harden XML parsing against billion-laughs DoS in extract_csproj and extract_lpk

stdlib ET does not cap entity expansion — a crafted .csproj or .lpk with nested
internal entities can exhaust memory. Pre-screen input bytes for <!DOCTYPE and
<!ENTITY before parsing (legitimate MSBuild/Lazarus files never contain these).
Also adds the missing 2 MiB size cap to extract_lpk (csproj already had one).
No new dependencies required.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

graphify/extract.py

@@ -7389,6 +7389,30 @@ def extract_delphi_form(path: Path) -> dict:
     return {"nodes": nodes, "edges": edges, "input_tokens": 0, "output_tokens": 0}
 
 
+# Size cap for project XML files we parse with stdlib ElementTree.
+# Real .csproj/.fsproj/.vbproj/.lpk files are well under 2 MiB; anything
+# larger is either malformed or hostile.
+_PROJECT_XML_MAX_BYTES = 2 * 1024 * 1024
+
+
+def _project_xml_is_safe(src: bytes) -> bool:
+    """Reject XML that declares DTDs or entities.
+
+    Stdlib ``xml.etree.ElementTree`` does not cap entity expansion, so a
+    crafted project file could trigger a billion-laughs style DoS. External
+    entity resolution is already disabled by pyexpat defaults, but rejecting
+    ``<!DOCTYPE`` / ``<!ENTITY`` outright is defense in depth.
+
+    Legitimate MSBuild and Lazarus package files never contain a DOCTYPE
+    or ENTITY declaration, so this is a zero-false-positive screen.
+    """
+    # Only the prolog can hold a DTD/internal subset, but be conservative
+    # and scan the full byte range -- these formats use ASCII tags so a
+    # case-insensitive substring match is sufficient.
+    lowered = src.lower()
+    return b"<!doctype" not in lowered and b"<!entity" not in lowered
+
+
 def extract_lazarus_package(path: Path) -> dict:
     """Extract package metadata from Lazarus .lpk package files (XML format).
 
@@ -7408,8 +7432,18 @@ def extract_lazarus_package(path: Path) -> dict:
     """
     try:
         import xml.etree.ElementTree as ET
-        text = path.read_text(encoding="utf-8", errors="replace")
-        xml_root = ET.fromstring(text)
+        src = path.read_bytes()
+    except OSError as e:
+        return {"nodes": [], "edges": [], "error": str(e)}
+
+    if len(src) > _PROJECT_XML_MAX_BYTES:
+        return {"nodes": [], "edges": [], "error": "package file too large"}
+    if not _project_xml_is_safe(src):
+        return {"nodes": [], "edges": [],
+                "error": "refusing XML with DOCTYPE/ENTITY declaration"}
+
+    try:
+        xml_root = ET.fromstring(src)
     except Exception as e:
         return {"nodes": [], "edges": [], "error": str(e)}
 
@@ -7798,8 +7832,11 @@ def extract_csproj(path: Path) -> dict:
     except OSError:
         return {"nodes": [], "edges": [], "error": f"cannot read {path}"}
 
-    if len(src) > 2_097_152:
+    if len(src) > _PROJECT_XML_MAX_BYTES:
         return {"nodes": [], "edges": [], "error": "project file too large"}
+    if not _project_xml_is_safe(src):
+        return {"nodes": [], "edges": [],
+                "error": "refusing XML with DOCTYPE/ENTITY declaration"}
 
     try:
         tree = ET.fromstring(src)