Boundaries that were supposed to hold

The security pass. Six findings, none reachable by visiting the site and
every one a boundary this codebase says it keeps.

A subagent is pinned to a list of read-only commands, in every mode,
unattended, with no card anybody could approve -- and `find *` was on it.
find writes files with -fprintf, runs programs with -exec and removes them
with -delete, and none of that needs a character the metacharacter guard
refuses. A page the model had just read could ask for a helper and get a
key into authorized_keys, from Plan mode, which promises to change
nothing. Refused in `subject()` rather than trimmed from the list: a
pattern cannot say "and no dangerous flags", and "this one looks
read-only" is exactly what put find there.

The loopback guard missed `0.0.0.0`, which is not is_loopback but does
connect to localhost -- so it answered a *decided* False and skipped the
DNS half too. The one spelling of "this machine" that walked past a guard
whose whole job is that sentence.

Twice in the update helper, which is the one place this deliberately
crosses a privilege boundary: root ran a script the service account owns,
and root sourced a file that account can replace. Either turns a
compromise of the web application into root. The first needed no
compromise at all -- a pull happens as the service user and root runs
whatever it fetched, so control of the branch was control of root. The
old test asserted that exact ExecStart line and had pinned it in place.

Push endpoints skipped check_url, the only outbound request that did. And
a chat could be filed in another account's folder, which hands over its
system prompt -- `_new_chat` resolved the folder, discarded it when it was
not the caller's, and stored the raw id anyway.

An existing helper install keeps the old wiring until install.sh is
re-run; update.sh now says so when it finds itself inside the checkout.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/services/agent/hosts.py

@@ -107,9 +107,17 @@ def _literal(host: str) -> bool | None:
     if text in ("localhost", "localhost.localdomain", "ip6-localhost", "ip6-loopback"):
         return True
     try:
-        return ipaddress.ip_address(text).is_loopback
+        address = ipaddress.ip_address(text)
     except ValueError:
         return None
+    # `is_unspecified` as well as `is_loopback`, because `0.0.0.0` and `::` are
+    # neither a real destination nor a refused one: connect() to either goes to
+    # loopback on Linux, so an SSH profile pointed at `0.0.0.0` reached this
+    # host's own sshd. `is_loopback` alone answered a decided **False**, which
+    # also short-circuited `resolves_here`, so the DNS half never ran either --
+    # the one spelling of "this machine" that walked past a guard whose whole
+    # job is that sentence.
+    return address.is_loopback or address.is_unspecified
 
 
 def is_loopback(host: str) -> bool:

src/lembas/services/agent/policy.py

@@ -111,6 +111,37 @@ POLICY: dict[str, dict[str, str]] = {
 # confined to `decide` and is worth doing; it is not done here.
 _UNSAFE = re.compile(r"[;&|<>`$\n\\()]")
 
+# Flags that turn a "read-only" command into one that writes or executes, on
+# tools whose *name* is on somebody's allow list.
+#
+# `_UNSAFE` stops a command line being composed out of two commands. It does
+# nothing about a single command that composes one itself, and several of the
+# obvious read-only tools do: `find -exec cmd +` runs a program, `-fprintf`
+# writes a file, `-delete` removes one, and `rg --pre` runs a preprocessor for
+# every file it opens. None of those needs a character `_UNSAFE` refuses, so
+# `find *` on an allow list -- which is what a subagent gets, in every mode --
+# was arbitrary write and arbitrary execution wearing a read-only name.
+#
+# Refused here rather than trimmed from the allow list alone, because the list
+# is the thing an administrator edits and "this one looks read-only" is exactly
+# the reasoning that put `find *` there. A pattern cannot express "and no
+# dangerous flags"; this can.
+#
+# Matched on the *normalised* line and word-bounded, so `docs/-exec-notes.md`
+# is fine -- the flag has to stand alone as an argument.
+#
+# It does catch `grep -rn -- -delete src/`, where the word is a search term
+# rather than a flag, and that is the right direction to be wrong in: a false
+# refusal here means the call falls through to the policy table and asks, which
+# costs one approval card. A false allow means an unattended helper writing
+# files. Nothing is *blocked* by this -- a reader in Auto still gets it, and in
+# any other mode they are shown it first, which is what they would want to be
+# shown.
+_ACTION = re.compile(
+    r"(?:^|\s)-(?:exec|execdir|ok|okdir|fprintf|fprint|fprint0|delete)(?=\s|$)"
+    r"|(?:^|\s)--(?:pre|search-zip|hostname-bin)(?=[\s=]|$)"
+)
+
 
 @dataclass(frozen=True)
 class Decision:
@@ -162,6 +193,8 @@ def subject(tool_name: str, command: str = "") -> str | None:
     if _UNSAFE.search(raw):
         return None
     line = " ".join(raw.split())
+    if _ACTION.search(line):
+        return None
     return line or None
 
 

src/lembas/services/push.py

@@ -76,8 +76,10 @@ from sqlalchemy import select
 from sqlalchemy.orm import Session as DBSession
 
 from lembas.db.models import PushSubscription, User
+from lembas.services import fetch as fetch_service
 from lembas.services import settings_store
 from lembas.services.crypto import decrypt, encrypt
+from lembas.services.fetch import FetchError
 
 log = logging.getLogger(__name__)
 
@@ -274,6 +276,18 @@ async def send_one(db: DBSession, subscription: PushSubscription, payload: dict[
         "Authorization": f"vapid t={token}, k={keys(db).public_b64}",
     }
 
+    # Again, on a stored value. The subscribe route checks it too, but the row
+    # outlives that check: a name that pointed at a push service when it was
+    # registered can point inside the network later, and this is the side that
+    # actually opens the socket. The same split `agent/hosts.py` makes.
+    try:
+        fetch_service.check_url(subscription.endpoint)
+    except FetchError as exc:
+        log.warning(
+            "refusing to push to %s: %s", _audience(subscription.endpoint), exc.message
+        )
+        return False
+
     try:
         async with httpx.AsyncClient(timeout=10.0) as client:
             response = await client.post(subscription.endpoint, content=encrypted, headers=headers)