Fix decodeCString to reject missing null terminator

async_postgres/pg_protocol.nim

@@ -329,17 +329,18 @@ proc decodeInt64*(buf: openArray[byte], offset: int): int64 =
 
 proc decodeCString*(buf: openArray[byte], offset: int): (string, int) =
   ## Decode a null-terminated string at the given offset. Returns (string, bytes consumed).
-  if offset > buf.len:
+  if offset >= buf.len:
     raise newException(ProtocolError, "decodeCString: offset past end of buffer")
   var i = offset
   while i < buf.len and buf[i] != 0:
     inc i
+  if i >= buf.len:
+    raise newException(ProtocolError, "decodeCString: missing null terminator")
   let slen = i - offset
   var s = newString(slen)
   if slen > 0:
     copyMem(addr s[0], unsafeAddr buf[offset], slen)
-  if i < buf.len:
-    inc i # skip null terminator
+  inc i # skip null terminator
   result = (s, i - offset)
 
 # Frontend message encoding

async_postgres/pg_replication.nim

@@ -230,15 +230,18 @@ proc currentPgTimestamp*(): int64 =
 
 proc decodeCStringAt(buf: openArray[byte], offset: int): (string, int) =
   ## Decode a null-terminated string at offset. Returns (string, next offset).
+  if offset >= buf.len:
+    raise newException(ProtocolError, "decodeCStringAt: offset past end of buffer")
   var i = offset
   while i < buf.len and buf[i] != 0:
     inc i
+  if i >= buf.len:
+    raise newException(ProtocolError, "decodeCStringAt: missing null terminator")
   let slen = i - offset
   var s = newString(slen)
   if slen > 0:
     copyMem(addr s[0], unsafeAddr buf[offset], slen)
-  if i < buf.len:
-    inc i # skip null
+  inc i # skip null
   (s, i)
 
 proc decodeTuple(buf: openArray[byte], offset: int): (seq[TupleField], int) =

tests/test_protocol.nim

@@ -48,9 +48,8 @@ suite "Byte helpers":
 
   test "decodeCString at end of buffer":
     let buf = @[byte('a'), 0'u8]
-    let (s, consumed) = decodeCString(buf, 2)
-    check s == ""
-    check consumed == 0
+    expect(ProtocolError):
+      discard decodeCString(buf, 2)
 
   test "decodeCString offset past end of buffer":
     let buf = @[byte('a'), 0'u8]
@@ -62,6 +61,11 @@ suite "Byte helpers":
     expect(ProtocolError):
       discard decodeCString(buf, 1)
 
+  test "decodeCString missing null terminator":
+    let buf = @[byte('h'), byte('i')]
+    expect(ProtocolError):
+      discard decodeCString(buf, 0)
+
 suite "Frontend encoding":
   test "encodeStartup - no type byte, version 3.0":
     let msg = encodeStartup("testuser", "testdb")