ASP.NET __VIEWSTATE crypto validation prone to replay attacks May 03 2005 01:38PM
Michal Zalewski (lcamtuf gmail com)
Good morning,

ASP.NET's extremely popular __VIEWSTATE functionality provides an automatic,
uniform method for storing current state of all webpage "controls" (including
form fields, database views, etc), so that user-entered data automagically
persists and is populated across newly rendered HTML, and so that current
selections of displayed database records are cached and do not need to be
looked up again after every operation.

The data is by *typically* stored on client side as base64-encoded, hidden
POST form field. By default, the field is protected from tampering by being
"signed" using SHA1 with machine-specific key and - although not discussed by
Microsoft - presumably either target .aspx filename or other file ID parameter.

Because information from controls such as DataGrid is by default "cached" in
__VIEWSTATE, enabling the user to tamper with this data would allow for
modifying item ID and price data in ASP.NET online shops, for example. This
possibility is discussed by Microsoft and others, with the conclusion that
SHA1/MD5 signing prevents it, and hence it is safe to rely on __VIEWSTATE
cached data in that regard:


Storing and then trusting database fields retrieved from user, even with
SHA1 validation, may send shivers down any infosec guy's or gal's spine,
but that's the way it'd seem to be done:

"DataGrid stores its contents in the view state so the page developer
doesn't need to rebind the database data to the DataGrid on each and
every page load, but only on the first one. The benefit is that the
database doesn't need to be accessed as often. "

Lacking proper documentation of __VIEWSTATE validation, and based
on my limited testing, I see two major problems with how the mechanism
is designed:

1) Injection-through-replay scenarios are still available.

"Signed" __VIEWSTATE is replayable for the system or cluster that
generated it, for that specific .aspx script:

a) The attacker may submit __VIEWSTATE acquired from one view, to a
wholly different view with a reasonably compatible layout of cached

For example, if a provider of online shopping solutions hosts two
distinct shops on a shared scripting platform - one shop with pet
supplies, and one with expensive watches - and both (within their
distinct databases or tables) happen to have item ID 123 - one for
$19.99 (Acme brand cat food), another for $1999.00 (Rolex), the
attacker may move __VIEWSTATE from buystuff.aspx?shop=catfood to
buystuff.aspx?shop=watches, and place an order item ID 123 for

b) Since there appears to be no expiration mechanism, user may keep
__VIEWSTATE data and reuse it after availability, pricing, or other
parameters of items or settings has changed, or ability to access
certain information has been restricted, effectively obtaining
a "backdoor" of sorts.

c) The attacker may obtain __VIEWSTATE after entering dangerous or
offensive contents or view settings within his session (some of which
may be not immediately visible), then redirect third parties through
a webpage with a specific __VIEWSTATE parameter to expose them to that
contents, or to impose these settings upon that viewer.

2) When NOT signed, resource starvation due to __VIEWSTATE complexity is

The format of the base64-encoded data is often summarised as "compressed
XML"; __VIEWSTATE uses a simplistic, hierarchical markup scheme: first
letter decides record type (triplet, pair, one of primitive types such
as strings or integers); values for that record are enclosed in <...>
that immediately follows that letter; fields of a record are separated by
';'; records can be nested with no limits.

The problem: __VIEWSTATE is sometimes used with no integrity protection
for valid reasons. If only user input fields are going to be cached,
there is no immediate problem with effects of __VIEWSTATE tampering
(with the possible exception of 1c), and performance benefits of avoiding
SHA1 are noticable.

Unfortunately, because of its hierarchical structure, and parsing based
on recursive function calls, it is trivial to consume excessive amounts
of resources by constructing a very deeply nested structure. Consider
this unix shell example:

perl -e '{print "t<" x 8000, ";;>" x 8000}' | mimencode | awk '{printf "%s", $0}'

Inserting the output of that sequence in place of __VIEWSTATE form field
value in a page then submitted to a webserver that has SHA1 integrity
checking disabled will consume excessive resources on the target machine
(in some cases, omitting the closing ";;>" sequence offers even better
effort-gain DoS ratio).

Possible mitigation method for problem #1: include secure session ID within
__VIEWSTATE data, correlate with existing session control data, or validate
and expire appropriately; or sacrifice storage and keep this data on server.

Mitigation method for problem #2: sacrifice CPU cycles to always validate
data; or store on server.

Michal Zalewski
SotW plug: http://lcamtuf.coredump.cx/silence/

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