[1.5.x] Fixed #23157 -- Removed O(n) algorithm when uploading duplicate file names.

This is a security fix. Disclosure following shortly.

docs/howto/custom-file-storage.txt

@@ -83,5 +83,14 @@ the provided filename into account. The ``name`` argument passed to this method
 will have already cleaned to a filename valid for the storage system, according
 to the ``get_valid_name()`` method described above.
 
-The code provided on ``Storage`` simply appends ``"_1"``, ``"_2"``, etc. to the
-filename until it finds one that's available in the destination directory.
+.. versionchanged:: 1.5.9
+
+    If a file with ``name`` already exists, an underscore plus a random 7
+    character alphanumeric string is appended to the filename before the
+    extension.
+
+    Previously, an underscore followed by a number (e.g. ``"_1"``, ``"_2"``,
+    etc.) was appended to the filename until an avaible name in the destination
+    directory was found. A malicious user could exploit this deterministic
+    algorithm to create a denial-of-service attack. This change was also made
+    in Django 1.4.14.

docs/ref/files/storage.txt

@@ -81,6 +81,17 @@ The Storage Class
         available for new content to be written to on the target storage
         system.
 
+        .. versionchanged:: 1.5.9
+
+        If a file with ``name`` already exists, an underscore plus a random 7
+        character alphanumeric string is appended to the filename before the
+        extension.
+
+        Previously, an underscore followed by a number (e.g. ``"_1"``, ``"_2"``,
+        etc.) was appended to the filename until an avaible name in the
+        destination directory was found. A malicious user could exploit this
+        deterministic algorithm to create a denial-of-service attack. This
+        change was also made in Django 1.4.14.
 
     .. method:: get_valid_name(name)
 

docs/releases/1.4.14.txt

@@ -18,3 +18,23 @@ To remedy this, URL reversing now ensures that no URL starts with two slashes
 (//), replacing the second slash with its URL encoded counterpart (%2F). This
 approach ensures that semantics stay the same, while making the URL relative to
 the domain and not to the scheme.
+
+File upload denial-of-service
+=============================
+
+Before this release, Django's file upload handing in its default configuration
+may degrade to producing a huge number of ``os.stat()`` system calls when a
+duplicate filename is uploaded. Since ``stat()`` may invoke IO, this may produce
+a huge data-dependent slowdown that slowly worsens over time. The net result is
+that given enough time, a user with the ability to upload files can cause poor
+performance in the upload handler, eventually causing it to become very slow
+simply by uploading 0-byte files. At this point, even a slow network connection
+and few HTTP requests would be all that is necessary to make a site unavailable.
+
+We've remedied the issue by changing the algorithm for generating file names
+if a file with the uploaded name already exists.
+:meth:`Storage.get_available_name()
+<django.core.files.storage.Storage.get_available_name>` now appends an
+underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
+rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
+``"_2"``, etc.).

docs/releases/1.5.9.txt

@@ -18,3 +18,23 @@ To remedy this, URL reversing now ensures that no URL starts with two slashes
 (//), replacing the second slash with its URL encoded counterpart (%2F). This
 approach ensures that semantics stay the same, while making the URL relative to
 the domain and not to the scheme.
+
+File upload denial-of-service
+=============================
+
+Before this release, Django's file upload handing in its default configuration
+may degrade to producing a huge number of ``os.stat()`` system calls when a
+duplicate filename is uploaded. Since ``stat()`` may invoke IO, this may produce
+a huge data-dependent slowdown that slowly worsens over time. The net result is
+that given enough time, a user with the ability to upload files can cause poor
+performance in the upload handler, eventually causing it to become very slow
+simply by uploading 0-byte files. At this point, even a slow network connection
+and few HTTP requests would be all that is necessary to make a site unavailable.
+
+We've remedied the issue by changing the algorithm for generating file names
+if a file with the uploaded name already exists.
+:meth:`Storage.get_available_name()
+<django.core.files.storage.Storage.get_available_name>` now appends an
+underscore plus a random 7 character alphanumeric string (e.g. ``"_x3a1gho"``),
+rather than iterating through an underscore followed by a number (e.g. ``"_1"``,
+``"_2"``, etc.).