EWS Managed API报错‘DTD在XML文档中被禁止’的原因及解决方法

在搜索Allitems文件夹时，遇到了**"DTD在XML文档中被禁止"**的错误。该代码此前正常运行多年，更换服务器后又正常运行了数月，当前突然触发此错误。

使用的示例代码如下：

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);

string exchangeURL = "ExchangeURL";

Uri myUri = new Uri(exchangeURL);
service.TraceEnabled = true;
service.Url = myUri;
service.UseDefaultCredentials = true;
service.Credentials = new WebCredentials("email", "password");
//service.AutodiscoverUrl(empMail.PrimaryEmail);
service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, empMail.EmployeeEmail);

//Certificate validation handling
ServicePointManager.ServerCertificateValidationCallback += (se, cert, chain, sslerror) =>
{
    return true;
};


EmailMessage message;
PropertySet propertySet;

Mailbox mailBox = new Mailbox("Email");
ItemView itemView = new ItemView(1);
itemView.Traversal = ItemTraversal.Shallow;

//Try to get 'Allitems' folder so that we could search items in all folders
FolderId fid = new FolderId(WellKnownFolderName.Root);
FolderView fv = new FolderView(1);
fv.Traversal = FolderTraversal.Shallow;

ExtendedPropertyDefinition epd = new ExtendedPropertyDefinition(13825, MapiPropertyType.Integer);
SearchFilter sf = new SearchFilter.IsEqualTo(epd, "2");
SearchFilter sf2 = new SearchFilter.IsEqualTo(FolderSchema.DisplayName, "allitems");
SearchFilter.SearchFilterCollection sfc = new SearchFilter.SearchFilterCollection(LogicalOperator.And);
sfc.Add(sf);
sfc.Add(sf2);
FindFoldersResults findAllItemsFolderresult = service.FindFolders(fid, sfc, fv);

• 服务器安全策略更新：Exchange服务器近期可能调整了XML解析配置，禁用DTD解析以防范注入类安全风险，导致客户端收到含DTD的响应时触发错误。
• EWS响应格式变化：服务器返回的XML响应中新增了DTD声明，但当前使用的EWS Managed API默认不允许解析DTD。
• 版本兼容性偏差：代码中指定的Exchange2013_SP1版本与服务器实际版本不匹配，引发响应解析兼容性问题。

1. 允许EWS API解析DTD #

在初始化ExchangeService前，修改XmlReaderSettings开启DTD解析：

// 先配置XML解析设置
System.Xml.XmlReaderSettings xmlSettings = new System.Xml.XmlReaderSettings();
xmlSettings.DtdProcessing = System.Xml.DtdProcessing.Parse;

// 初始化ExchangeService
ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);

// 若上述方法不生效，通过反射修改API内部的XmlReader配置
var bindingFlags = System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic;
var internalSettings = service.GetType().GetField("_xmlReaderSettings", bindingFlags) as System.Xml.XmlReaderSettings;
if (internalSettings != null)
{
    internalSettings.DtdProcessing = System.Xml.DtdProcessing.Parse;
}

2. 匹配Exchange服务器版本 #

确认当前Exchange服务器的实际版本，将代码中的ExchangeVersion参数更新为对应版本，例如服务器为Exchange 2016时：

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2016);

3. 直接绑定AllItems文件夹 #

放弃通过搜索属性查找Allitems的方式，直接使用WellKnownFolderName.AllItems绑定文件夹，这是官方推荐的可靠方法：

// 替换原有的文件夹查找逻辑
Folder allItemsFolder = Folder.Bind(service, WellKnownFolderName.AllItems, new PropertySet(BasePropertySet.IdOnly));

4. 协调服务器端配置 #

联系Exchange管理员，确认是否近期修改了XML安全策略，可尝试调整服务器配置，使EWS响应不包含DTD声明，或允许特定客户端的DTD解析请求。

内容的提问来源于stack exchange，提问作者tushar