--- guix-1.5.0/guix/scripts/perform-download.scm
+++ guix-1.5.0/guix/scripts/perform-download.scm
@@ -97,16 +97,46 @@ a list of wrapper procedures for safely calling the list of procedures that
   (call-with-port (open file (logior O_NOFOLLOW O_RDONLY))
     proc))
 
+(define %allowed-local-download-directories
+  ;; Allow only the explicitly prepared offline distfiles directory.  Other
+  ;; local file URLs remain forbidden.
+  '("/external/distfiles"))
+
+(define (path-within-directory? path directory)
+  (or (string=? path directory)
+      (string-prefix? (string-append directory "/") path)))
+
+(define (allowed-local-file? path)
+  (let ((canon-path (false-if-exception (canonicalize-path path))))
+    (and canon-path
+         (let loop ((directories %allowed-local-download-directories))
+           (match directories
+             (() #f)
+             ((directory rest ...)
+              (let ((canon-directory
+                     (false-if-exception (canonicalize-path directory))))
+                (if (and canon-directory
+                         (path-within-directory? canon-path canon-directory))
+                    #t
+                    (loop rest)))))))))
+
+(define (file-uri-allowed? url)
+  (let ((uri (string->uri url)))
+    (and uri
+         (eq? (uri-scheme uri) 'file)
+         (allowed-local-file? (uri-path uri)))))
+
 (define (assert-non-local-urls url)
   "Exit if URL (or any element of URL if it is a list) is either not a valid
 URL or is a URL for a local file."
   (for-each (lambda (url)
-              (let ((scheme (and=> (string->uri url) uri-scheme)))
-                (unless scheme
+              (let ((uri (string->uri url)))
+                (unless uri
                   (leave (G_ "URL ~S can't be decoded~%") url))
-                (when (eq? scheme 'file)
+                (when (and (eq? (uri-scheme uri) 'file)
+                           (not (file-uri-allowed? url)))
                   (leave (G_ "URL ~S is for a local file~%") url))))
             (if (list? url)
                 url
                 (list url))))