Вставить это правило
<rule name="Imported Rule 0" stopProcessing="true">
<match url="^(img|css|files|js)(.*)$" />
<action type="Rewrite" url="/app/webroot/{R:1}{R:2}" appendQueryString="false" />
</rule>
вместе с правилами сайта для cakephp.org:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="Imported Rule 0" stopProcessing="true">
<match url="^(img|css|files|js)(.*)$" />
<action type="Rewrite" url="/app/webroot/{R:1}{R:2}" appendQueryString="false" />
</rule>
<rule name="Imported Rule 1" stopProcessing="true">
<match url="^(.*)$" ignoreCase="false" />
<conditions logicalGrouping="MatchAll">
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" ignoreCase="false" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsFile" ignoreCase="false" negate="true" />
</conditions>
<action type="Rewrite" url="index.php?url={R:1}" appendQueryString="true" />
</rule>
<rule name="Imported Rule 3" stopProcessing="true">
<match url="^$" ignoreCase="false" />
<action type="Rewrite" url="/app/webroot/" />
</rule>
<rule name="Imported Rule 2" stopProcessing="true">
<match url="(.*)" ignoreCase="false" />
<action type="Rewrite" url="/app/webroot/{R:1}" />
</rule>
<rule name="Imported Rule 1-1" stopProcessing="true">
<match url="^$" ignoreCase="false" />
<action type="Rewrite" url="/" />
</rule>
<rule name="Imported Rule 2-1" stopProcessing="true">
<match url="(.*)" ignoreCase="false" />
<action type="Rewrite" url="/{R:1}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>