2 files changed, 13 insertions, 7 deletions
diff --git a/Changelog b/Changelog
index 1fd6762..5252fd2 100644
--- a/Changelog
+++ b/Changelog
@@ -28,6 +28,8 @@ lacme (0.3) upstream;
     vulnerability.
   - lacme(1), lacme-accountd(1): fix version number shown with
     --version.
+  - client: remove potential race when creating ACME challenge response
+    files.
 
  -- Guilhem Moulin <guilhem@guilhem.org>  Sun, 19 Feb 2017 13:08:41 +0100
 
diff --git a/client b/client
index cd94ed8..333ae3b 100755
--- a/client
+++ b/client
@@ -44,10 +44,13 @@ use warnings;
 
 my $PROTOCOL_VERSION = 1;
 
-use LWP::UserAgent ();
+use Errno 'EEXIST';
+use Fcntl qw/O_CREAT O_EXCL O_WRONLY/;
+use Digest::SHA qw/sha256 sha256_hex/;
 use MIME::Base64 qw/encode_base64 encode_base64url/;
+
+use LWP::UserAgent ();
 use JSON ();
-use Digest::SHA qw/sha256 sha256_hex/;
 
 use Config::Tiny ();
 
@@ -266,18 +269,19 @@ elsif ($COMMAND eq 'new-cert') {
                                @{request_json_decode($r)->{challenges} // []};
         die "Missing 'http-01' challenge in server response" unless defined $challenge;
         die "Invalid challenge token ".($challenge->{token} // '')."\n"
+            # ensure we don't write outside the cwd
             unless ($challenge->{token} // '') =~ /\A[A-Za-z0-9_\-]+\z/;
 
         my $keyAuthorization = $challenge->{token}.'.'.$JWK_thumbprint;
 
         # serve $keyAuthorization at http://$domain/.well-known/acme-challenge/$challenge->{token}
-        if (-e $challenge->{token}) {
-            print STDERR "WARNING: File exists: $challenge->{token}\n";
-        }
-        else {
-            open my $fh, '>', $challenge->{token} or die "Can't open $challenge->{token}: $!";
+        if (sysopen(my $fh, $challenge->{token}, O_CREAT|O_EXCL|O_WRONLY, 0644)) {
             $fh->print($keyAuthorization);
             $fh->close() or die "Can't close: $!";
+        } elsif ($! == EEXIST) {
+            print STDERR "WARNING: File exists: $challenge->{token}\n";
+        } else {
+            die "Can't open $challenge->{token}: $!";
         }
 
         $r = acme($challenge->{uri}, {