diff --git a/GHC/Conc/Sync.hs b/GHC/Conc/Sync.hs
--- a/GHC/Conc/Sync.hs
+++ b/GHC/Conc/Sync.hs
@@ -1,4 +1,3 @@
-{-# LANGUAGE CPP #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE RankNTypes #-}
@@ -392,13 +391,14 @@
 getNumProcessors = fmap fromIntegral c_getNumberOfProcessors
 
 foreign import ccall unsafe "getNumberOfProcessors"
-  c_getNumberOfProcessors :: IO CUInt
+  c_getNumberOfProcessors :: IO Word32
 
 -- | Returns the number of sparks currently in the local spark pool
 numSparks :: IO Int
 numSparks = IO $ \s -> case numSparks# s of (# s', n #) -> (# s', I# n #)
 
-foreign import ccall "&enabled_capabilities" enabled_capabilities :: Ptr CInt
+foreign import ccall "&enabled_capabilities"
+  enabled_capabilities :: Ptr Word32
 
 childHandler :: SomeException -> IO ()
 childHandler err = catch (real_handler err) childHandler
diff --git a/GHC/Event/KQueue.hsc b/GHC/Event/KQueue.hsc
--- a/GHC/Event/KQueue.hsc
+++ b/GHC/Event/KQueue.hsc
@@ -44,8 +44,8 @@
 import GHC.Real (quotRem, fromIntegral)
 import GHC.Show (Show(show))
 import GHC.Event.Internal (Timeout(..))
-import System.Posix.Internals (c_close)
-import System.Posix.Types (Fd(..))
+import System.Posix.Internals (c_close, c_getpid)
+import System.Posix.Types (Fd(..), CPid)
 import qualified GHC.Event.Array as A
 
 #if defined(netbsd_HOST_OS)
@@ -73,19 +73,26 @@
 data KQueue = KQueue {
       kqueueFd     :: {-# UNPACK #-} !KQueueFd
     , kqueueEvents :: {-# UNPACK #-} !(A.Array Event)
+    , kqueuePid    :: {-# UNPACK #-} !CPid -- ^ pid, used to detect forks
     }
 
 new :: IO E.Backend
 new = do
   kqfd <- kqueue
   events <- A.new 64
-  let !be = E.backend poll modifyFd modifyFdOnce delete (KQueue kqfd events)
+  pid <- c_getpid
+  let !be = E.backend poll modifyFd modifyFdOnce delete (KQueue kqfd events pid)
   return be
 
 delete :: KQueue -> IO ()
 delete kq = do
-  _ <- c_close . fromKQueueFd . kqueueFd $ kq
-  return ()
+  -- detect forks: the queue isn't inherited by a child process created with
+  -- fork. Hence we mustn't try to close the old fd or we might close a random
+  -- one (e.g. the one used by timerfd, cf #24672).
+  pid <- c_getpid
+  when (pid == kqueuePid kq) $ do
+    _ <- c_close . fromKQueueFd . kqueueFd $ kq
+    return ()
 
 modifyFd :: KQueue -> Fd -> E.Event -> E.Event -> IO Bool
 modifyFd kq fd oevt nevt = do
diff --git a/base.cabal b/base.cabal
--- a/base.cabal
+++ b/base.cabal
@@ -1,6 +1,6 @@
 cabal-version:  3.0
 name:           base
-version:        4.18.2.1
+version:        4.18.3.0
 -- NOTE: Don't forget to update ./changelog.md
 
 license:        BSD-3-Clause
diff --git a/changelog.md b/changelog.md
--- a/changelog.md
+++ b/changelog.md
@@ -1,5 +1,9 @@
 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
+## 4.18.3.0 *January 2025*
+  * Shipped with GHC 9.6.7.
+  * Fix interaction between `fork` and the `kqueue`-based IO manager ([#24672](https://gitlab.haskell.org/ghc/ghc/-/issues/24672)).
+
 ## 4.18.2.1 *April 2024*
   * Various documentation improvements