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security: escape exception messages in default HTML error response #1385

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29790c2
🛡️ Sentinel: [HIGH] Fix XSS in default HTML error response
google-labs-jules[bot] Jun 11, 2026
ca08704
security: add spec for escaping HTML in default error response
renich Jun 17, 2026
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4 changes: 4 additions & 0 deletions .jules/sentinel.md
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@@ -0,0 +1,4 @@
## 2026-06-11 - [XSS in Default Error Handler]
**Vulnerability:** Reflected Cross-Site Scripting (XSS) in default HTML error response.
**Learning:** The default error controller generated raw HTML incorporating the unescaped Exception message. In Crystal, `Exception#message` can be `nil` (`String?`), so safely interpolating it into HTML requires `HTML.escape(@ex.message || "")` and explicitly requiring the `"html"` module.
**Prevention:** Always escape user-controllable or dynamic string data, including exception messages, before interpolating it into an HTML template or string.
13 changes: 13 additions & 0 deletions spec/amber/pipes/error_spec.cr
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Expand Up @@ -25,6 +25,19 @@ module Amber

response.status_code.should eq 500
end

it "escapes exception messages in the HTML response body" do
error = Error.new
error.next = ->(_context : HTTP::Server::Context) { raise "<script>alert('xss')</script>" }
request = HTTP::Request.new("GET", "/")
request.headers["Accept"] = "text/html"

response = create_request_and_return_io(error, request)

response.status_code.should eq 500
response.body.should contain("&lt;script&gt;alert(&#39;xss&#39;)&lt;/script&gt;")
response.body.should_not contain("<script>")
end
end
end
end
3 changes: 2 additions & 1 deletion src/amber/controller/error.cr
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@@ -1,5 +1,6 @@
require "./base"
require "../exceptions/page"
require "html"

module Amber::Controller
class Error < Base
Expand Down Expand Up @@ -48,7 +49,7 @@ module Amber::Controller
if Amber.env.development?
Amber::Exceptions::Page.new(context, @ex)
else
"<html><body><pre>#{@ex.message}</pre></body></html>"
"<html><body><pre>#{HTML.escape(@ex.message || "")}</pre></body></html>"
end
end
end
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