Intercept HTTP request body from chrome extension

While you may not be able to intercept, you can use standard AJAX approach to duct-tape it. Instead of making the href request see if you can make an asynchronous call and save it to an HTML object that isn't presented. Then scrape/read/parse/whatever your body criteria is, and if it passes, push that body object back up to the current window/page.

Storing the content in a suppressed element and then using that same element for content would allow you to avoid making duplicate calls. The downside is that you will get the full content for stuff you won't end up using. That may or may not be a bandwidth/speed performance issue.

Here is what I did

1. I used the requestBody to get the post requests body
2. I used a decoder the parse the body into a string

Here is an example

chrome.webRequest.onBeforeRequest.addListener(
    function(details) {
        if(details.method == "POST")
        // Use this to decode the body of your post
            var postedString = decodeURIComponent(String.fromCharCode.apply(null,
                                      new Uint8Array(details.requestBody.raw[0].bytes)));
           console.log(postedString)

    },
    {urls: ["<all_urls>"]},
    ["blocking", "requestBody"]
);

This functionality has been added to the API now, see the documentation.

In order to access the body you need to do the following:

chrome.webRequest.onBeforeRequest.addListener(
    function(details)
    {
        console.log(details.requestBody);
    },
    {urls: ["https://myurlhere.com/*"]},
    ['requestBody']
);