I had thought any template.HTML-typed value will be output directly. template.CSS works. But why string does not.

This may be a bug as I'd expect css function syntax to be allowed.

However, if you use template.CSS instead of template.HTML as the type of ColorRGB it works correctly.

I don't know this area, but if this is ultimately deemed not a bug, somebody should consider what documentation could be added to clarify this for future users.

It happens because rgb() looks like unsafe JavaScript, which template.HTML tries to prevent. See the documentation on the ZgotmlpZ error. The documentation for template.CSS notes that that type is there to mark the content as safe, and that the source should be trusted.

rgb() is actually a safe css function. More functions can be found: https://www.w3schools.com/cssref/css_functions.asp

(a better way may be find css standards)

I don't think encouraging people to use template.CSS is a good idea. It can only be a workaround. Using string will be safer in case some careless bug may introduce unsafe code.

@ianlancetaylor does this need to be fixed? I'll have a look at it if it needs one.

@jamdagni86 Frankly, I don't know.

CC @mikesamuel @stjj89

cssValueFilter is the function that is rejecting these. It's conservative and does not make any attempt to recognize safe functions.

@daviddengcn said

rgb() is actually a safe css function.

I agree that rgb, rgba, hsl, and hsla are safe functions. They're just not recognized as such.

The tests (code) give a sense of what is currently recognized.

@mikesamuel - one approach to fix this is to figure out if the value is a safe CSS function call. If it is, then we'll have to make sure the function call is of a valid syntax. Do we have a parser somewhere which I can use?

If you had other thoughts, please let me know.

@jamdagni86 the current implementation just ranges over the decoded bytes and detects potentially dangerous characters, so there is no parser to use.

Switching to a parser to allow certain functions would imply a complete rewrite of that logic, and would very likely introduce issues and potential vulnerabilities in code that is currently protected by that check (e.g. existing code with a globally defined and vulnerable rgb javascript function or something similar).

I don't think switching to an allowed-list can be easily done in a secure way, and would probably not be considered backward compatible.

Apparently we will get more functions:

• acos()
• asin()
• atan()
• atan2()
• calc()
• clamp()
• cos()
• hypot()
• max()
• min()
• pow()
• sin()
• sqrt()
• tan()

And this is not considering the already existing ones:

• attr() ← Dangerous
• calc()
• cubic-bezier()
• hsl()
• hsla()
• linear-gradient()
• radial-gradient()
• repeating-linear-gradient()
• repeating-radial-gradient()
• rgb()
• rgba()
• var() ← Dangerous

These are some options we could go for:

1. invest some time in finding a scalable way to support CSS functions;
2. document that in order to use those functions the CSS type should be used and that user-controlled data should not be passed to it, but this might become a footgun;
3. ATM the parameters accepted by these functions are colors, numbers and percentages, we could offer helper functions to escape/sanitize them before they get promoted to the CSS type without supporting it in the cssValueFilter.

Are there any opinions on how to approach this?

document that in order to use those functions the CSS type should be used and that user-controlled data should not be passed to it, but this might become a footgun;

Documentation for this already exists, actually. In the CSS type docs, the rgba function is one example of an acceptable CSS value.

CSS typed strings are already foot-cannons in the html/template design, so I don't think pointing users this way makes it any worse.

invest some time in finding a scalable way to support CSS functions;

I'm not sure how difficult CSS parsing is, but I wonder how added complexity (and potential for bugs) this will add the html/template.

Do you think it is reasonable to just add something to the doc for the CSS type?

In my opinion since most of the inputs are numbers this is very unlikely to cause vulnerabilities even if users have to disable escaping.

@empijei I think it's safe to assume that numeric inputs to functions are safe. I don't see any greater risk for info leakage due to numbers in functions than numbers directly in positioning rules.

For functions that take strings, we'd need to know whether the string will be assumed to be a URL.

An easy first cut would be to just

• Allow functions on a whitelist including those you listed above.
• Allow function arguments that are not quoted strings