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Date: Mon, 30 Jul 2012 19:21:28 -0600
From: Kurt Seifried <kseifried@...hat.com>
To: oss-security@...ts.openwall.com
Subject: Re: CVE Request: Django 1.3.1 and 1.4.0 security issues
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On 07/30/2012 07:16 PM, Kurt Seifried wrote:
> https://www.djangoproject.com/weblog/2012/jul/30/security-releases-issued/
>
> Security releases issued
>
> Today the Django team is issuing multiple releases -- Django 1.3.2
> and Django 1.4.1 -- to remedy security issues reported to us.
>
> All users are encouraged to upgrade Django immediately.
>
> ========================================= Cross-site scripting in
> authentication views
>
> The login() and logout() views provided in Django's authentication
> framework make use of the common "POST-redirect-GET" pattern; a
> configurable querystring parameter can be used to specify the
> location to redirect to on successful submission. Currently, those
> views perform basic validation to ensure that the redirect location
> does not specify a different domain.
>
> However, this validation does not check the scheme of the target
> URL; armed with this knowledge, an attacker can craft, for example,
> a data: scheme URL which will execute JavaScript.
>
> Some browsers are known to currently provide protection against
> this issue: Google Chrome in particular explicitly disallows
> redirects to data: scheme URLs. However, several other major
> browsers do permit such redirects.
>
> After careful consideration of this issue, we have decided that
> the safest course of action involves a slight break to backwards
> compatibility. Although temporary mitigation could be achieved
> through more stringent validation in the relevant views, the root
> issue lies in Django's HTTP response classes, which currently do
> not perform any validation of redirect targets. The fact that some
> major browsers already disallow certain URL schemes in redirects
> indicates that the impact of this change is likely to be minimal.
>
> As such, the following change is being made despite breaking API
> compatibility:
>
> django.http.HttpResponseRedirect and
> django.http.HttpResponsePermanentRedirect now subclass a common
> base class, django.http.HttpResponseRedirectBase. That base class
> defines an explicit whitelist of allowed URL schemes. Attempts to
> instantiate a redirect with a URL of a scheme not in the whitelist
> will raise the exception
> django.core.exceptions.SuspiciousOperation, which is already
> employed for similar purposes in other parts of Django's codebase
> (e.g., to warn of possible session tampering).
>
> End-user code which issues redirects is unlikely to be affected
> unless it either explicitly requires redirecting to an unsupported
> scheme, or accepts the target URL from a user-supplied parameter.
>
> In the former case, subclassing the appropriate redirect class
> (HttpResponseRedirect for status code 302,
> HttpResponsePermanentRedirect for status code 301) and overriding
> the allowed_schemes list will be sufficient. The default value of
> allowed_schemes is ['http', 'https', 'ftp'].
>
> In the latter case, code which accepts user-supplied parameters
> can attempt to instantiate the redirect, catch the
> SuspiciousOperation exception, and fall back to an alternate
> location as needed.
>
> At present, Django's authentication views will leave this
> exception uncaught. This means site administrators will receive
> error reports if/when that exception is raised. It is likely that
> future Django releases will begin catching this exception, after
> allowing some time for users of Django to observe behavior and
> judge their exposure to potential issues.
Please use CVE-2012-3442 for this issue.
> ========================================= Denial-of-service in
> image validation
>
> Django's form system includes field types for handling file
> uploads, including a field class -- django.forms.ImageField -- for
> uploading images, which can perform some validation of image
> formats.
>
> Part of that validation involves detecting corrupted image files,
> using routines provided by the Python Imaging Library (PIL).
>
> The check as it currently exists in Django is vulnerable, however,
> because it will read the entire image file, including
> decompressing compressed formats as needed. It is trivially
> possible to craft a reasonably-sized file which, when decompressed
> in this fashion, grows to enormous size, consuming available memory
> and offering the ability to perform a denial-of-service attack.
>
> To mitigate this, image validation will now make use of PIL's
> Image.verify() method, which performs some validation checks but
> does not decompress or read the entire image file.
Please use CVE-2012-3443 for this issue.
> ========================================= Denial-of-service via
> get_image_dimensions()
>
> Django's image-handling facilities also include helper methods to
> determine the dimensions of an image. Currently, the process for
> this involves reading a 1024-byte chunk from the start of the file,
> and passing to PIL to determine the dimensions; if insufficient
> data is provided, further 1024-byte chunks are read until PIL is
> able to return a definite answer.
>
> While this works well for image formats which store enough
> information in their headers to determine dimensions, it can result
> in large quantities of read/process cycles for formats which do
> not. In particular, larger TIFF images can require tens of
> thousands of such cycles, tying up or timing out worker
> processes/threads and consuming enough server resources to result
> in an effective denial-of-service.
>
> To mitigate this, the algorithm for determining image dimensions
> is being changed; the initial attempt will still use a 1024-byte
> chunk, but the chunk size will be doubled on each successive read.
> Testing has demonstrated that this reduces time to process TIFF
> files by multiple orders of magnitude.
Please use CVE-2012-3444 for this issue.
- --
Kurt Seifried Red Hat Security Response Team (SRT)
PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993
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