Re: [exim/exim] [Bug]: ${utf8_domain_to_alabel:} doe not validate result (Issue #3226)

On Thu, May 14, 2026 at 03:20:50PM +0100, jgh via Exim-dev wrote:

> What is the problem environment ?
> ---------------------------------
> 
> commit hash 1b3209b
> 
> What happened?
> --------------
> 
> Fed with \xe2 \x80 \80 (which is a space-class thing) it returns a space.
> This is unfortunate in a domain, and not legitimate per IDNA2008

Indeed "enquad" (U+2000 <https://www.compart.com/en/unicode/U+2000>) is
a disallowed code point.  So a robust U-label to A-label encoder should
reject a U-label with that code point as disallowed.

I've just put together a fresh IDNA2008 implementation in Haskell,
this reports:

  1. Valid domain with some Greek letters in its second label:

    l0.xn--l1--x-f9dhi.l2 ([LDH,ULABEL,LDH])

  2. Invalid domain with an "enquad" in its second label:

    ErrLabelInvalid (IdnaLoc {idnaLabelIndex = 1, idnaByteOffset = -1}) (DisallowedCodepoint 8192)

[ The specific byte offset is difficult to report in U-labels that may
  be subject to NFC-normalisation, and possible additional mappings. ]

-- 
    Viktor.  🇺🇦 Слава Україні!

module Main (main) where
import qualified Data.ByteString as B
import qualified Data.Text as T
import qualified Data.Text.Encoding as T
import qualified Data.Text.IO as T
import Text.IDNA2008

main :: IO ()
main = do
    cvt okdom
    cvt baddom
  where
    okdom = T.pack "l0.l1-αβγ-x.l2"
    baddom = T.pack "l0.l1-" <> enquad <> T.pack "-x.l2"
      where
        enquad = T.decodeUtf8 $ B.pack [0xe2, 0x80, 0x80]

    cvt name = case parseDomain idnLabelForms name of
        Right (dom, inf) -> do
            T.putStr $ domainToAscii dom
            putStr " (" >> putStr (show inf) >> putStrLn ")"
        Left err -> print err

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