Harden emoji SVG proxy responses (#2842)

routes/emoji_routes.py

@@ -16,7 +16,7 @@ from pathlib import Path
 
 import httpx
 from fastapi import APIRouter
-from fastapi.responses import FileResponse, Response
+from fastapi.responses import Response
 
 logger = logging.getLogger(__name__)
 
@@ -26,12 +26,42 @@ _CACHE_DIR = Path(__file__).resolve().parent.parent / "data" / "emoji_cache"
 _OPENMOJI_BASE = "https://cdn.jsdelivr.net/npm/openmoji@15.0.0/black/svg"
 # codepoints like "1f600" or "1f468-200d-1f469-200d-1f467" (lowercase hex, '-' joined)
 _CODE_RE = re.compile(r"^[0-9a-f]{2,6}(?:-[0-9a-f]{2,6})*$")
-_SVG_HEADERS = {"Cache-Control": "public, max-age=31536000, immutable"}
+_MAX_SVG_BYTES = 256 * 1024
+_BLOCKED_SVG_RE = re.compile(
+    br"<\s*(?:script|foreignObject|iframe|object|embed|image)\b|"
+    br"\bon[a-z0-9_-]+\s*=",
+    re.IGNORECASE,
+)
+_EXTERNAL_REF_RE = re.compile(
+    br"\b(?:href|xlink:href)\s*=\s*['\"](?:https?:|//|data:|javascript:)",
+    re.IGNORECASE,
+)
+_SVG_SECURITY_HEADERS = {
+    "X-Content-Type-Options": "nosniff",
+    "Content-Security-Policy": "sandbox",
+    "Cross-Origin-Resource-Policy": "same-origin",
+}
+_SVG_HEADERS = {
+    "Cache-Control": "public, max-age=31536000, immutable",
+    **_SVG_SECURITY_HEADERS,
+}
 # Returned when a codepoint is unknown/unreachable: an empty (transparent) SVG,
 # so the CSS mask renders nothing instead of a solid box. Not cached, so a later
 # request can still pick up the real glyph once the CDN is reachable.
 _BLANK_SVG = b'<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1 1"></svg>'
-_BLANK_HEADERS = {"Cache-Control": "no-store"}
+_BLANK_HEADERS = {"Cache-Control": "no-store", **_SVG_SECURITY_HEADERS}
+
+
+def _is_safe_svg(content: bytes) -> bool:
+    if not isinstance(content, bytes) or not content:
+        return False
+    if len(content) > _MAX_SVG_BYTES:
+        return False
+    if b"<svg" not in content[:256].lower():
+        return False
+    if _BLOCKED_SVG_RE.search(content) or _EXTERNAL_REF_RE.search(content):
+        return False
+    return True
 
 
 def setup_emoji_routes() -> APIRouter:
@@ -49,14 +79,21 @@ def setup_emoji_routes() -> APIRouter:
         _CACHE_DIR.mkdir(parents=True, exist_ok=True)
         fp = _CACHE_DIR / f"{code}.svg"
         if fp.exists():
-            return FileResponse(fp, media_type="image/svg+xml", headers=_SVG_HEADERS)
+            try:
+                content = fp.read_bytes()
+                if _is_safe_svg(content):
+                    return Response(content, media_type="image/svg+xml", headers=_SVG_HEADERS)
+                fp.unlink(missing_ok=True)
+            except Exception as e:
+                logger.warning("emoji cache read %s failed: %s", code, e)
+            return _blank()
 
         # First time we've seen this emoji — fetch the OpenMoji black SVG + cache
         # it. OpenMoji filenames are the codepoints uppercased.
         try:
             async with httpx.AsyncClient(timeout=8.0) as client:
                 r = await client.get(f"{_OPENMOJI_BASE}/{code.upper()}.svg")
-            if r.status_code == 200 and b"<svg" in r.content[:256]:
+            if r.status_code == 200 and _is_safe_svg(r.content):
                 try:
                     fp.write_bytes(r.content)
                 except Exception: