Code: xmlParsePEReference(xmlParserCtxtPtr ctxt)
{
    const xmlChar *name;
    xmlEntityPtr entity = NULL;
    xmlParserInputPtr input;

    if (RAW != '%')
        return;
    NEXT;
    name = xmlParseName(ctxt);
    if (name == NULL) {
	xmlFatalErrMsg(ctxt, XML_ERR_PEREF_NO_NAME, "PEReference: no name\n");
	return;
    }
    if (xmlParserDebugEntities)
	xmlGenericError(xmlGenericErrorContext,
		"PEReference: %s\n", name);
    if (RAW != ';') {
	xmlFatalErr(ctxt, XML_ERR_PEREF_SEMICOL_MISSING, NULL);
        return;
    }

    NEXT;

    /*
     * Increate the number of entity references parsed
     */
    ctxt->nbentities++;

    /*
     * Request the entity from SAX
     */
    if ((ctxt->sax != NULL) &&
	(ctxt->sax->getParameterEntity != NULL))
	entity = ctxt->sax->getParameterEntity(ctxt->userData, name);
    if (ctxt->instate == XML_PARSER_EOF)
	return;
    if (entity == NULL) {
	/*
	 * [ WFC: Entity Declared ]
	 * In a document without any DTD, a document with only an
	 * internal DTD subset which contains no parameter entity
	 * references, or a document with "standalone='yes'", ...
	 * ... The declaration of a parameter entity must precede
	 * any reference to it...
	 */
	if ((ctxt->standalone == 1) ||
	    ((ctxt->hasExternalSubset == 0) &&
	     (ctxt->hasPErefs == 0))) {
	    xmlFatalErrMsgStr(ctxt, XML_ERR_UNDECLARED_ENTITY,
			      "PEReference: %%%s; not found\n",
			      name);
	} else {
	    /*
	     * [ VC: Entity Declared ]
	     * In a document with an external subset or external
	     * parameter entities with "standalone='no'", ...
	     * ... The declaration of a parameter entity must
	     * precede any reference to it...
	     */
            if ((ctxt->validate) && (ctxt->vctxt.error != NULL)) {
                xmlValidityError(ctxt, XML_WAR_UNDECLARED_ENTITY,
                                 "PEReference: %%%s; not found\n",
                                 name, NULL);
            } else
                xmlWarningMsg(ctxt, XML_WAR_UNDECLARED_ENTITY,
                              "PEReference: %%%s; not found\n",
                              name, NULL);
            ctxt->valid = 0;
	}
	xmlParserEntityCheck(ctxt, 0, NULL, 0);
    } else {
	/*
	 * Internal checking in case the entity quest barfed
	 */
	if ((entity->etype != XML_INTERNAL_PARAMETER_ENTITY) &&
	    (entity->etype != XML_EXTERNAL_PARAMETER_ENTITY)) {
	    xmlWarningMsg(ctxt, XML_WAR_UNDECLARED_ENTITY,
		  "Internal: %%%s; is not a parameter entity\n",
			  name, NULL);
	} else {
            xmlChar start[4];
            xmlCharEncoding enc;

	    if ((entity->etype == XML_EXTERNAL_PARAMETER_ENTITY) &&
	        ((ctxt->options & XML_PARSE_NOENT) == 0) &&
		((ctxt->options & XML_PARSE_DTDVALID) == 0) &&
		((ctxt->options & XML_PARSE_DTDLOAD) == 0) &&
		((ctxt->options & XML_PARSE_DTDATTR) == 0) &&
		(ctxt->replaceEntities == 0) &&
		(ctxt->validate == 0))
 		return;
 
 	    input = xmlNewEntityInputStream(ctxt, entity);
	    if (xmlPushInput(ctxt, input) < 0)
 		return;
 
 	    if (entity->etype == XML_EXTERNAL_PARAMETER_ENTITY) {
                 /*
                 * Get the 4 first bytes and decode the charset
                 * if enc != XML_CHAR_ENCODING_NONE
                 * plug some encoding conversion routines.
                 * Note that, since we may have some non-UTF8
                 * encoding (like UTF16, bug 135229), the 'length'
                 * is not known, but we can calculate based upon
                 * the amount of data in the buffer.
                 */
                GROW
                if (ctxt->instate == XML_PARSER_EOF)
                    return;
                if ((ctxt->input->end - ctxt->input->cur)>=4) {
                    start[0] = RAW;
                    start[1] = NXT(1);
                    start[2] = NXT(2);
                    start[3] = NXT(3);
                    enc = xmlDetectCharEncoding(start, 4);
                    if (enc != XML_CHAR_ENCODING_NONE) {
                        xmlSwitchEncoding(ctxt, enc);
                    }
                }

                if ((CMP5(CUR_PTR, '<', '?', 'x', 'm', 'l')) &&
                    (IS_BLANK_CH(NXT(5)))) {
                    xmlParseTextDecl(ctxt);
                }
            }
	}
    }
    ctxt->hasPErefs = 1;
}

Vulnerability Type: 
CWE ID: CWE-835
Summary: parser.c in libxml2 before 2.9.5 does not prevent infinite recursion in parameter entities.
Commit Message: Detect infinite recursion in parameter entities

When expanding a parameter entity in a DTD, infinite recursion could
lead to an infinite loop or memory exhaustion.

Thanks to Wei Lei for the first of many reports.

Fixes bug 759579.