Injecting tags into HTML

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EricLaw

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Mar 31, 2009, 7:37:07 PM3/31/09

to Fiddler

Someone asked how to inject HTML tags into the HEAD section of a
document using a Fiddler extension. Here's a code sample:

private bool insertIntoHTMLHead(Session oSession, string
sToInsert)
{
if ((null == oSession.responseBodyBytes) || (null ==
oSession.oResponse) || (null == oSession.oResponse.headers)){
return false;
}

// If HTML
if (oSession.oResponse.headers.ExistsAndContains("Content-
Type", "html"))
{
// Remove any content-encoding so we can properly scan
the response
oSession.utilDecodeResponse();
Encoding oEnc = Utilities.getResponseBodyEncoding
(oSession);
string sBody = Utilities.GetStringFromArrayRemovingBOM
(oSession.responseBodyBytes, oEnc);
int iStart = 0;
int iHeadPtr = 0;
do
{
iHeadPtr = sBody.IndexOf("<head", iStart, Math.Min
(16384, sBody.Length-iStart), StringComparison.OrdinalIgnoreCase); //
Find <HEAD within first 16k
if (0 > iHeadPtr) return false;

if ((sBody.Length > (iHeadPtr + 5) &&
(sBody[iHeadPtr + 5] != '>') &&
((int)(sBody[iHeadPtr + 5]) > 0x21) && (int)
(sBody[iHeadPtr + 5]) < 0x7a))
{
// Someone tried to trick us with a "<headNOT"
or similar tag.
iStart = iHeadPtr + 5;
iHeadPtr = -1;
}
} while (0 > iHeadPtr);

iStart = iHeadPtr + 1;
int iInsertPtr = sBody.IndexOf(">", iStart, Math.Min
(2048, sBody.Length - iStart),
StringComparison.OrdinalIgnoreCase); // Find > within next 2k
if (0 > iHeadPtr) { return false; }

oSession.responseBodyBytes = oEnc.GetBytes(sBody.Insert
(iInsertPtr + 1, sToInsert));
oSession.oResponse.headers["Content-Length"] =
oSession.responseBodyBytes.LongLength.ToString();
return true;
}
return false;
}

Call it like so:

insertIntoHTMLHead(oSession, "\r\n\r\n");

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