw /etc/chat-gateway/upstream.key
$ sudo chmod 600 /etc/chat-gateway/upstream.key
$ sudo sed -i '/^Environment=UPSTREAM_KEY=/d' /etc/systemd/system/chat-gateway.service
$ sudo systemctl daemon-reload

The gateway will now find the key through UPSTREAM_KEY_FILE, which is readable by its own account and nobody else. (LoadCredential=upstream_key:/etc/chat-gateway/upstream.key would be the stronger choice; the gateway reads a credential in preference to the file.)

3. Turn on authentication and turn off header logging, then restart so a fresh invocation begins:

$ sudo sed -i 's/^REQUIRE_TOKEN=.*/REQUIRE_TOKEN=1/; s/^LOG_HEADERS=.*/LOG_HEADERS=0/' \
    /etc/chat-gateway/gateway.env
$ sudo systemctl restart chat-gateway

4. Verify authentication, and that the upstream still accepts the key — a 200 proves both, since the upstream refuses requests without it:

$ curl -s -o /dev/null -w '%{http_code}\n' -d '{"prompt":"ping"}' localhost:8090/ask
401
$ tok=$(sudo head -1 /etc/chat-gateway/clients.token)
$ curl -s -H "Authorization: Bearer $tok" -d '{"prompt":"ping"}' localhost:8090/ask
{"answer": "stub answer for: ping"}

Had step 2 broken the key lookup, this would be a 502 from the gateway: upstream unavailable. (For a lab test this is fine; on a shared machine note that $tok on curl's command line is visible in ps while it runs. curl -H @file reads headers from a file instead — sudo curl -s -H @/root/auth.hdr … with a 0600 header file works identically.)

5. Verify the unit is clean, from the unprivileged view.

$ sudo -u nobody systemctl show chat-gateway | grep -c sk-norboten
0
$ sudo -u nobody systemctl cat chat-gateway | grep -c sk-norboten
0
$ sudo grep -rlF -f /etc/chat-gateway/upstream.key /etc 2>/dev/null | xargs -r sudo stat -c '%a %U %n'
600 chatgw /etc/chat-gateway/upstream.key
600 root /etc/model-api/key

6. Add the rate limit at the proxy. The zone goes in the http context — a file in conf.d/ is included there — and the limit in the location:

$ echo 'limit_req_zone $binary_remote_addr zone=chatgw:10m rate=2r/s;' | \
    sudo tee /etc/nginx/conf.d/chat-limit.conf
$ sudo tee /etc/nginx/conf.d/chat-proxy.conf >/dev/null <<'CONF'
server {
    listen 8090;
    server_name _;

    location / {
        limit_req zone=chatgw burst=5 nodelay;
        limit_req_status 429;
        proxy_pass http://127.0.0.1:8100;
        proxy_set_header Authorization $http_authorization;
    }
}
CONF
$ sudo nginx -t && sudo systemctl reload nginx
nginx: configuration file /etc/nginx/nginx.conf test is successful

7. Test it with a real burst.

$ seq 40 | xargs -P 10 -I{} curl -s -o /dev/null -w '%{http_code}\n' \
    -H "Authorization: Bearer $tok" -d '{"prompt":"ping"}' localhost:8090/ask | sort | uniq -c
      6 200
     34 429
$ sleep 3 ; curl -s -o /dev/null -w '%{http_code}\n' -H "Authorization: Bearer $tok" \
    -d '{"prompt":"ping"}' localhost:8090/ask
200
$ sudo tail -1 /var/log/nginx/error.log
… [error] 1391#1391: *134 limiting requests, excess: 5.864 by zone "chatgw", client: 127.0.0.1, …

nginx logs each rejection at error level with how far over the burst the request was. On a busy proxy that is a lot of lines; limit_req_log_level warn moves them down a level.

Rejections under a burst, and normal service once the bucket drains — both halves matter.

8. Check this run's log for secrets. Generate some traffic first, so the check is meaningful:

$ id=$(systemctl show -p InvocationID --value chat-gateway)
$ sudo journalctl _SYSTEMD_INVOCATION_ID=$id --no-pager | grep -cF -f /etc/chat-gateway/clients.token
0
$ sudo journalctl -u chat-gateway --no-pager | grep -cF -f /etc/chat-gateway/clients.token
1
$ sudo journalctl -u chat-gateway --no-pager | grep -m1 'request headers' | cut -c1-120
… gateway.py[962]: request headers: {'Host': '127.0.0.1:8100', 'Connection': 'close', 'Content-Length': '17', …

This run is clean; the history is not — on the lab machine one earlier request was enough to put a client token in the journal, inside that header dump. (Searching with -F -f again, so the search does not add a copy.) Purge it if policy requires (journalctl --rotate then --vacuum-time=1s), and — separately — rotate the key and the client token, because both were readable for as long as that history exists.

9. Reboot, and repeat the outside checks.

$ sudo reboot
$ systemctl is-active chat-gateway nginx model-api
$ sudo -u nobody systemctl show chat-gateway | grep -c sk-norboten
$ curl -s -o /dev/null -w '%{http_code}\n' -d '{"prompt":"ping"}' localhost:8090/ask      # 401
$ seq 40 | xargs -P 10 -I{} curl -s -o /dev/null -w '%{http_code}\n' \
    -H "Authorization: Bearer $tok" -d '{"prompt":"ping"}' localhost:8090/ask | sort | uniq -c

Common wrong turns

Overriding Environment=UPSTREAM_KEY in a drop-in instead of deleting it. The process no longer receives the key, and systemctl cat still prints the original unit file with the key in it — to any account. A drop-in layers on top of a unit; it does not redact it.

Moving the key to the ExecStart line. ExecStart=/opt/chat-gateway/gateway.py --key sk-… is worse than Environment=: the command line is visible in ps and /proc/<pid>/cmdline for as long as the process runs, to every user.

Moving the key into gateway.env and leaving that file at 0644. EnvironmentFile= keeps the value out of systemctl show, and a world-readable file hands it straight back. The file that holds the secret needs the private mode, whichever file it is.

chmod 600 without chown. A 0600 root:root key file is private — and unreadable by the gateway, which runs as chatgw. The upstream then rejects the gateway, and the symptom is a 502 that looks unrelated. Own it by the service, or use LoadCredential= so that systemd reads it as root.

Stopping at the file permissions. The review listed four findings; the key file is one. A gateway with a private key and no authentication is still a free inference endpoint for anyone who can reach the port.

Returning 403 to anonymous requests. 403 means "authenticated, and not allowed". A missing token is 401, and clients — and this lab's check — distinguish them.

Comparing tokens with ==. It works, and it returns earlier the sooner a byte differs, which is a timing side channel. Use a constant-time comparison (hmac.compare_digest).

limit_req with no burst. The default burst is zero, so any client that sends two requests within 500 ms (at 2r/s) is rejected. Real clients — a page loading, a retry — are refused for normal behaviour.

Forgetting limit_req_status 429. The default is 503, which tells clients and monitoring the gateway is broken rather than that the caller is too fast. Well-behaved clients back off on 429 and retry on 503 — the opposite of what you want.

Putting limit_req_zone inside a server block. It is only valid in the http context; nginx -t refuses it. A separate file in conf.d/ is the tidy way, since those files are included inside http.

Testing the limit with a sequential loop. If each request takes longer than the rate interval, a plain for loop never exceeds the limit and you conclude it is not working. Send a real concurrent burst (xargs -P).

Limiting by IP behind a load balancer. Every request arrives from the balancer's address, so one bucket throttles every user at once. Use real_ip_header/set_real_ip_from, or key the zone on the client token.

Turning off header logging and assuming the journal is clean. The fix covers new entries only. The old ones — and every copy already forwarded to central logging — still contain the tokens.

Treating the fix as complete without rotating. A secret that was readable is compromised, whether or not anyone is known to have read it. Close the leaks, then replace the key and the client tokens.

Cheat sheet

# what an unprivileged account can see
sudo -u nobody systemctl show UNIT | grep -i key
sudo -u nobody systemctl cat UNIT  | grep -i key
ps -eo args | grep '[s]ecret'                  # command lines are public
sudo cat /proc/PID/environ | tr '\0' '\n'      # the process environment (owner and root)
sudo grep -rlF -f /path/to/secretfile /etc     # every copy under /etc — NOT grep "$SECRET":
                                               #   sudo logs the command line, secret included

# homes for a secret, best last
chown SVC:SVC /etc/app/key ; chmod 600 /etc/app/key     # a private file, read by path
# EnvironmentFile=/etc/app/env        with the env file 0600 root:root
# LoadCredential=name:/etc/app/key    → $CREDENTIALS_DIRECTORY/name, this unit only
systemd-creds encrypt --name=name plain.txt /etc/credstore.encrypted/name   # + LoadCredentialEncrypted=
# never: Environment= in a unit, or a flag on ExecStart=
sed -i '/^Environment=SECRET=/d' /etc/systemd/system/UNIT.service ; systemctl daemon-reload

# authentication
curl -s -o /dev/null -w '%{http_code}\n' URL                          # expect 401 anonymously
curl -s -H "Authorization: Bearer $tok" URL                          # expect 200
# 401 = who are you?   403 = I know who you are, and no.
# compare tokens in constant time: hmac.compare_digest(a, b)

# nginx rate limiting
# http{}:     limit_req_zone $binary_remote_addr zone=NAME:10m rate=2r/s;
#             limit_req_zone $http_authorization zone=pertoken:10m rate=2r/s;   (per client token)
# location{}: limit_req zone=NAME burst=5 nodelay;
#             limit_req_status 429;            (default is 503)
#             limit_req_dry_run on;            (log, do not reject — for choosing numbers)
#             limit_conn_zone / limit_conn     (concurrency rather than rate)
nginx -t && systemctl reload nginx
seq 40 | xargs -P 10 -I{} curl -s -o /dev/null -w '%{http_code}\n' URL | sort | uniq -c

# logs
id=$(systemctl show -p InvocationID --value UNIT)
journalctl _SYSTEMD_INVOCATION_ID=$id --no-pager       # this run only
journalctl -u UNIT --no-pager | grep -cF -f SECRETFILE  # all of history (pattern from a file)
journalctl --rotate ; journalctl --vacuum-time=1s      # purge (deletes ALL history)
getent group adm systemd-journal                       # who can read the journal
# never log: Authorization, Cookie, Set-Cookie, X-Api-Key, auth request bodies

# after any leak
# rotate the key at the provider, revoke the old one, reissue client tokens

Going deeper

Where this lab's hints send you for reading, beyond this journal. In the TUI, h shows a hint's reading and l opens a journal section; the lab's page on the site lists the same.

• Nine letters and a refusal (lab journal hello) — How the kernel picks a triad
• The log lines that were never written down (topic journal logging-journald) — Everything a service prints becomes a journal entry

Manual pages: man 5 systemd.exec.

Documentation:

• https://nginx.org/en/docs/http/ngx_http_limit_req_module.html

The whole subject, end to end: the topic journal The log lines that were never written down (logging-journald) — the mechanism, the symptoms and their causes, exercises, and sources.

Review

1. An API key is in a unit's Environment= line and the unit file is 0644. Name two commands an unprivileged user can use to read it, and explain why a drop-in that blanks the variable does not fix the finding.

   systemctl show -p Environment <unit> and systemctl cat <unit> both work without privilege. A drop-in stops the process receiving the value, but a drop-in is layered on top of the unit rather than replacing it, so systemctl cat still prints the original file with the key in it. The line has to be deleted from the unit file.

2. Why is a secret on the ExecStart= command line worse than one in the environment?

   Command lines are readable by every user through ps and /proc/<pid>/cmdline for as long as the process runs. /proc/<pid>/environ is readable only by the process owner and root. (Both are worse than a private file or a credential.)

3. Compare a 0600 key file read by path, EnvironmentFile=, and LoadCredential=. What does each still expose?

   A private file read by path exposes the secret to the service account, which must own it or be able to read it. EnvironmentFile= keeps it out of systemctl show, but the value ends up in the process environment and is inherited by every child process. LoadCredential= lets systemd read the source file as root and gives this unit alone a private in-memory copy, with nothing in the environment — the source can stay 0600 root:root.

4. You chmod 600 the key file and the gateway starts returning 502. What happened?

   The file is still owned by root, and the gateway runs as chatgw, so it can no longer read the key. It calls the upstream without a valid key and gets refused. chown the file to the service account, or use LoadCredential=.

5. What is the difference between 401 and 403, and which does an anonymous request to a protected endpoint deserve?

   401 means the request lacks valid credentials — authenticate and try again. 403 means the caller is known and is not permitted, so retrying with the same credentials is pointless. An anonymous request gets 401.

6. Explain rate=2r/s, burst=5 and nodelay in terms of the leaky bucket, and say why limit_req_status 429 matters.

   The bucket drains at one request every 500 ms per key; burst=5 lets up to five requests queue above that before the bucket overflows and requests are rejected; nodelay serves the burst immediately rather than spacing it out to the rate. The default rejection status is 503, which signals an outage — 429 tells clients to slow down, which is what well-behaved clients back off on.

7. Why can limiting by $binary_remote_addr throttle an entire company at once, and what are two alternatives?

   Behind a load balancer or a shared NAT every request carries the same source address, so everyone shares one bucket. Restore the client address with set_real_ip_from/real_ip_header, or key the zone on something that identifies the client, such as $http_authorization.

8. Header logging is off, this run's journal is clean, and every file is private. Why is the job not finished?

   The old journal entries, and any copies forwarded to central logging, still contain the client tokens, and the key was readable for as long as the file and unit were exposed. Closing a leak does not un-leak anything: purge what you can, then rotate the upstream key and reissue the client tokens.