Test: untrusted HTML runs no JS and fetches nothing

.github/workflows/docker.yml

@@ -80,6 +80,9 @@ jobs:
       - name: Render a page
         run: make smoke DOCKER_IMAGE=dockette/docshot:test
 
+      - name: Check HTML isolation
+        run: make test-isolation DOCKER_IMAGE=dockette/docshot:test
+
       # Informational: the base image (LibreOffice, Chromium) carries CVEs we cannot fix here;
       # the weekly rebuild picks up upstream fixes. Pinned by digest: scanners are a supply-chain target too.
       - name: Scan image (Trivy)

Makefile

@@ -5,7 +5,7 @@ DOCKER_IMAGE ?= dockette/docshot:latest
 VERSION := $(shell cat VERSION)
 
 .DEFAULT_GOAL := up
-.PHONY: test-formats build run up down restart logs ps validate test test-params test-all formats test-unit lint smoke version release
+.PHONY: test-formats build run up down restart logs ps validate test test-params test-all formats test-unit lint smoke test-isolation version release
 
 build:            ## Build the public image (docker buildx)
 	docker buildx build --build-arg VERSION=$(VERSION) -t ${DOCKER_IMAGE} .
@@ -21,7 +21,7 @@ test-unit:        ## Go vet + unit tests (in a golang container)
 
 lint:             ## gofmt, staticcheck, govulncheck, shellcheck, hadolint
 	$(GO_DOCKER) sh -c 'test -z "$$(gofmt -l .)" && go run honnef.co/go/tools/cmd/staticcheck@452d5bb86b45 -tags nodynamic ./... && go run golang.org/x/vuln/cmd/govulncheck@v1.8.0 -tags nodynamic ./...'
-	docker run --rm -v $(CURDIR):/mnt -w /mnt koalaman/shellcheck:stable -S warning docshot.sh test/formats/gen.sh
+	docker run --rm -v $(CURDIR):/mnt -w /mnt koalaman/shellcheck:stable -S warning docshot.sh test/formats/gen.sh test/isolation.sh
 	docker run --rm -i -v $(CURDIR)/.hadolint.yaml:/.config/hadolint.yaml:ro hadolint/hadolint < Dockerfile
 
 smoke:            ## Start the public image, wait for /readyz, render Markdown to PNG
@@ -30,6 +30,9 @@ smoke:            ## Start the public image, wait for /readyz, render Markdown t
 	ct=$$(printf '# docshot\n\nsmoke test\n' | curl -sS -m 120 -o /dev/null -w '%{http_code} %{content_type}' --data-binary @- 'http://localhost:39994/upload?name=smoke.md'); \
 	docker rm -f docshot-smoke >/dev/null; echo "smoke: $$ct"; [ "$$ct" = "200 image/png" ]
 
+test-isolation:   ## Untrusted HTML: no JavaScript, no remote fetches (starts ${DOCKER_IMAGE})
+	test/isolation.sh ${DOCKER_IMAGE}
+
 version:          ## Print VERSION
 	@echo $(VERSION)
 

test/isolation.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+# Untrusted HTML must neither run JavaScript nor fetch anything (CDN scripts/styles, public images, file://).
+# Each probe page is rendered next to an identical page without the probe: the PNGs must be byte-identical.
+# Usage: test/isolation.sh [image]   (default dockette/docshot:latest)
+IMAGE=${1:-dockette/docshot:latest}
+PORT=39995
+NAME=docshot-isolation
+DIR=$(mktemp -d)
+trap 'docker rm -f $NAME >/dev/null 2>&1; rm -rf "$DIR"' EXIT
+
+docker rm -f $NAME >/dev/null 2>&1
+docker run -d --name $NAME -p $PORT:3000 "$IMAGE" >/dev/null || exit 1
+i=0
+until curl -fsS -m 2 "http://localhost:$PORT/readyz" >/dev/null 2>&1; do
+	i=$((i + 1)); [ $i -gt 60 ] && { echo "isolation: docshot did not become ready"; exit 1; }
+	sleep 2
+done
+
+page() { printf '<!doctype html><html><head><style>body{background:#fff;font:20px sans-serif}</style>%s</head><body><h1>isolation</h1>%s</body></html>' "$1" "$2"; }
+render() { curl -sS -m 120 --data-binary @- "http://localhost:$PORT/upload?name=p.html" -o "$DIR/$1.png"; }
+
+fail=0
+probe() { # name, probe head, probe body, control head, control body
+	page "$2" "$3" | render "$1-probe"
+	page "$4" "$5" | render "$1-control"
+	if [ -s "$DIR/$1-control.png" ] && cmp -s "$DIR/$1-probe.png" "$DIR/$1-control.png"; then
+		echo "isolation: $1 ok"
+	else
+		echo "isolation: $1 FAILED (probe changed the rendered page)"; fail=1
+	fi
+}
+
+probe script '' '<script>document.body.style.background="#f00"</script>' '' ''
+probe cdn-script '<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>' '' '' ''
+probe cdn-style '<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap/5.3.3/css/bootstrap.min.css">' '' '' ''
+probe remote-image '' '<img src="https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png" width="272" height="92">' '' '<img src="data:," width="272" height="92">'
+probe file-iframe '' '<iframe src="file:///etc/passwd" width="400" height="100"></iframe>' '' '<iframe src="file:///nonexistent" width="400" height="100"></iframe>'
+
+exit $fail