Correction to BID 29112 "Apache Server HTML Injection and UTF-7 XSSVulnerability"

[William A. Rowe, Jr.]

HTTP User and Desktop Security Communities;

With respect to http://www.securityfocus.com/bid/29112

Per http://www.ietf.org/rfc/rfc2616.txt

3.7.1 Canonicalization and Text Defaults
[...]
The "charset" parameter is used with some media types to define the
character set (section 3.4) of the data. When no explicit charset
parameter is provided by the sender, media subtypes of the "text"
type are defined to have a default charset value of "ISO-8859-1" when
received via HTTP. Data in character sets other than "ISO-8859-1" or
its subsets MUST be labeled with an appropriate charset value. See
section 3.4.1 for compatibility problems.

Internet Explorer's autodetection of UTF-7 clearly violates this
specification, introducing the opportunity for myriad similar attacks.
These are literally everywhere on the web today, we can trust the kids
to continue to explore this vector until it is fixed by Microsoft.

There are several workarounds in Apache HTTP Server to dodge this particular
vulnerability on your own sites, including

AddDefaultCharset ISO-8859-1

and by enabling multilanguage error docs (each translation with an explicit
charset) by simply uncommenting this Include of the default httpd.conf file;

# Multi-language error messages
Include conf/extra/httpd-multilang-errordoc.conf

All releases after Jan 2 include fixes across the board to add an explicit
charset iso-8859-1 to the built in Apache HTTP modules to compensate for
Microsoft's vulnerability, including released versions 2.2.8, 2.0.63, and
1.3.41. This does not affect third party modules you may be loading,
applications hosted-on or proxied-through HTTP Server, etc.

However this vulnerability should clearly be labeled as a flaw in Internet
Explorer. If the browsers under your supervision continue to enable the
autodetection of UTF-7, your users remain at risk. As all ISO, UTF-8 and
related charsets were 7-bit clean, it's clear that Microsoft err'ed on
the side of accepting UTF-7 charset for automatic detection, contrary to
to the behavior dictated by RFC 2616.

[William A. Rowe, Jr.]

William A. Rowe, Jr. wrote:
> With respect to http://www.securityfocus.com/bid/29112
>
> All releases after Jan 2 include fixes across the board to add an explicit
> charset iso-8859-1 to the built in Apache HTTP modules to compensate for
> Microsoft's vulnerability, including released versions 2.2.8, 2.0.63, and
> 1.3.41. This does not affect third party modules you may be loading,
> applications hosted-on or proxied-through HTTP Server, etc.

Having reviewed the vulnerability history again, those versions corrected
the unexploitable bug which echoed the supplied HTTP method (CVE-2007-6203).

In fact, the 1.3.34. 2.0.55 and all 2.2 releases resolved the Apache flaw
and tag all canned error responses with charset=iso-8859-1, as identified by
lament hero with bid 29112. So why would he be so clueless as to report
these later versions as affected? Simply, by observation.

My colleague Adam Qualset of Covalent has researched and confirmed that all
versions of IE are exploitable with UTF-7 in the presence of a Content-type
charset field. As originally cited...

> However this vulnerability should clearly be labeled as a flaw in Internet
> Explorer. If the browsers under your supervision continue to enable the
> autodetection of UTF-7, your users remain at risk. As all ISO, UTF-8 and
> related charsets were 7-bit clean, it's clear that Microsoft err'ed on
> the side of accepting UTF-7 charset for automatic detection, contrary to
> to the behavior dictated by RFC 2616.

So this statement stands, and exploits even via IIS itself should be trivial
to construct for the enthusiastic student.

I encourage the securityfocus team to update this vulnerability report
appropriately, and also please note that the cited homepage for References
should be http://httpd.apache.org/ with respect to any Apache HTTP Server
issues.

Many thanks,

Bill