this fixes https://github.com/gorhill/uMatrix/issues/129

src/js/traffic.js

@@ -380,12 +380,7 @@ var onHeadersReceived = function(details) {
 
     µb.updateBadgeAsync(tabId);
 
-    details.responseHeaders.push({
+    return { 'responseHeaders': foilInlineScripts(details.responseHeaders) };
-        'name': 'Content-Security-Policy',
-        'value': "script-src 'unsafe-eval' *"
-    });
-
-    return { 'responseHeaders': details.responseHeaders };
 };
 
 /******************************************************************************/
@@ -431,12 +426,110 @@ var onRootFrameHeadersReceived = function(details) {
 
     µb.updateBadgeAsync(tabId);
 
-    details.responseHeaders.push({
+    return { 'responseHeaders': foilInlineScripts(details.responseHeaders) };
-        'name': 'Content-Security-Policy',
+};
-        'value': "script-src 'unsafe-eval' *"
-    });
 
-    return { 'responseHeaders': details.responseHeaders };
+/******************************************************************************/
+
+var foilInlineScripts = function(headers) {
+    // Below is copy-pasta from uMatrix's project.
+
+    // If javascript is not allowed, say so through a `Content-Security-Policy`
+    // directive.
+    // We block only inline-script tags, all the external javascript will be
+    // blocked by our request handler.
+
+    // https://github.com/gorhill/uMatrix/issues/129
+    // https://github.com/gorhill/uMatrix/issues/320
+    //   Modernize CSP injection:
+    //   - Do not overwrite blindly possibly already present CSP header
+    //   - Add CSP directive to block inline script ONLY if needed
+    //   - If we end up modifying an existing CSP, strip out `report-uri`
+    //     to prevent spurious CSP violations.
+
+    // Is there a CSP header present?
+    // If not, inject a script-src CSP directive to prevent inline javascript
+    // from executing.
+    var i = headerIndexFromName('content-security-policy', headers);
+    if ( i === -1 ) {
+        headers.push({
+            'name': 'Content-Security-Policy',
+            'value': "script-src 'unsafe-eval' *"
+        });
+        return headers;
+    }
+
+    // A CSP header is already present.
+    // Remove the CSP header, we will re-inject it after processing it.
+    // TODO: We are currently forced to add the CSP header at the end of the
+    //       headers array, because this is what the platform specific code
+    //       expect (Firefox).
+    var csp = headers.splice(i, 1)[0].value.trim();
+
+    // Is there a script-src directive in the CSP header?
+    // If not, we simply need to append our script-src directive.
+    // https://github.com/gorhill/uMatrix/issues/320
+    //   Since we are modifying an existing CSP header, we need to strip out
+    //   'report-uri' if it is present, to prevent spurious reporting of CSP
+    //   violation, and thus the leakage of information to the remote site.
+    var matches = reScriptsrc.exec(csp);
+    if ( matches === null ) {
+        csp += "; script-src 'unsafe-eval' *";
+        headers.push({
+            'name': 'Content-Security-Policy',
+            'value': csp.replace(reReporturi, '')
+        });
+        return headers;
+    }
+
+    // A `script-src' directive is already present. Extract it.
+    var scriptsrc = matches[0];
+
+    // Is there at least one 'unsafe-inline' or 'nonce-' token in the
+    // script-src?
+    // If not we have no further processing to perform: inline scripts are
+    // already forbidden by the site.
+    if ( reUnsafeinline.test(scriptsrc) === false ) {
+        headers.push({
+            'name': 'Content-Security-Policy',
+            'value': csp
+        });
+        return headers;
+    }
+
+    // There are tokens enabling inline script tags in the script-src
+    // directive, so we have to strip them out.
+    // Strip out whole script-src directive, remove the offending tokens
+    // from it, then append the resulting script-src directive to the original
+    // CSP header.
+    // https://github.com/gorhill/uMatrix/issues/320
+    //   Since we are modifying an existing CSP header, we need to strip out
+    //   'report-uri' if it is present, to prevent spurious reporting of CSP
+    //   violation, and thus the leakage of information to the remote site.
+    csp = csp.replace(reScriptsrc, '') + scriptsrc.replace(reUnsafeinline, '');
+    headers.push({
+        'name': 'Content-Security-Policy',
+        'value': csp.replace(reReporturi, '')
+    });
+    return headers;
+};
+
+var reReporturi = /report-uri[^;]*;?\s*/;
+var reScriptsrc = /script-src[^;]*;?\s*/;
+var reUnsafeinline = /'unsafe-inline'\s*|'nonce-[^']+'\s*/g;
+
+/******************************************************************************/
+
+// Caller must ensure headerName is normalized to lower case.
+
+var headerIndexFromName = function(headerName, headers) {
+    var i = headers.length;
+    while ( i-- ) {
+        if ( headers[i].name.toLowerCase() === headerName ) {
+            return i;
+        }
+    }
+    return -1;
 };
 
 /******************************************************************************/