docs(users/Profpatsch/netencode): Parser security considerations

Netencode parsers should probably set an upper length limit.

Change-Id: Ibe65f2b59058106b720867a83435bf45660f1adf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5908
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/netencode/README.md

@@ -102,6 +102,24 @@ Similar to records, lists start with the length of their whole encoded content.
 * The list with text `foo` followed by i3 `-42`: `[14:t3:foo,i3:-42,]`
 * The list with `Some` and `None` tags: `[33:<4:Some|t3:foo,<4None|u,<4None|u,]`
 
+## parser security considerations
+
+The length field is a decimal number that is not length-restricted,
+meaning an attacker could give an infinitely long length (or extremely long)
+thus overflowing your parser if you are not careful.
+
+You should thus put a practical length limit to the length of length fields,
+which implicitely enforces a length limit on how long the value itself can be.
+
+Start by defining a max value length in bytes.
+Then count the number of decimals in that number.
+
+So if your max length is 1024 bytes, your length field can be a maximum `count_digits(1024) == 4` bytes long.
+
+Thus, if you restrict your parser to a length field of 4 bytes,
+it should also never parse anything longer than 1024 bytes for the value
+(plus 1 byte for the type tag, 4 bytes for the length, and 2 bytes for the separator & ending character).
+
 ## motivation
 
 TODO