diff --git a/libxml.h b/libxml.h
index 4558b70..88e515f 100644
--- a/libxml.h
+++ b/libxml.h
@@ -9,6 +9,8 @@
 #ifndef __XML_LIBXML_H__
 #define __XML_LIBXML_H__
 
+#include <libxml/xmlstring.h>
+
 #ifndef NO_LARGEFILE_SOURCE
 #ifndef _LARGEFILE_SOURCE
 #define _LARGEFILE_SOURCE
@@ -93,6 +95,7 @@
 int __xmlRandom(void);
 #endif
 
+XMLPUBFUN xmlChar * XMLCALL xmlEscapeFormatString(xmlChar **msg);
 int xmlNop(void);
 
 #ifdef IN_LIBXML
diff --git a/relaxng.c b/relaxng.c
index 345f354..56a3344 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -2215,7 +2215,8 @@
         snprintf(msg, 1000, "Unknown error code %d\n", err);
     }
     msg[1000 - 1] = 0;
-    return (xmlStrdup((xmlChar *) msg));
+    xmlChar *result = xmlCharStrdup(msg);
+    return (xmlEscapeFormatString(&result));
 }
 
 /**
diff --git a/xmlschemas.c b/xmlschemas.c
index 76f2119..e1b3a4f 100644
--- a/xmlschemas.c
+++ b/xmlschemas.c
@@ -1769,7 +1769,7 @@
     }
     FREE_AND_NULL(str)
 
-    return (*buf);
+    return (xmlEscapeFormatString(buf));
 }
 
 /**
@@ -2249,6 +2249,13 @@
 	TODO
 	return (NULL);
     }
+
+    /*
+     * xmlSchemaFormatItemForReport() also returns an escaped format
+     * string, so do this before calling it below (in the future).
+     */
+    xmlEscapeFormatString(msg);
+
     /*
     * VAL TODO: The output of the given schema component is currently
     * disabled.
@@ -2476,11 +2483,13 @@
 	msg = xmlStrcat(msg, BAD_CAST " '");
 	if (type->builtInType != 0) {
 	    msg = xmlStrcat(msg, BAD_CAST "xs:");
-	    msg = xmlStrcat(msg, type->name);
-	} else
-	    msg = xmlStrcat(msg,
-		xmlSchemaFormatQName(&str,
-		    type->targetNamespace, type->name));
+	    str = xmlStrdup(type->name);
+	} else {
+	    const xmlChar *qName = xmlSchemaFormatQName(&str, type->targetNamespace, type->name);
+	    if (!str)
+		str = xmlStrdup(qName);
+	}
+	msg = xmlStrcat(msg, xmlEscapeFormatString(&str));
 	msg = xmlStrcat(msg, BAD_CAST "'");
 	FREE_AND_NULL(str);
     }
@@ -2617,7 +2626,7 @@
 		str = xmlStrcat(str, BAD_CAST ", ");
 	}
 	str = xmlStrcat(str, BAD_CAST " ).\n");
-	msg = xmlStrcat(msg, BAD_CAST str);
+	msg = xmlStrcat(msg, xmlEscapeFormatString(&str));
 	FREE_AND_NULL(str)
     } else
       msg = xmlStrcat(msg, BAD_CAST "\n");
@@ -3141,11 +3150,13 @@
 		msg = xmlStrcat(msg, BAD_CAST " '");
 		if (type->builtInType != 0) {
 		    msg = xmlStrcat(msg, BAD_CAST "xs:");
-		    msg = xmlStrcat(msg, type->name);
-		} else
-		    msg = xmlStrcat(msg,
-			xmlSchemaFormatQName(&str,
-			    type->targetNamespace, type->name));
+		    str = xmlStrdup(type->name);
+		} else {
+		    const xmlChar *qName = xmlSchemaFormatQName(&str, type->targetNamespace, type->name);
+		    if (!str)
+			str = xmlStrdup(qName);
+		}
+		msg = xmlStrcat(msg, xmlEscapeFormatString(&str));
 		msg = xmlStrcat(msg, BAD_CAST "'.");
 		FREE_AND_NULL(str);
 	    }
@@ -3158,7 +3169,9 @@
 	}
 	if (expected) {
 	    msg = xmlStrcat(msg, BAD_CAST " Expected is '");
-	    msg = xmlStrcat(msg, BAD_CAST expected);
+	    xmlChar *expectedEscaped = xmlCharStrdup(expected);
+	    msg = xmlStrcat(msg, xmlEscapeFormatString(&expectedEscaped));
+	    FREE_AND_NULL(expectedEscaped);
 	    msg = xmlStrcat(msg, BAD_CAST "'.\n");
 	} else
 	    msg = xmlStrcat(msg, BAD_CAST "\n");
diff --git a/xmlstring.c b/xmlstring.c
index 42e380f..cc85777 100644
--- a/xmlstring.c
+++ b/xmlstring.c
@@ -987,5 +987,60 @@
     return(xmlUTF8Strndup(utf, len));
 }
 
+/**
+ * xmlEscapeFormatString:
+ * @msg:  a pointer to the string in which to escape '%' characters.
+ * Must be a heap-allocated buffer created by libxml2 that may be
+ * returned, or that may be freed and replaced.
+ *
+ * Replaces the string pointed to by 'msg' with an escaped string.
+ * Returns the same string with all '%' characters escaped.
+ */
+xmlChar *
+xmlEscapeFormatString(xmlChar **msg)
+{
+    xmlChar *msgPtr = NULL;
+    xmlChar *result = NULL;
+    xmlChar *resultPtr = NULL;
+    size_t count = 0;
+    size_t msgLen = 0;
+    size_t resultLen = 0;
+
+    if (!msg || !*msg)
+        return(NULL);
+
+    for (msgPtr = *msg; *msgPtr != '\0'; ++msgPtr) {
+        ++msgLen;
+        if (*msgPtr == '%')
+            ++count;
+    }
+
+    if (count == 0)
+        return(*msg);
+
+    resultLen = msgLen + count + 1;
+    result = (xmlChar *) xmlMallocAtomic(resultLen * sizeof(xmlChar));
+    if (result == NULL) {
+        /* Clear *msg to prevent format string vulnerabilities in
+           out-of-memory situations. */
+        xmlFree(*msg);
+        *msg = NULL;
+        xmlErrMemory(NULL, NULL);
+        return(NULL);
+    }
+
+    for (msgPtr = *msg, resultPtr = result; *msgPtr != '\0'; ++msgPtr, ++resultPtr) {
+        *resultPtr = *msgPtr;
+        if (*msgPtr == '%')
+            *(++resultPtr) = '%';
+    }
+    result[resultLen - 1] = '\0';
+
+    xmlFree(*msg);
+    *msg = result;
+
+    return *msg;
+}
+
 #define bottom_xmlstring
 #include "elfgcchack.h"