diff --git a/mercurial/narrowspec.py b/mercurial/narrowspec.py
--- a/mercurial/narrowspec.py
+++ b/mercurial/narrowspec.py
@@ -20,6 +20,15 @@
 
 FILENAME = 'narrowspec'
 
+# Pattern prefixes that are allowed in narrow patterns. This list MUST
+# only contain patterns that are fast and safe to evaluate. Keep in mind
+# that patterns are supplied by clients and executed on remote servers
+# as part of wire protocol commands.
+VALID_PREFIXES = (
+    b'path:',
+    b'rootfilesin:',
+)
+
 def parseserverpatterns(text):
     """Parses the narrowspec format that's returned by the server."""
     includepats = set()
@@ -82,8 +91,42 @@
     return '%s:%s' % normalizesplitpattern(kind, pat)
 
 def parsepatterns(pats):
-    """Parses a list of patterns into a typed pattern set."""
-    return set(normalizepattern(p) for p in pats)
+    """Parses an iterable of patterns into a typed pattern set.
+
+    Patterns are assumed to be ``path:`` if no prefix is present.
+    For safety and performance reasons, only some prefixes are allowed.
+    See ``validatepatterns()``.
+
+    This function should be used on patterns that come from the user to
+    normalize and validate them to the internal data structure used for
+    representing patterns.
+    """
+    res = {normalizepattern(orig) for orig in pats}
+    validatepatterns(res)
+    return res
+
+def validatepatterns(pats):
+    """Validate that patterns are in the expected data structure and format.
+
+    And that is a set of normalized patterns beginning with ``path:`` or
+    ``rootfilesin:``.
+
+    This function should be used to validate internal data structures
+    and patterns that are loaded from sources that use the internal,
+    prefixed pattern representation (but can't necessarily be fully trusted).
+    """
+    if not isinstance(pats, set):
+        raise error.ProgrammingError('narrow patterns should be a set; '
+                                     'got %r' % pats)
+
+    for pat in pats:
+        if not pat.startswith(VALID_PREFIXES):
+            # Use a Mercurial exception because this can happen due to user
+            # bugs (e.g. manually updating spec file).
+            raise error.Abort(_('invalid prefix on narrow pattern: %s') % pat,
+                              hint=_('narrow patterns must begin with one of '
+                                     'the following: %s') %
+                                   ', '.join(VALID_PREFIXES))
 
 def format(includes, excludes):
     output = '[include]\n'
diff --git a/tests/test-narrow-clone.t b/tests/test-narrow-clone.t
--- a/tests/test-narrow-clone.t
+++ b/tests/test-narrow-clone.t
@@ -16,6 +16,20 @@
   $ for x in `$TESTDIR/seq.py 20`; do echo $x > "t$x"; hg add "t$x"; hg commit -m "Commit test $x"; done
   $ cd ../../..
 
+Only path: and rootfilesin: pattern prefixes are allowed
+
+  $ hg clone --narrow ssh://user@dummy/master badnarrow --noupdate --include 'glob:**'
+  requesting all changes
+  abort: invalid prefix on narrow pattern: glob:**
+  (narrow patterns must begin with one of the following: path:, rootfilesin:)
+  [255]
+
+  $ hg clone --narrow ssh://user@dummy/master badnarrow --noupdate --exclude 'set:ignored'
+  requesting all changes
+  abort: invalid prefix on narrow pattern: set:ignored
+  (narrow patterns must begin with one of the following: path:, rootfilesin:)
+  [255]
+
 narrow clone a file, f10
 
   $ hg clone --narrow ssh://user@dummy/master narrow --noupdate --include "dir/src/f10"
@@ -254,3 +268,17 @@
   I path:dir/tests
   I path:file:dir/src/f12
   $ cd ..
+
+Narrow spec with invalid patterns is rejected
+
+  $ cat > narrowspecs <<EOF
+  > [include]
+  > glob:**
+  > EOF
+
+  $ hg clone ssh://user@dummy/master badspecfile --narrowspec narrowspecs
+  reading narrowspec from '$TESTTMP/narrowspecs'
+  requesting all changes
+  abort: invalid prefix on narrow pattern: glob:**
+  (narrow patterns must begin with one of the following: path:, rootfilesin:)
+  [255]
diff --git a/tests/test-narrow-patterns.t b/tests/test-narrow-patterns.t
--- a/tests/test-narrow-patterns.t
+++ b/tests/test-narrow-patterns.t
@@ -427,3 +427,15 @@
   |
   o  0 2a4f0c3b67da... root
   
+
+Illegal patterns are rejected
+
+  $ hg tracked --addinclude glob:**
+  abort: invalid prefix on narrow pattern: glob:**
+  (narrow patterns must begin with one of the following: path:, rootfilesin:)
+  [255]
+
+  $ hg tracked --addexclude set:ignored
+  abort: invalid prefix on narrow pattern: set:ignored
+  (narrow patterns must begin with one of the following: path:, rootfilesin:)
+  [255]