The TemporaryFile() function has a security leak -- because the

filenames generated are easily predictable, it is possible to trick an
unsuspecting program into overwriting another file by creating a
symbolic link with the predicted name.  Fix this by using the
low-level os.open() function with the O_EXCL flag and mode 0700.  On
non-Unix platforms, presumably there are no symbolic links so the
problem doesn't exist.  The explicit test for Unix (posix, actually)
makes it possible to change the non-Unix logic to work without a
try-except clause.

The mktemp() file is as unsafe as ever.

1 files changed, 6 insertions, 5 deletions

diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 6a2730a..140eebc 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -126,11 +126,12 @@ class TemporaryFileWrapper:
 
 def TemporaryFile(mode='w+b', bufsize=-1, suffix=""):
     name = mktemp(suffix)
-    file = open(name, mode, bufsize)
-    try:
+    if os.name == 'posix':
+        # Unix -- be very careful
+        fd = os.open(name, os.O_RDWR|os.O_CREAT|os.O_EXCL, 0700)
         os.unlink(name)
-    except os.error:
+        return os.fdopen(fd, mode, bufsize)
+    else:
         # Non-unix -- can't unlink file that's still open, use wrapper
+        file = open(name, mode, bufsize)
         return TemporaryFileWrapper(file, name)
-    else:
-        return file