XXE lab: Parse an uploaded invoice without resolving external entities

lxml only reads external entities when the parser is told to. Build the XMLParser with resolve_entities=False so a declared entity is left as an unresolved reference, not fetched.

Add no_network=True and load_dtd=False. A text blocklist for "file://" misses a bare absolute path and other schemes; configure the parser instead of inspecting the string.

from lxml import etree


def parse_invoice(xml):
    parser = etree.XMLParser(resolve_entities=False, no_network=True, load_dtd=False)
    root = etree.fromstring(xml.encode("utf-8"), parser)
    return {
        "supplier": root.findtext("supplier") or "",
        "total": root.findtext("total") or "",
    }

With resolve_entities=False, no_network=True and load_dtd=False, an ordinary invoice returns its supplier and total, and & and the numeric é still expand (lxml always resolves the predefined and character entities). A DOCTYPE entity pointing at file://…/secret.txt or at a bare absolute path is left unresolved, so the supplier comes back empty rather than carrying the file’s contents; a network entity such as the cloud-metadata URL is never fetched. A harmless DOCTYPE with no entity still parses. External general entities are only read when a parser opts in with resolve_entities=True (lxml’s default since 5.0 is resolve_entities='internal'), so the fix is to opt out explicitly and keep DTD loading and network access off.

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The browser and the lab download run the same cases, including cases that catch common wrong fixes, such as comparing with one fixture value instead of the real rule. The download includes the complete test adapter and cryptographic fixtures.