diff --git a/.gitea/scripts/sop-tier-check.sh b/.gitea/scripts/sop-tier-check.sh
index d1bd2c235..d9d16839a 100755
--- a/.gitea/scripts/sop-tier-check.sh
+++ b/.gitea/scripts/sop-tier-check.sh
@@ -290,48 +290,75 @@ debug "approvers: $(echo "$APPROVERS" | tr '\n' ' ')"
 # Pre/post spaces ensure case patterns *${_t}* match even when the name
 # is the first or last entry (bash case *word* needs delimiters on both sides).
 #
-# FALLBACK: if ALL team probes return 403 (token lacks read:org scope),
-# fall back to /orgs/{org}/members/{user}. This returns 204 for any org
-# member — a superset of team membership. Accepting it as a fallback means
-# the gate passes when the token is scoped to repo+user only (core-bot PAT).
-# This is safe because: (a) org membership is a prerequisite for every
-# eligible team; (b) the AND-composition of internal#189 still requires
-# multiple independent approvers; (c) any token with read:repository can
-# see the approving reviews, so bypass requires a colluding approver.
+# FAIL-CLOSED AUTHORIZATION (security: SOP tier gate is an AUTHORIZATION gate).
+#
+# This used to fall back to /orgs/{org}/members/{user} whenever every team
+# probe failed and credit any org member as a member of EVERY queried team.
+# That was a privilege-escalation: org membership is NOT team membership, so
+# a 403/visibility/token-scope gap on the team probes silently promoted a
+# plain org member to satisfy tier:high (ceo). An inability-to-verify became
+# an authorization GRANT. The fallback is REMOVED — org membership must never
+# satisfy a team-gated tier.
+#
+# A team-membership probe has exactly three meaningful outcomes:
+#   200 / 204  → the user IS a member of that team       (credit it)
+#   404        → the user is definitively NOT a member    (no credit, verified)
+#   anything else (403 / 401 / 5xx / curl failure / non-numeric)
+#              → membership CANNOT be read                 (cannot-verify)
+#
+# Per the dev-sop fail-closed rule (inability-to-verify = failure, never a
+# pass — and here, never an authorization grant), a cannot-verify outcome on
+# ANY probe is a HARD infra failure: we publish a loud cannot-verify error and
+# exit non-zero. We do NOT proceed to evaluate the tier expression on a partial
+# / unverifiable membership picture, because doing so could let an unverifiable
+# approver's clause silently fail-or-pass on incomplete data. Fix the token
+# scope (read:organization) or the runner network — not the gate.
 declare -A APPROVER_TEAMS
+_verify_failed=""   # accumulates "<user>:<team>(HTTP <code>)" for probes we could not read
 for U in $APPROVERS; do
   [ "$U" = "$PR_AUTHOR" ] && debug "skip self-review by $U" && continue
-  _any_team_success="no"
   for T in "${!TEAM_ID[@]}"; do
     ID="${TEAM_ID[$T]}"
+    set +e
     CODE=$(curl -sS -o /dev/null -w '%{http_code}' -H "$AUTH" \
       "${API}/teams/${ID}/members/${U}")
-    debug "probe: $U in team $T (id=$ID) → HTTP $CODE"
-    if [ "$CODE" = "200" ] || [ "$CODE" = "204" ]; then
-      APPROVER_TEAMS[$U]="${APPROVER_TEAMS[$U]:- } ${APPROVER_TEAMS[$U]:+ }$T "
-      debug "$U qualifies for team $T"
-      _any_team_success="yes"
+    _curl_exit=$?
+    set -e
+    debug "probe: $U in team $T (id=$ID) → HTTP $CODE (curl exit=$_curl_exit)"
+    if [ "$_curl_exit" -ne 0 ]; then
+      # curl itself failed (DNS, connection refused, timeout) — unreachable.
+      _verify_failed="${_verify_failed}${_verify_failed:+, }${U}:${T}(curl exit ${_curl_exit})"
+      continue
     fi
-  done
-  # Fallback: if every team probe returned 403, try org membership.
-  # "??" teams were never resolved to IDs so they never entered the loop.
-  # If the user is an org member, credit them as being in each queried team
-  # (engineers, managers, ceo are all org-level). This is safe because org
-  # membership is a prerequisite for all three, and bypass requires a colluding
-  # approver (same risk as before the AND-composition).
-  if [ "$_any_team_success" = "no" ]; then
-    ORG_CODE=$(curl -sS -o /dev/null -w '%{http_code}' -H "$AUTH" \
-      "${API}/orgs/${OWNER}/members/${U}")
-    debug "probe: $U in org $OWNER (fallback) → HTTP $ORG_CODE"
-    if [ "$ORG_CODE" = "204" ]; then
-      for T in "${!TEAM_ID[@]}"; do
+    case "$CODE" in
+      200|204)
         APPROVER_TEAMS[$U]="${APPROVER_TEAMS[$U]:- } ${APPROVER_TEAMS[$U]:+ }$T "
-      done
-      debug "$U credited as org member for all queried teams (fallback — token may lack read:org)"
-    fi
-  fi
+        debug "$U qualifies for team $T"
+        ;;
+      404)
+        # Definitively not a member of this team — a verified negative.
+        debug "$U is NOT a member of team $T (verified 404)"
+        ;;
+      *)
+        # 403/401/5xx/etc — membership is unreadable. Do NOT treat as "not a
+        # member" and do NOT fall back to org membership. This is cannot-verify.
+        _verify_failed="${_verify_failed}${_verify_failed:+, }${U}:${T}(HTTP ${CODE})"
+        ;;
+    esac
+  done
 done
 
+# Fail-closed: if ANY membership probe could not be read, we cannot make an
+# authorization decision. Publish a loud cannot-verify / infra-failed status
+# and exit non-zero. Never grant the tier on unverifiable membership.
+if [ -n "$_verify_failed" ]; then
+  echo "::error::sop-tier-check CANNOT VERIFY team membership — gate FAILS CLOSED."
+  echo "::error::Unreadable membership probe(s): ${_verify_failed}"
+  echo "::error::A team-membership probe returned 403/401/5xx (or curl failed). The SOP tier gate is an authorization gate; an inability to verify team membership is treated as a FAILURE, never a pass. Org membership is NOT team membership and is never credited as a fallback."
+  echo "::error::Fix: ensure GITEA_TOKEN (SOP_TIER_CHECK_TOKEN) has read:organization scope and the Gitea API is reachable from the runner, then re-run. Do NOT relax this gate."
+  exit 1
+fi
+
 # 7. Evaluate the tier expression.
 #
 # legacy OR-gate: use the simplified loop from before internal#189.
diff --git a/.gitea/scripts/tests/test_sop_tier_check_authz.sh b/.gitea/scripts/tests/test_sop_tier_check_authz.sh
new file mode 100755
index 000000000..53f84f6ee
--- /dev/null
+++ b/.gitea/scripts/tests/test_sop_tier_check_authz.sh
@@ -0,0 +1,272 @@
+#!/usr/bin/env bash
+# Security regression test for the SOP tier-gate AUTHORIZATION bypass.
+#
+# Bug (fixed in fix/sop-tier-authz-no-org-fallback):
+#   sop-tier-check.sh probed team membership at /teams/{id}/members/{user}.
+#   If EVERY team probe failed (e.g. 403 — token lacks read:organization, or
+#   any visibility/flakiness gap), it FELL BACK to /orgs/{org}/members/{user}
+#   and credited that org member as a member of EVERY queried team. The
+#   evaluator then treated those synthetic memberships as real, so a plain
+#   NON-CEO org member satisfied tier:high (ceo). A visibility/auth gap became
+#   a real highest-tier authorization PASS — privilege escalation.
+#
+# Fix (fail-closed authorization):
+#   - The org-member ⇒ "member of all teams" fallback is REMOVED. Org
+#     membership is never credited as team membership.
+#   - A team probe that returns anything other than 200/204 (member) or 404
+#     (verified non-member) is a CANNOT-VERIFY condition: the gate fails loud
+#     (exit 1) with a cannot-verify status and never grants the tier.
+#
+# Method: this is a true end-to-end test. It prepends a fake `curl` to PATH
+# that serves canned Gitea API responses keyed by URL, then runs the REAL
+# sop-tier-check.sh. The fake exercises the genuine probe→credit→evaluate
+# path — no logic is re-implemented in the test.
+
+set -euo pipefail
+
+THIS_DIR="$(cd "$(dirname "$0")" && pwd)"
+SCRIPT_DIR="$(cd "$THIS_DIR/.." && pwd)"
+SCRIPT="$SCRIPT_DIR/sop-tier-check.sh"
+
+command -v jq >/dev/null 2>&1 || { echo "::error::jq required but not found"; exit 1; }
+[ -f "$SCRIPT" ] || { echo "::error::sop-tier-check.sh not found at $SCRIPT — test must fail loudly if the script is absent"; exit 1; }
+
+# sop-tier-check.sh uses `declare -A` (associative arrays), which require
+# bash >= 4. CI runners (Ubuntu) ship bash 5; macOS ships 3.2. Resolve a
+# bash >= 4 to run the script under.
+pick_bash() {
+  local c
+  for c in bash /opt/homebrew/bin/bash /usr/local/bin/bash /bin/bash; do
+    local p; p="$(command -v "$c" 2>/dev/null || true)"
+    [ -n "$p" ] || continue
+    local maj; maj="$("$p" -c 'echo "${BASH_VERSINFO[0]}"' 2>/dev/null || echo 0)"
+    if [ "${maj:-0}" -ge 4 ]; then echo "$p"; return 0; fi
+  done
+  return 1
+}
+BASH4="$(pick_bash)" || { echo "::error::need bash >= 4 to run sop-tier-check.sh (associative arrays); none found"; exit 1; }
+echo "using bash: $BASH4 ($("$BASH4" -c 'echo $BASH_VERSION'))"
+
+PASS=0
+FAIL=0
+
+assert_eq() {
+  local label="$1" expected="$2" got="$3"
+  if [ "$expected" = "$got" ]; then
+    echo "  PASS  $label"
+    PASS=$((PASS + 1))
+  else
+    echo "  FAIL  $label"
+    echo "        expected: <$expected>"
+    echo "        got:      <$got>"
+    FAIL=$((FAIL + 1))
+  fi
+}
+
+assert_contains() {
+  local label="$1" haystack="$2" needle="$3"
+  if printf '%s' "$haystack" | grep -qF -- "$needle"; then
+    echo "  PASS  $label"
+    PASS=$((PASS + 1))
+  else
+    echo "  FAIL  $label (missing substring: <$needle>)"
+    FAIL=$((FAIL + 1))
+  fi
+}
+
+assert_not_contains() {
+  local label="$1" haystack="$2" needle="$3"
+  if printf '%s' "$haystack" | grep -qF -- "$needle"; then
+    echo "  FAIL  $label (unexpected substring present: <$needle>)"
+    FAIL=$((FAIL + 1))
+  else
+    echo "  PASS  $label"
+    PASS=$((PASS + 1))
+  fi
+}
+
+# ---------------------------------------------------------------------------
+# Fake-curl harness.
+#
+# The real script calls curl in two shapes:
+#   (a) body capture:   curl -sS -H AUTH URL                 -> prints JSON body
+#   (b) http-code:      curl -sS -o FILE -w '%{http_code}' -H AUTH URL
+#   (c) http-code only: curl -sS -o /dev/null -w '%{http_code}' -H AUTH URL
+#
+# Our fake reads the URL (last non-flag arg), looks up a response in fixture
+# files under $FIXDIR, and emits body and/or http-code accordingly.
+# ---------------------------------------------------------------------------
+
+make_harness() {
+  # $1 = scenario dir to populate with fixtures
+  local FIXDIR="$1"
+  local BIN="$FIXDIR/bin"
+  mkdir -p "$BIN"
+  cat > "$BIN/curl" <<'FAKE'
+#!/usr/bin/env bash
+# Fake curl for sop-tier-check authz tests. Looks up canned responses by URL.
+set -u
+FIXDIR="${SOP_TEST_FIXDIR:?SOP_TEST_FIXDIR unset}"
+
+url=""
+out=""
+want_code="no"
+prev=""
+for a in "$@"; do
+  case "$prev" in
+    -o) out="$a" ;;
+  esac
+  case "$a" in
+    http*://*) url="$a" ;;
+    '%{http_code}') want_code="yes" ;;
+  esac
+  # -w '%{http_code}' arrives as the value of the -w flag
+  if [ "$prev" = "-w" ] && [ "$a" = '%{http_code}' ]; then want_code="yes"; fi
+  prev="$a"
+done
+
+# Map URL -> fixture key (a filename-safe slug).
+# We only need the path after /api/v1.
+path="${url#*/api/v1}"
+slug="$(printf '%s' "$path" | tr '/?=&' '____')"
+
+body_file="$FIXDIR/body${slug}"
+code_file="$FIXDIR/code${slug}"
+
+# Emit body to -o target (or capture for stdout) when a body fixture exists.
+body=""
+if [ -f "$body_file" ]; then body="$(cat "$body_file")"; fi
+if [ -n "$out" ]; then
+  printf '%s' "$body" > "$out"
+else
+  printf '%s' "$body"
+fi
+
+# Emit http code when requested.
+if [ "$want_code" = "yes" ]; then
+  if [ -f "$code_file" ]; then
+    printf '%s' "$(cat "$code_file")"
+  else
+    printf '200'
+  fi
+fi
+exit 0
+FAKE
+  chmod +x "$BIN/curl"
+  echo "$BIN"
+}
+
+# Common fixtures shared by scenarios. $1 = FIXDIR, $2 = approver login,
+# $3 = tier label name (e.g. tier:high), $4 = teams JSON.
+seed_common() {
+  local FIXDIR="$1" approver="$2" tier="$3" teams_json="$4"
+  mkdir -p "$FIXDIR"
+  # /user -> whoami
+  printf '%s' '{"login":"sop-bot"}' > "$FIXDIR/body_user"
+  # PR head sha
+  printf '%s' '{"head":{"sha":"headsha1"}}' \
+    > "$FIXDIR/body_repos_molecule-ai_molecule-core_pulls_42"
+  # labels
+  printf '%s' "[{\"name\":\"$tier\"}]" \
+    > "$FIXDIR/body_repos_molecule-ai_molecule-core_issues_42_labels"
+  # org teams list
+  printf '%s' "$teams_json" > "$FIXDIR/body_orgs_molecule-ai_teams"
+  printf '%s' '200' > "$FIXDIR/code_orgs_molecule-ai_teams"
+  # reviews: one APPROVED on current head by $approver
+  printf '%s' "[{\"state\":\"APPROVED\",\"commit_id\":\"headsha1\",\"user\":{\"login\":\"$approver\"}}]" \
+    > "$FIXDIR/body_repos_molecule-ai_molecule-core_pulls_42_reviews"
+}
+
+run_script() {
+  # $1 = FIXDIR (must contain bin/curl). Returns combined stdout+stderr; sets RC.
+  local FIXDIR="$1"
+  local BIN="$FIXDIR/bin"
+  set +e
+  OUT=$(
+    SOP_TEST_FIXDIR="$FIXDIR" \
+    PATH="$BIN:$PATH" \
+    GITEA_TOKEN="faketoken" \
+    GITEA_HOST="git.moleculesai.app" \
+    REPO="molecule-ai/molecule-core" \
+    PR_NUMBER="42" \
+    PR_AUTHOR="pr-author" \
+    SOP_DEBUG="0" \
+    SOP_LEGACY_CHECK="0" \
+    "$BASH4" "$SCRIPT" 2>&1
+  )
+  RC=$?
+  set -e
+  printf '%s' "$OUT"
+  return $RC
+}
+
+TEAMS_JSON='[{"name":"ceo","id":10},{"name":"engineers","id":11},{"name":"managers","id":12}]'
+
+echo "=============================================================="
+echo "Scenario 1: tier:high, team probe 403 (cannot read), approver"
+echo "            is a plain org member but NOT in ceo team."
+echo "            EXPECT: tier NOT granted (fail-closed cannot-verify)."
+echo "=============================================================="
+S1="$(mktemp -d)"
+make_harness "$S1" >/dev/null
+seed_common "$S1" "org-only-bob" "tier:high" "$TEAMS_JSON"
+# Team membership probe for ceo (id=10) returns 403 — cannot read.
+printf '%s' '403' > "$S1/code_teams_10_members_org-only-bob"
+# The OLD bug path: org membership probe would 204 and synthetic-credit.
+printf '%s' '204' > "$S1/code_orgs_molecule-ai_members_org-only-bob"
+set +e
+OUT1="$(run_script "$S1")"; RC1=$?
+set -e
+echo "$OUT1" | sed 's/^/    /'
+echo "    (exit=$RC1)"
+assert_eq "S1 exit non-zero (tier NOT granted)" "1" "$([ "$RC1" -ne 0 ] && echo 1 || echo 0)"
+assert_not_contains "S1 did NOT print PASSED" "$OUT1" "sop-tier-check PASSED"
+assert_contains "S1 cannot-verify error surfaced" "$OUT1" "CANNOT VERIFY"
+assert_contains "S1 names the unreadable probe (403)" "$OUT1" "HTTP 403"
+rm -rf "$S1"
+
+echo
+echo "=============================================================="
+echo "Scenario 2: tier:high, genuine ceo team member (probe 204)."
+echo "            EXPECT: tier GRANTED."
+echo "=============================================================="
+S2="$(mktemp -d)"
+make_harness "$S2" >/dev/null
+seed_common "$S2" "real-ceo" "tier:high" "$TEAMS_JSON"
+printf '%s' '204' > "$S2/code_teams_10_members_real-ceo"   # ceo team: member
+set +e
+OUT2="$(run_script "$S2")"; RC2=$?
+set -e
+echo "$OUT2" | sed 's/^/    /'
+echo "    (exit=$RC2)"
+assert_eq "S2 exit zero (granted)" "0" "$RC2"
+assert_contains "S2 printed PASSED" "$OUT2" "sop-tier-check PASSED"
+rm -rf "$S2"
+
+echo
+echo "=============================================================="
+echo "Scenario 3: tier:high, approver is an org member but a VERIFIED"
+echo "            non-member of ceo (team probe 404). Org probe would"
+echo "            204 — must NEVER be synthetic-credited."
+echo "            EXPECT: tier NOT granted (clause FAIL), no fallback."
+echo "=============================================================="
+S3="$(mktemp -d)"
+make_harness "$S3" >/dev/null
+seed_common "$S3" "org-member-carol" "tier:high" "$TEAMS_JSON"
+printf '%s' '404' > "$S3/code_teams_10_members_org-member-carol"  # verified NOT in ceo
+printf '%s' '204' > "$S3/code_orgs_molecule-ai_members_org-member-carol" # org member (must be ignored)
+set +e
+OUT3="$(run_script "$S3")"; RC3=$?
+set -e
+echo "$OUT3" | sed 's/^/    /'
+echo "    (exit=$RC3)"
+assert_eq "S3 exit non-zero (tier NOT granted)" "1" "$([ "$RC3" -ne 0 ] && echo 1 || echo 0)"
+assert_not_contains "S3 did NOT print PASSED" "$OUT3" "sop-tier-check PASSED"
+assert_contains "S3 reported a real clause FAIL (not cannot-verify)" "$OUT3" "FAILED for tier:high"
+assert_not_contains "S3 did NOT cannot-verify (404 is a verified negative)" "$OUT3" "CANNOT VERIFY"
+rm -rf "$S3"
+
+echo
+echo "------"
+echo "PASS=$PASS FAIL=$FAIL"
+[ "$FAIL" -eq 0 ]