diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Revision history for leb128-serialize
 
+## 1.2 -- 2020-11-20
+
+* Add `Data.Serialize.LEB128.Lenient` for a decoder that
+  allows overlong encodings.
+  For now, a simple code copy; may be refactored later.
+
 ## 1.1 -- 2020-06-08
 
 * Fix check for overlong encodings; it was not strict enough
diff --git a/leb128-cereal.cabal b/leb128-cereal.cabal
--- a/leb128-cereal.cabal
+++ b/leb128-cereal.cabal
@@ -1,6 +1,6 @@
 cabal-version:       >=1.10
 name:                leb128-cereal
-version:             1.1
+version:             1.2
 synopsis:            LEB128 and SLEB128 encoding
 description:
    This module implements encoding and decoding of 'Natural' and 'Integer'
@@ -22,6 +22,7 @@
 
 library
   exposed-modules:     Data.Serialize.LEB128
+  exposed-modules:     Data.Serialize.LEB128.Lenient
   build-depends:       base >=4.10 && <5
   build-depends:       bytestring >= 0.10
   build-depends:       cereal >= 0.5
@@ -30,6 +31,17 @@
   ghc-options:         -Wall -Wno-name-shadowing
 
 test-suite test
+  default-language: Haskell2010
+  type: exitcode-stdio-1.0
+  main-is: test.hs
+  build-depends: base >= 4 && < 5
+  build-depends: tasty >= 0.7
+  build-depends: tasty-quickcheck
+  build-depends: tasty-hunit
+  build-depends: leb128-cereal
+  build-depends: bytestring
+
+test-suite test-lenient
   default-language: Haskell2010
   type: exitcode-stdio-1.0
   main-is: test.hs
diff --git a/src/Data/Serialize/LEB128.hs b/src/Data/Serialize/LEB128.hs
--- a/src/Data/Serialize/LEB128.hs
+++ b/src/Data/Serialize/LEB128.hs
@@ -24,6 +24,7 @@
 --
 -- The decoders will fail if the input is not in canonical representation,
 -- i.e. longer than necessary.
+-- Use "Data.Serialize.LEB128.Lenient" if you need the strict semantics.
 --
 -- This code is inspired by Andreas Klebinger's LEB128 implementation in GHC.
 module Data.Serialize.LEB128
@@ -126,20 +127,12 @@
 
 -- | SLEB128-encodes an integer via a builder
 buildSLEB128 :: SLEB128 a => a -> B.Builder
-buildSLEB128 = go
+buildSLEB128 val
+  | val >= -64 && val < 64 = stopByte
+  | otherwise = goByte <> buildSLEB128 (shiftR val 7)
   where
-    go val = do
-        let !byte = fromIntegral (clearBit val 7) :: Word8
-        let !val' = val `unsafeShiftR` 7
-        let !signBit = testBit byte 6
-        let !done =
-                -- Unsigned value, val' == 0 and and last value can
-                -- be discriminated from a negative number.
-                (val' == 0 && not signBit) ||
-                -- Signed value,
-                (val' == -1 && signBit)
-        let !byte' = if done then byte else setBit byte 7
-        B.word8 byte' <> if done then mempty else go val'
+  stopByte = B.word8 (fromIntegral $ clearBit val 7)
+  goByte = B.word8 (fromIntegral $ setBit val 7)
 
 {-# SPECIALIZE buildSLEB128 :: Integer -> B.Builder #-}
 {-# SPECIALIZE buildSLEB128 :: Int -> B.Builder #-}
diff --git a/src/Data/Serialize/LEB128/Lenient.hs b/src/Data/Serialize/LEB128/Lenient.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Serialize/LEB128/Lenient.hs
@@ -0,0 +1,228 @@
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE AllowAmbiguousTypes #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# OPTIONS_GHC -Wno-dodgy-imports #-}
+{-# OPTIONS_GHC -O2 #-}
+-- {-# OPTIONS_GHC -ddump-simpl -dsuppress-unfoldings -dsuppress-idinfo -dsuppress-module-prefixes -ddump-to-file #-}
+
+-- |
+-- Module      : Data.Serialize.LEB128
+-- Description : LEB128 encoding
+-- License     : MIT
+-- Maintainer  : Joachim Breitner
+--
+-- | This module implements encoding and decoding of 'Natural' and 'Integer'
+-- values according to LEB128 and SLEB128. See
+-- https://en.wikipedia.org/wiki/LEB128 for a specification.
+--
+-- The module provides conversion to and from strict bytestrings.
+--
+-- Additionally, to integrate these into your own parsers and serializers, you
+-- can use the interfaces based on 'B.Builder' as well as @cereal@'s 'G.Get'
+-- and 'P.Put' monad.
+--
+-- The decoders in this module will accept over-long representations.
+-- Use "Data.Serialize.LEB128" if you need the strict semantics.
+--
+-- This code is inspired by Andreas Klebinger's LEB128 implementation in GHC.
+module Data.Serialize.LEB128.Lenient
+    (
+    -- * The class of encodable and decodable types
+      LEB128
+    , SLEB128
+    -- * Bytestring-based interface
+    , toLEB128
+    , fromLEB128
+    , toSLEB128
+    , fromSLEB128
+    -- * Builder interface
+    , buildLEB128
+    , buildSLEB128
+    -- * Cereal interface
+    , getLEB128
+    , getSLEB128
+    , putLEB128
+    , putSLEB128
+    ) where
+
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Lazy as BSL
+import qualified Data.ByteString.Builder as B
+import qualified Data.ByteString.Builder.Extra as B
+import qualified Data.Serialize.Get as G
+import qualified Data.Serialize.Put as P
+import Numeric.Natural
+import Control.Applicative
+import Control.Monad
+import Data.Bits
+import Data.Word
+import Data.Int
+import Data.Maybe
+import Data.Monoid ((<>))
+import Prelude hiding ((<>))
+
+-- | Unsigned number types can be LEB128-encoded
+class (Bits a, Num a, Integral a) => LEB128 a where
+instance LEB128 Natural
+instance LEB128 Word
+instance LEB128 Word8
+instance LEB128 Word16
+instance LEB128 Word32
+instance LEB128 Word64
+
+-- | Signed number types can be SLEB128-encoded
+class (Bits a, Num a, Integral a) => SLEB128 a
+instance SLEB128 Integer
+instance SLEB128 Int
+instance SLEB128 Int8
+instance SLEB128 Int16
+instance SLEB128 Int32
+instance SLEB128 Int64
+
+-- | LEB128-encodes a natural number to a strict bytestring
+toLEB128 :: LEB128 a => a -> BS.ByteString
+toLEB128 = BSL.toStrict . B.toLazyByteStringWith (B.safeStrategy 32 32) BSL.empty . buildLEB128
+
+{-# SPECIALIZE toLEB128 :: Natural -> BS.ByteString #-}
+{-# SPECIALIZE toLEB128 :: Word -> BS.ByteString #-}
+{-# SPECIALIZE toLEB128 :: Word8 -> BS.ByteString #-}
+{-# SPECIALIZE toLEB128 :: Word16 -> BS.ByteString #-}
+{-# SPECIALIZE toLEB128 :: Word32 -> BS.ByteString #-}
+{-# SPECIALIZE toLEB128 :: Word64 -> BS.ByteString #-}
+
+-- | SLEB128-encodes an integer to a strict bytestring
+toSLEB128 :: SLEB128 a => a -> BS.ByteString
+toSLEB128 = BSL.toStrict . B.toLazyByteStringWith (B.safeStrategy 32 32) BSL.empty . buildSLEB128
+
+{-# SPECIALIZE toSLEB128 :: Integer -> BS.ByteString #-}
+{-# SPECIALIZE toSLEB128 :: Int -> BS.ByteString #-}
+{-# SPECIALIZE toSLEB128 :: Int8 -> BS.ByteString #-}
+{-# SPECIALIZE toSLEB128 :: Int16 -> BS.ByteString #-}
+{-# SPECIALIZE toSLEB128 :: Int32 -> BS.ByteString #-}
+{-# SPECIALIZE toSLEB128 :: Int64 -> BS.ByteString #-}
+
+-- | LEB128-encodes a natural number via a builder
+buildLEB128 :: LEB128 a => a -> B.Builder
+buildLEB128 = go
+  where
+    go i
+      | i <= 127
+      = B.word8 (fromIntegral i :: Word8)
+      | otherwise =
+        -- bit 7 (8th bit) indicates more to come.
+        B.word8 (setBit (fromIntegral i) 7) <> go (i `unsafeShiftR` 7)
+
+{-# SPECIALIZE buildLEB128 :: Natural -> B.Builder #-}
+{-# SPECIALIZE buildLEB128 :: Word -> B.Builder #-}
+{-# SPECIALIZE buildLEB128 :: Word8 -> B.Builder #-}
+{-# SPECIALIZE buildLEB128 :: Word16 -> B.Builder #-}
+{-# SPECIALIZE buildLEB128 :: Word32 -> B.Builder #-}
+{-# SPECIALIZE buildLEB128 :: Word64 -> B.Builder #-}
+
+-- This gets inlined for the specialied variants
+isFinite :: forall a. Bits a => Bool
+isFinite = isJust (bitSizeMaybe (undefined :: a))
+
+-- | SLEB128-encodes an integer via a builder
+buildSLEB128 :: SLEB128 a => a -> B.Builder
+buildSLEB128 val
+  | val >= -64 && val < 64 = stopByte
+  | otherwise = goByte <> buildSLEB128 (shiftR val 7)
+  where
+  stopByte = B.word8 (fromIntegral $ clearBit val 7)
+  goByte = B.word8 (fromIntegral $ setBit val 7)
+
+{-# SPECIALIZE buildSLEB128 :: Integer -> B.Builder #-}
+{-# SPECIALIZE buildSLEB128 :: Int -> B.Builder #-}
+{-# SPECIALIZE buildSLEB128 :: Int8 -> B.Builder #-}
+{-# SPECIALIZE buildSLEB128 :: Int16 -> B.Builder #-}
+{-# SPECIALIZE buildSLEB128 :: Int32 -> B.Builder #-}
+{-# SPECIALIZE buildSLEB128 :: Int64 -> B.Builder #-}
+
+-- | LEB128-encodes a natural number in @cereal@'s 'P.Put' monad
+putLEB128 :: LEB128 a => P.Putter a
+putLEB128 = P.putBuilder . buildLEB128
+{-# INLINE putLEB128 #-}
+
+-- | SLEB128-encodes an integer in @cereal@'s 'P.Put' monad
+putSLEB128 :: SLEB128 a => P.Putter a
+putSLEB128 = P.putBuilder . buildSLEB128
+{-# INLINE putSLEB128 #-}
+
+-- | LEB128-decodes a natural number from a strict bytestring
+fromLEB128 :: LEB128 a => BS.ByteString -> Either String a
+fromLEB128 = runComplete getLEB128
+{-# INLINE fromLEB128 #-}
+
+-- | SLEB128-decodes an integer from a strict bytestring
+fromSLEB128 :: SLEB128 a => BS.ByteString -> Either String a
+fromSLEB128 = runComplete getSLEB128
+{-# INLINE fromSLEB128 #-}
+
+runComplete :: G.Get a -> BS.ByteString -> Either String a
+runComplete p bs = do
+    (x,r) <- G.runGetState p bs 0
+    unless (BS.null r) $ Left "extra bytes in input"
+    return x
+
+-- | LEB128-decodes a natural number via @cereal@
+getLEB128 :: forall a. LEB128 a => G.Get a
+getLEB128 = G.label "LEB128" $ go 0 0
+  where
+    go :: Int -> a -> G.Get a
+    go !shift !w = do
+      byte <- G.getWord8 <|> fail "short encoding"
+      let !byteVal = fromIntegral (clearBit byte 7)
+      case bitSizeMaybe w of
+          Just bs | shift > bs -> fail "overflow"
+          _ -> return ()
+      when (isFinite @a) $
+        unless (byteVal `unsafeShiftL` shift `unsafeShiftR` shift == byteVal) $
+          fail "overflow"
+      let !val = w .|. (byteVal `unsafeShiftL` shift)
+      let !shift' = shift+7
+      if hasMore byte
+        then go shift' val
+        else return $! val
+
+    hasMore b = testBit b 7
+
+{-# SPECIALIZE getLEB128 :: G.Get Natural #-}
+{-# SPECIALIZE getLEB128 :: G.Get Word #-}
+{-# SPECIALIZE getLEB128 :: G.Get Word8 #-}
+{-# SPECIALIZE getLEB128 :: G.Get Word16 #-}
+{-# SPECIALIZE getLEB128 :: G.Get Word32 #-}
+{-# SPECIALIZE getLEB128 :: G.Get Word64 #-}
+
+-- | SLEB128-decodes an integer via @cereal@
+getSLEB128 :: forall a. SLEB128 a => G.Get a
+getSLEB128 = G.label "SLEB128" $ go 0 0 0
+  where
+    go :: Word8 -> Int -> a -> G.Get a
+    go !prev !shift !w = do
+        byte <- G.getWord8 <|> fail "short encoding"
+        let !byteVal = fromIntegral (clearBit byte 7)
+        case bitSizeMaybe w of
+            Just bs | shift > bs -> fail "overflow"
+            _ -> return ()
+        when (isFinite @a) $
+          unless ((byteVal `unsafeShiftL` shift `unsafeShiftR` shift) .&. 0x7f == byteVal) $
+            fail "overflow"
+        let !val = w .|. (byteVal `unsafeShiftL` shift)
+        let !shift' = shift+7
+        if hasMore byte
+            then go byte shift' val
+            else if signed byte
+              then return $! val - bit shift'
+              else return $! val
+
+    hasMore b = testBit b 7
+    signed b = testBit b 6
+
+{-# SPECIALIZE getSLEB128 :: G.Get Integer #-}
+{-# SPECIALIZE getSLEB128 :: G.Get Int #-}
+{-# SPECIALIZE getSLEB128 :: G.Get Int8 #-}
+{-# SPECIALIZE getSLEB128 :: G.Get Int16 #-}
+{-# SPECIALIZE getSLEB128 :: G.Get Int32 #-}
+{-# SPECIALIZE getSLEB128 :: G.Get Int64 #-}