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Port a Brakeman update to content_tag for Rails 6 #1552

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Dec 9, 2024
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Port a Brakeman update to content_tag for Rails 6 #1552

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12 changes: 2 additions & 10 deletions cheatsheets/Ruby_on_Rails_Cheat_Sheet.md
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Original file line number Diff line number Diff line change
Expand Up @@ -66,19 +66,11 @@ By default, protection against XSS comes as the default behavior. When string da

# Wrong! Do not do this!
<%= @product.name.html_safe %>

# Wrong! Do not do this!
<%= content_tag @product.name %>
```

Unfortunately, any field that uses `raw`, `html_safe`, `content_tag` or similar like this will be a potential XSS target. Note that there are also widespread misunderstandings about `html_safe()`.

[This writeup](https://stackoverflow.com/questions/4251284/raw-vs-html-safe-vs-h-to-unescape-html) describes the underlying SafeBuffer mechanism in detail. Other tags that change the way strings are prepared for output can introduce similar issues, including content_tag.
Unfortunately, any field that uses `raw`, `html_safe` or similar like this will be a potential XSS target. Note that there are also widespread misunderstandings about `html_safe()`.

``` ruby
content_tag("/><script>alert('hack!');</script>") # XSS example
# produces: </><script>alert('hack!');</script>><//><script>alert('hack!');</script>>
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@Greg-Myers-SB Greg-Myers-SB Nov 29, 2024 •
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On Rails 8.0

$ rails c
class MyClass
  include ActionView::Helpers::TagHelper
end
=> MyClass
foo = MyClass.new
=> #<MyClass:0x000000012ec9caa0>
foo.content_tag("/><script>alert('hack!');</script>")
=> "<___script_alert__hack______script_></___script_alert__hack______script_>"

```
[This writeup](https://stackoverflow.com/questions/4251284/raw-vs-html-safe-vs-h-to-unescape-html) describes the underlying SafeBuffer mechanism in detail. Other tags that change the way strings are prepared for output can introduce similar issues.

The method `html_safe` of String is somewhat confusingly named. It means that we know for sure the content of the string is safe to include in HTML without escaping. **This method itself is un-safe!**

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