diff --git a/mercurial/hgweb/hgweb_mod.py b/mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py
+++ b/mercurial/hgweb/hgweb_mod.py
@@ -312,7 +312,7 @@
 
         # This state is global across all threads.
         encoding.encoding = rctx.config('web', 'encoding')
-        rctx.repo.ui.environ = wsgireq.env
+        rctx.repo.ui.environ = req.rawenv
 
         if rctx.csp:
             # hgwebdir may have added CSP header. Since we generate our own,
diff --git a/mercurial/hgweb/request.py b/mercurial/hgweb/request.py
--- a/mercurial/hgweb/request.py
+++ b/mercurial/hgweb/request.py
@@ -149,6 +149,8 @@
     headers = attr.ib()
     # Request body input stream.
     bodyfh = attr.ib()
+    # WSGI environment dict, unmodified.
+    rawenv = attr.ib()
 
 def parserequestfromenv(env, bodyfh, reponame=None, altbaseurl=None):
     """Parse URL components from environment variables.
@@ -342,7 +344,8 @@
                          querystring=querystring,
                          qsparams=qsparams,
                          headers=headers,
-                         bodyfh=bodyfh)
+                         bodyfh=bodyfh,
+                         rawenv=env)
 
 class offsettrackingwriter(object):
     """A file object like object that is append only and tracks write count.