Prevent access to external files/URLs via XML entity references.

xml_parse() would attempt to fetch external files or URLs as needed to
resolve DTD and entity references in an XML value, thus allowing
unprivileged database users to attempt to fetch data with the privileges
of the database server.  While the external data wouldn't get returned
directly to the user, portions of it could be exposed in error messages
if the data didn't parse as valid XML; and in any case the mere ability
to check existence of a file might be useful to an attacker.

The ideal solution to this would still allow fetching of references that
are listed in the host system's XML catalogs, so that documents can be
validated according to installed DTDs.  However, doing that with the
available libxml2 APIs appears complex and error-prone, so we're not going
to risk it in a security patch that necessarily hasn't gotten wide review.
So this patch merely shuts off all access, causing any external fetch to
silently expand to an empty string.  A future patch may improve this.

In HEAD and 9.2, also suppress warnings about undefined entities, which
would otherwise occur as a result of not loading referenced DTDs.  Previous
branches don't show such warnings anyway, due to different error handling
arrangements.

Credit to Noah Misch for first reporting the problem, and for much work
towards a solution, though this simplistic approach was not his preference.
Also thanks to Daniel Veillard for consultation.

Security: CVE-2012-3489

diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
index 0c0d3554c3285ea0e5bca3af17ef2e6cd12e0cd9..d8b892b61637b85fa15680ca02a34e777ac1ae3c 100644 (file)
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -48,6 +48,7 @@
 #ifdef USE_LIBXML
 #include 
 #include 
+#include 
 #include 
 #include 
 #include 
@@ -86,6 +87,8 @@ int           xmloption;
 
 static StringInfo xml_err_buf = NULL;
 
+static xmlParserInputPtr xmlPgEntityLoader(const char *URL, const char *ID,
+                 xmlParserCtxtPtr ctxt);
 static void xml_errorHandler(void *ctxt, const char *msg,...);
 static void xml_ereport_by_code(int level, int sqlcode,
                    const char *msg, int errcode);
@@ -886,6 +889,9 @@ pg_xml_init(void)
        /* Now that xml_err_buf exists, safe to call xml_errorHandler */
        xmlSetGenericErrorFunc(NULL, xml_errorHandler);
 
+       /* set up our entity loader, too */
+       xmlSetExternalEntityLoader(xmlPgEntityLoader);
+
 #ifdef USE_LIBXMLCONTEXT
        /* Set up memory allocation our way, too */
        xml_memory_init();
@@ -910,6 +916,9 @@ pg_xml_init(void)
         * about, anyway.
         */
        xmlSetGenericErrorFunc(NULL, xml_errorHandler);
+
+       /* set up our entity loader, too */
+       xmlSetExternalEntityLoader(xmlPgEntityLoader);
    }
 }
 
@@ -1322,6 +1331,25 @@ xml_pstrdup(const char *string)
 #endif   /* USE_LIBXMLCONTEXT */
 
 
+/*
+ * xmlPgEntityLoader --- entity loader callback function
+ *
+ * Silently prevent any external entity URL from being loaded.  We don't want
+ * to throw an error, so instead make the entity appear to expand to an empty
+ * string.
+ *
+ * We would prefer to allow loading entities that exist in the system's
+ * global XML catalog; but the available libxml2 APIs make that a complex
+ * and fragile task.  For now, just shut down all external access.
+ */
+static xmlParserInputPtr
+xmlPgEntityLoader(const char *URL, const char *ID,
+                 xmlParserCtxtPtr ctxt)
+{
+   return xmlNewStringInputStream(ctxt, (const xmlChar *) "");
+}
+
+
 /*
  * xml_ereport --- report an XML-related error
  *

diff --git a/src/test/regress/expected/xml.out b/src/test/regress/expected/xml.out
index ecca5896a70c2afe483e936a3b6eb2eb334881b4..7db5f917bed1189cb3246b1c73c48e29a0d661db 100644 (file)
--- a/src/test/regress/expected/xml.out
+++ b/src/test/regress/expected/xml.out
@@ -502,3 +502,23 @@ SELECT xpath('//b', 'one two three etc');
  {two,etc}
 (1 row)
 
+-- External entity references should not leak filesystem information.
+SELECT XMLPARSE(DOCUMENT ']>&c;');
+                            xmlparse                             
+-----------------------------------------------------------------
+ ]>&c;
+(1 row)
+
+SELECT XMLPARSE(DOCUMENT ']>&c;');
+                               xmlparse                                
+-----------------------------------------------------------------------
+ ]>&c;
+(1 row)
+
+-- This might or might not load the requested DTD, but it mustn't throw error.
+SELECT XMLPARSE(DOCUMENT ' ');
+                                                                       xmlparse                                                                       
+------------------------------------------------------------------------------------------------------------------------------------------------------
+  
+(1 row)
+

diff --git a/src/test/regress/expected/xml_1.out b/src/test/regress/expected/xml_1.out
index d542b0689a9dbc386bfb8c57237d923224488383..d654056b08b2020ed4ff4b995cacd0a5eb5f986d 100644 (file)
--- a/src/test/regress/expected/xml_1.out
+++ b/src/test/regress/expected/xml_1.out
@@ -456,3 +456,17 @@ LINE 1: SELECT xpath('//b', 'one two three etc'...
                             ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+-- External entity references should not leak filesystem information.
+SELECT XMLPARSE(DOCUMENT ']>&c;');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT XMLPARSE(DOCUMENT ']>&c;');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+-- This might or might not load the requested DTD, but it mustn't throw error.
+SELECT XMLPARSE(DOCUMENT ' ');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.

diff --git a/src/test/regress/sql/xml.sql b/src/test/regress/sql/xml.sql
index 086eedd27004edde15ab540280b3469dde846692..1e5026b04c8f6c74476c89f0af6b46d1ac3a1181 100644 (file)
--- a/src/test/regress/sql/xml.sql
+++ b/src/test/regress/sql/xml.sql
@@ -163,3 +163,9 @@ SELECT xpath('', '');
 SELECT xpath('//text()', 'number one');
 SELECT xpath('//loc:piece/@id', 'number one', ARRAY[ARRAY['loc', 'http://127.0.0.1']]);
 SELECT xpath('//b', 'one two three etc');
+
+-- External entity references should not leak filesystem information.
+SELECT XMLPARSE(DOCUMENT ']>&c;');
+SELECT XMLPARSE(DOCUMENT ']>&c;');
+-- This might or might not load the requested DTD, but it mustn't throw error.
+SELECT XMLPARSE(DOCUMENT ' ');