diff --git a/changes/bug17404 b/changes/bug17404
new file mode 100644
index 000000000..d524f6662
--- /dev/null
+++ b/changes/bug17404
@@ -0,0 +1,6 @@
+  o Major bugfixes (security, correctness):
+    - Fix a programming error that could cause us to read 4 bytes before
+      the beginning of an openssl string. This could be used to provoke
+      a crash on systems with an unusual malloc implementation, or
+      systems with unsual hardening installed. Fixes bug 17404; bugfix
+      on 0.2.3.6-alpha.
diff --git a/src/common/tortls.c b/src/common/tortls.c
index 97a82bf6e..379f7347d 100644
--- a/src/common/tortls.c
+++ b/src/common/tortls.c
@@ -2739,6 +2739,10 @@ dn_indicates_v3_cert(X509_NAME *name)
   len = ASN1_STRING_to_UTF8(&s, str);
   if (len < 0)
     return 0;
+  if (len < 4) {
+    OPENSSL_free(s);
+    return 1;
+  }
   r = fast_memneq(s + len - 4, ".net", 4);
   OPENSSL_free(s);
   return r;