{"id":176,"date":"2019-09-21T15:59:49","date_gmt":"2019-09-21T12:59:49","guid":{"rendered":"https:\/\/lyubomir-angelov.com\/?p=176"},"modified":"2019-09-21T15:59:50","modified_gmt":"2019-09-21T12:59:50","slug":"how-browsers-are-parsing-html-script-tags","status":"publish","type":"post","link":"https:\/\/lyubomir-angelov.com\/blog\/how-browsers-are-parsing-html-script-tags\/","title":{"rendered":"How browsers are parsing HTML script tags"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n

The project that caused this article is React and headless architecture, a lot of API calls are returning JSONs.
We had a problem, sometimes we had white page because of a lot of JS errors.<\/p>\n

Do you know how browsers are parsing the HTML and especially the script<\/strong> tags?<\/p>\n

It is sequentially, line by line, character by character.
Look at the below HTML, what do you think will happen if you run this code?<\/p>\n

<\/p>\n

    <html>\n        <head>\n        <\/head>\n        <body>\n            <script>\n                let myJsonBackendResponse = {"source": "<\/script><script>alert(1)<\/script>"};\n            <\/script>\n        <\/body>\n    <\/html>\n<\/code><\/pre>\n
If you answered that it won’t<\/strong> run the alert()<\/strong> you are wrong.
Browsers will read the above closing script<\/strong> tag in the JSON, close the first open tag, then opens new, execute the alert()<\/strong> and then stop the reading till you close it \ud83d\ude42<\/p>\n
In order to fix it we have 2 options, remove the closing tag from the response OR split it however you want and it won’t be parsed as closing tag<\/p>\n
    <html>\n        <head>\n        <\/head>\n        <body>\n            <script>\n                let myJsonBackendResponse = {"source": "<\/" + "script><script>alert(1)<\/" + "script>"};\n            <\/script>\n        <\/body>\n    <\/html>\n<\/code><\/pre>\n
Our case was that one of the APIs is returning closing script tag (probably you should not have this) and breaks everything.<\/p>\n
Hope it is helpful for someone!<\/p>","protected":false},"excerpt":{"rendered":"
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