1 files changed, 8 insertions, 1 deletions
diff --git a/Lib/ssl.py b/Lib/ssl.py
index 36f3098312..8af22c7e4c 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -160,9 +160,16 @@ class CertificateError(ValueError):
     pass
 
 
-def _dnsname_to_pat(dn):
+def _dnsname_to_pat(dn, max_wildcards=1):
     pats = []
     for frag in dn.split(r'.'):
+        if frag.count('*') > max_wildcards:
+            # Issue #17980: avoid denials of service by refusing more
+            # than one wildcard per fragment.  A survery of established
+            # policy among SSL implementations showed it to be a
+            # reasonable choice.
+            raise CertificateError(
+                "too many wildcards in certificate DNS name: " + repr(dn))
         if frag == '*':
             # When '*' is a fragment by itself, it matches a non-empty dotless
             # fragment.