Prefix Serialized JSON in ColdFusion
By Pete Freitag
When ColdFusion 8 added the ability to return data from remote functions formatted with JSON they also added some settings that allow you to put a prefix on the JSON string.
Why would I want to prefix my JSON?
The reason this setting exists is to prevent a hack called JSON hijacking. Services such as GMail, and twitter have suffered from JSON hijacking.
It works by embedding a script tag pointing to the JSON url on the attack site, eg hacker-site.com:
__defineSetter__ (works on firefox) method.
So this brings us back to our question Why would I want to prefix JSON?. When you prefix with
// it effectively makes the script evaluate as a comment, and these exploits won't work. Google takes a more nasty approach, they use
while(1); as their JSON prefix, this will put the victim's browser in an infinite loop.
How do I enable a JSON Prefix in ColdFusion?
ColdFusion 8, and 9 added a setting in the ColdFusion administrator called Prefix serialized JSON with: which allows to to enter a prefix (the default being
It can also be toggled on in the
Application.cfc by adding the following inside the
<cfset this.secureJSON = true> <cfset this.secureJSONPrefix = "//">
And finally you can enable the prefix
cffunction call using the
Will this break my code?
It might, if you are only using this feature with ColdFusion's ajax tags then it will automatically remove the prefix for you. If you are calling remote methods with
The prefix will also be added when you call the
SerializeJSON function. There is currently no argument in SerializeJSON to toggle this behavior, I have filed an enhancement request for such as setting.
Update: Adobe has added the argument to SerializeJSON:
serializeJSON(var[, serializeQueryByColumns[, useSecureJSONPrefix[, useCustomSerializer]]]) in ColdFusion 11.
Checkout Phil Haack's blog for more info about these vulnerabilities.
Prefix Serialized JSON in ColdFusion was first published on October 20, 2009.
If you like reading about json, hijacking, coldfusion, security, cffunction, or cfcomponent then you might also like:
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