diff --git a/async_postgres/pg_protocol.nim b/async_postgres/pg_protocol.nim
index 49c8df6..6dee54b 100644
--- a/async_postgres/pg_protocol.nim
+++ b/async_postgres/pg_protocol.nim
@@ -307,6 +307,16 @@ const
     ## tiny. Bounding by message length alone still buys a 16-byte `string`
     ## header per wire byte: 1 GiB in, 16 GiB preallocated, OutOfMemDefect.
 
+  MaxErrorOrNoticeFields* = 128
+    ## Upper bound on fields in a single ErrorResponse/NoticeResponse. Only
+    ## about 20 field codes exist in the protocol; 128 leaves ample room for
+    ## future codes while capping worst-case allocation.
+
+  MaxSaslMechanisms* = 64
+    ## Upper bound on advertised SASL mechanisms in AuthenticationSASL. Only
+    ## SCRAM-SHA-256 and SCRAM-SHA-256-PLUS are standardised; 64 leaves room
+    ## for future mechanisms while bounding pre-auth allocation.
+
 func makeBinarySafeLookup(): array[BinarySafeMaxOid + 1, bool] {.compileTime.} =
   for oid in BinarySafeOids:
     result[oid] = true
@@ -816,6 +826,12 @@ proc parseAuthentication(body: openArray[byte]): BackendMessage =
       offset += consumed
       if mechanism.len == 0:
         break
+      # Bound advertised list before adding to survive a hostile pre-auth server.
+      if result.saslMechanisms.len >= MaxSaslMechanisms:
+        raise newException(
+          PgProtocolError,
+          "AuthenticationSASL: mechanism count exceeds maximum of " & $MaxSaslMechanisms,
+        )
       result.saslMechanisms.add(mechanism)
   of 11:
     # SASLContinue
@@ -877,6 +893,7 @@ proc parseErrorOrNotice(body: openArray[byte], isError: bool): BackendMessage =
     result = BackendMessage(kind: bmkNoticeResponse)
     result.noticeFields = @[]
   var offset = 0
+  var fieldCount = 0
   while offset < body.len:
     let fieldType = char(body[offset])
     inc offset
@@ -884,6 +901,15 @@ proc parseErrorOrNotice(body: openArray[byte], isError: bool): BackendMessage =
       break
     let (value, consumed) = decodeCString(body, offset)
     offset += consumed
+    # Bound field count before allocating: a hostile ~1 GiB body could otherwise
+    # produce hundreds of millions of two-byte fields, amplifying allocation.
+    if fieldCount >= MaxErrorOrNoticeFields:
+      let name = if isError: "ErrorResponse" else: "NoticeResponse"
+      raise newException(
+        PgProtocolError,
+        name & ": field count exceeds maximum of " & $MaxErrorOrNoticeFields,
+      )
+    inc fieldCount
     let field = ErrorField(code: fieldType, value: value)
     if isError:
       result.errorFields.add(field)
diff --git a/async_postgres/pg_replication.nim b/async_postgres/pg_replication.nim
index bbd5621..d276761 100644
--- a/async_postgres/pg_replication.nim
+++ b/async_postgres/pg_replication.nim
@@ -586,6 +586,14 @@ proc identifySystem*(
   if results.len == 0 or results[0].rowCount == 0:
     raise newException(PgConnectionError, "IDENTIFY_SYSTEM returned no results")
   let qr = results[0]
+  # Guard fixed-column access so a malformed server response surfaces as a
+  # catchable PgConnectionError instead of an uncatchable IndexDefect from
+  # `cellInfo`.
+  if qr.fields.len < 3:
+    raise newException(
+      PgConnectionError,
+      "IDENTIFY_SYSTEM returned " & $qr.fields.len & " columns, expected >= 3",
+    )
   let row = initRow(qr.data, 0)
   var info = SystemInfo()
   info.systemId = row.getStr(0)
@@ -598,6 +606,11 @@ proc identifySystem*(
   return info
 
 proc decodeCreateSlotRow(qr: QueryResult): ReplicationSlotInfo =
+  if qr.fields.len < 2:
+    raise newException(
+      PgConnectionError,
+      "CREATE_REPLICATION_SLOT returned " & $qr.fields.len & " columns, expected >= 2",
+    )
   let row = initRow(qr.data, 0)
   result.slotName = row.getStr(0)
   result.consistentPoint = parseLsn(row.getStr(1))
@@ -652,6 +665,11 @@ proc readReplicationSlot*(
   if results.len == 0 or results[0].rowCount == 0:
     raise newException(PgConnectionError, "READ_REPLICATION_SLOT returned no results")
   let qr = results[0]
+  if qr.fields.len < 2:
+    raise newException(
+      PgConnectionError,
+      "READ_REPLICATION_SLOT returned " & $qr.fields.len & " columns, expected >= 2",
+    )
   let row = initRow(qr.data, 0)
   var info = ReplicationSlotInfo()
   # READ_REPLICATION_SLOT returns: slot_type, restart_lsn, restart_tli
@@ -675,6 +693,11 @@ proc timelineHistory*(
   if results.len == 0 or results[0].rowCount == 0:
     raise newException(PgConnectionError, "TIMELINE_HISTORY returned no results")
   let qr = results[0]
+  if qr.fields.len < 2:
+    raise newException(
+      PgConnectionError,
+      "TIMELINE_HISTORY returned " & $qr.fields.len & " columns, expected >= 2",
+    )
   let row = initRow(qr.data, 0)
   var info = TimelineHistory()
   if not row.isNull(0):
diff --git a/tests/test_protocol.nim b/tests/test_protocol.nim
index 7900209..f2225f2 100644
--- a/tests/test_protocol.nim
+++ b/tests/test_protocol.nim
@@ -1239,6 +1239,36 @@ suite "Backend decoding - edge cases":
     check res.state == psIncomplete
     check buf == original
 
+  test "ErrorResponse rejects excessive field count":
+    var body: seq[byte] = @[]
+    for _ in 0 .. MaxErrorOrNoticeFields:
+      body.add(byte('S'))
+      body.addCString("x")
+    body.add(0'u8)
+    var buf = buildMsg('E', body)
+    expect PgProtocolError:
+      discard parseBackendMessage(buf)
+
+  test "NoticeResponse rejects excessive field count":
+    var body: seq[byte] = @[]
+    for _ in 0 .. MaxErrorOrNoticeFields:
+      body.add(byte('S'))
+      body.addCString("x")
+    body.add(0'u8)
+    var buf = buildMsg('N', body)
+    expect PgProtocolError:
+      discard parseBackendMessage(buf)
+
+  test "AuthenticationSASL rejects excessive mechanism count":
+    var body: seq[byte] = @[]
+    body.addInt32(10)
+    for i in 0 .. MaxSaslMechanisms:
+      body.addCString("MECH-" & $i)
+    body.add(0'u8)
+    var buf = buildMsg('R', body)
+    expect PgProtocolError:
+      discard parseBackendMessage(buf)
+
 suite "Frontend encoding - edge cases":
   test "encodeBind with result formats":
     let msg = encodeBind("", "", @[0'i16], @[some(@[byte('1')])], @[1'i16])
diff --git a/tests/test_replication.nim b/tests/test_replication.nim
index b9a05c2..3bc3db9 100644
--- a/tests/test_replication.nim
+++ b/tests/test_replication.nim
@@ -781,3 +781,8 @@ suite "decodeCreateSlotRow":
     check info.consistentPoint == parseLsn("0/16B3740")
     check info.snapshotName == ""
     check info.outputPlugin == ""
+
+  test "missing required columns raises catchable PgConnectionError":
+    let qr = mkSlotQr(["my_slot"], 1)
+    expect(PgConnectionError):
+      discard decodeCreateSlotRow(qr)