--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -1658,6 +1658,10 @@
     the write lock while determining what data to transfer.
     (default: True)
 
+``uncompressedallowsecret``
+    Whether to allow stream clones when the repository contains secret
+    changesets. (default: False)
+
 ``preferuncompressed``
     When set, clients will try to use the uncompressed streaming
     protocol. (default: False)

--- a/mercurial/streamclone.py
+++ b/mercurial/streamclone.py
@@ -13,6 +13,7 @@
 from . import (
     branchmap,
     error,
+    phases,
     store,
     util,
 )
@@ -162,9 +163,18 @@
 
         repo.invalidate()
 
-def allowservergeneration(ui):
+def allowservergeneration(repo):
     """Whether streaming clones are allowed from the server."""
-    return ui.configbool('server', 'uncompressed', True, untrusted=True)
+    if not repo.ui.configbool('server', 'uncompressed', True, untrusted=True):
+        return False
+
+    # The way stream clone works makes it impossible to hide secret changesets.
+    # So don't allow this by default.
+    secret = phases.hassecret(repo)
+    if secret:
+        return repo.ui.configbool('server', 'uncompressedallowsecret', False)
+
+    return True
 
 # This is it's own function so extensions can override it.
 def _walkstreamfiles(repo):

--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -754,7 +754,7 @@
     """
     # copy to prevent modification of the global list
     caps = list(wireprotocaps)
-    if streamclone.allowservergeneration(repo.ui):
+    if streamclone.allowservergeneration(repo):
         if repo.ui.configbool('server', 'preferuncompressed', False):
             caps.append('stream-preferred')
         requiredformats = repo.requirements & repo.supportedformats
@@ -946,7 +946,7 @@
     capability with a value representing the version and flags of the repo
     it is serving. Client checks to see if it understands the format.
     '''
-    if not streamclone.allowservergeneration(repo.ui):
+    if not streamclone.allowservergeneration(repo):
         return '1\n'
 
     def getstream(it):

--- a/tests/test-clone-uncompressed.t
+++ b/tests/test-clone-uncompressed.t
@@ -49,6 +49,77 @@
   bundle2-input-bundle: 1 parts total
   checking for updated bookmarks
 
+Cannot stream clone when there are secret changesets
+
+  $ hg -R server phase --force --secret -r tip
+  $ hg clone --uncompressed -U http://localhost:$HGPORT secret-denied
+  warning: stream clone requested but server has them disabled
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+
+  $ killdaemons.py
+
+Streaming of secrets can be overridden by server config
+
+  $ cd server
+  $ hg --config server.uncompressedallowsecret=true serve -p $HGPORT -d --pid-file=hg.pid
+  $ cat hg.pid > $DAEMON_PIDS
+  $ cd ..
+
+  $ hg clone --uncompressed -U http://localhost:$HGPORT secret-allowed
+  streaming all changes
+  1027 files to transfer, 96.3 KB of data
+  transferred 96.3 KB in * seconds (*/sec) (glob)
+  searching for changes
+  no changes found
+
+  $ killdaemons.py
+
+Verify interaction between preferuncompressed and secret presence
+
+  $ cd server
+  $ hg --config server.preferuncompressed=true serve -p $HGPORT -d --pid-file=hg.pid
+  $ cat hg.pid > $DAEMON_PIDS
+  $ cd ..
+
+  $ hg clone -U http://localhost:$HGPORT preferuncompressed-secret
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+
+  $ killdaemons.py
+
+Clone not allowed when full bundles disabled and can't serve secrets
+
+  $ cd server
+  $ hg --config server.disablefullbundle=true serve -p $HGPORT -d --pid-file=hg.pid
+  $ cat hg.pid > $DAEMON_PIDS
+  $ cd ..
+
+  $ hg clone --uncompressed http://localhost:$HGPORT secret-full-disabled
+  warning: stream clone requested but server has them disabled
+  requesting all changes
+  remote: abort: server has pull-based clones disabled
+  abort: pull failed on remote
+  (remove --pull if specified or upgrade Mercurial)
+  [255]
+
+Local stream clone with secrets involved
+(This is just a test over behavior: if you have access to the repo's files,
+there is no security so it isn't important to prevent a clone here.)
+
+  $ hg clone -U --uncompressed server local-secret
+  warning: stream clone requested but server has them disabled
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
 
 Stream clone while repo is changing: