HTML Comment Box, an embeddable third-party comments widget, contained a stored cross-site scripting (XSS) flaw that let an attacker’s comment run JavaScript on pages using the widget. Karim Rahal reproduced a filter bypass and reported it through Detectify Crowdsource; he said the developer fixed the issue within a couple of hours. Search results suggested wide use, but they do not establish how many sites were vulnerable or compromised.
What was the HTML Comment Box vulnerability?
HTML Comment Box let site owners add a comments section to their pages. The security issue was stored XSS: attacker-controlled comment content could be saved and later rendered by the widget in a way that executed as JavaScript in visitors’ browsers. That meant a successful comment could affect people who subsequently loaded a page displaying it, rather than only the person who submitted it.
In a January 18, 2017 Detectify Labs write-up, Karim Rahal described reproducing the bypass after Ibram Marzouk found a similar stored-XSS pattern in PasteCoin comments. Rahal connected that pattern to HTML Comment Box. The report documents that the payload executed; it does not document confirmed data theft, account takeovers, or named site compromises.
How did the filter bypass work?
The widget attempted to filter comment input, but Rahal found that its handling of tag delimiters and attributes could be circumvented. In his account, doubled greater-than and less-than characters evaded the tag filter; a semicolon closed an attribute, and double slashes commented out following JavaScript in the relevant context.
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The reduced example published by Rahal was ">><<img src=x onerror=alert(1);//>>". The image’s error handler is the part that runs JavaScript when the image fails to load. This illustrates the filter bypass; it should not be treated as a general-purpose payload for every site, because execution depends on how a particular page and widget render the input.
How many websites used the vulnerable widget?
Contemporaneous reports offered search-result counts as indicators of apparent adoption, not audited totals. Rahal said a Google search query for sites using the third-party comments section returned about 2,000,000 results. SecurityWeek reported more than 760,000 results from its own search and noted that many results were duplicates.
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| Report | Search-result estimate | What it establishes |
|---|---|---|
| Karim Rahal, Detectify Labs, January 2017 | About 2,000,000 Google results | A search estimate for sites appearing to use the widget; not a confirmed vulnerable-site or victim count. |
| SecurityWeek, January 2017 | More than 760,000 Google results | A separate search estimate; SecurityWeek noted duplicate results. |
Search-result counts can include duplicates and pages that do not represent a distinct, active, vulnerable installation. Neither figure shows how many sites accepted a malicious comment or suffered an attack.
Was the flaw fixed?
According to Rahal’s Detectify Labs account, the issue was disclosed through Detectify Crowdsource and the developer fixed it within a couple of hours after being contacted. That is the reported response to the 2017 disclosure; the available incident reporting does not establish the widget’s present-day status or provide a current inventory of affected deployments.
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Was this the first XSS issue reported in HTML Comment Box?
No. SecurityWeek reported that Rafay Baloch and Deepankar Arora identified persistent and reflected XSS flaws in HTML Comment Box in 2013. Those earlier reports are distinct from Rahal’s 2017 filter-bypass account, but they show that cross-site scripting had been reported in the widget before.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What website owners can learn from the incident
Embedding a third-party comments tool gives its rendering code a role in pages visitors trust. Filtering input alone is not a reliable defense: applications need robust sanitization and context-appropriate output handling, and operators need a way to identify and respond to weaknesses in embedded dependencies. The 2017 reporting supports those lessons, but does not provide comparative product testing or evidence about the current security of other comment services.
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