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The principle of least privilege states that every program and every user of the system should operate using the least set of privileges necessary to complete the job [Saltzer 1974, Saltzer 1975]. The Build Security In website [DHS 2006] provides additional definitions of this principle. Executing with minimal privileges mitigates against exploitation in case a vulnerability is discovered in the code.

Noncompliant Code Example

An application may spawn another process as part of its normal course of action. On Windows, the newly-spawned process automatically receives the same privileges as the parent process [MSDN]. By allowing the child process to run in the same security context as the parent process, the attack surface for the application is extended to the child process. Furthermore, this example allows the child process to inherit handles from the parent process by passing TRUE to the bInheritsHandles parameter.

#include <Windows.h>
 
void launch_notepad(void) {
  PROCESS_INFORMATION pi;
  STARTUPINFO si;
 
  ZeroMemory(&si, sizeof(si));
  si.cb = sizeof( si );
  if (CreateProcess(TEXT("C:\\Windows\\Notepad.exe"), NULL, NULL, NULL, TRUE,
                    0, NULL, NULL, &si, &pi )) {
    /* Process has been created; work with the process and wait for it to
       terminate. */
    WaitForSingleObject(pi.hProcess, INFINITE);
    CloseHandle(pi.hThread);
    CloseHandle(pi.hProcess);
  }
}

It is possible that the act of calling launch_notepad() will give the user an elevated Notepad application (from which the user could execute Explorer.exe), allowing the user access to all user's files, change system settings, and so on.

Compliant Solution

By using the Windows Integrity Mechanism [MSDN] when creating the process, you can assign an integrity level to the launched child process. Doing so allows you to execute the child process with a specific set of privileges instead of defaulting to the parent process's security level.

#include <Windows.h>
#include <sddl.h>
 
static void launch_notepad_as_user(HANDLE token) {
  PROCESS_INFORMATION pi;
  STARTUPINFO si;
 
  ZeroMemory(&si, sizeof(si));
  si.cb = sizeof( si );
  if (CreateProcessAsUser(token, TEXT("C:\\Windows\\Notepad.exe"), NULL, NULL,
                          NULL, FALSE, 0, NULL, NULL, &si, &pi )) {
    /* Process has been created; work with the process and wait for it to
       terminate. */
    WaitForSingleObject(pi.hProcess, INFINITE);
    CloseHandle(pi.hThread);
    CloseHandle(pi.hProcess);
  }
}
 
static BOOL adjust_token_integrity_level(HANDLE token, const char *sid) {
  /* Convert the string SID to a SID *, then adjust the token's
     privileges. */
  BOOL ret;
  PSID psd = NULL;
  if (ConvertStringSidToSidA(sid, &psd)) {
    TOKEN_MANDATORY_LABEL tml;
    
    ZeroMemory(&tml, sizeof(tml));
    tml.Label.Attributes = SE_GROUP_INTEGRITY;
    tml.Label.Sid = psd;
 
    ret = SetTokenInformation(token, TokenIntegrityLevel, &tml,
                              sizeof(tml) + GetLengthSid(psd));
    
    LocalFree(psd);
  }
  return ret;
}
 
void launch_notepad(void) {
  /* Low level; see table for integrity level string names */
  const char *requested_sid = "S-1-16-4096";
  HANDLE token_cur, token_dup;
  /* Get the current process' security token as a starting point, then modify
     a duplicate so that it runs with a fixed integrity level. */
  if (OpenProcessToken(GetCurrentProcess(), TOKEN_DUPLICATE |
                                            TOKEN_ADJUST_DEFAULT |
                                            TOKEN_QUERY |
                                            TOKEN_ASSIGN_PRIMARY,
                                            &token_cur)) {
    if (DuplicateTokenEx(token_cur, 0, NULL, SecurityImpersonation,
                         TokenPrimary, &token_dup)) {
      if (adjust_token_integrity_level(token_dup, requested_sid))
        launch_notepad_as_user(token_dup);
      CloseHandle(token_dup);
    }
    CloseHandle(token_cur);
  }
}

The compliant solution demonstrates how to launch notepad.exe using a low integrity level, regardless of what privilege level the parent process is running from. It also disallows handle inheritance by passing FALSE to the bInheritsHandles parameter, because notepad.exe does not require access to any of the process's handles.

Possible values for the integrity level SID strings are listed in the following table:

Integrity level SID	Name
S-1-16-4096	Mandatory Label\Low Mandatory Level
S-1-16-8192	Mandatory Label\Medium Mandatory Level
S-1-16-12288	Mandatory Label\High Mandatory Level
S-1-16-16384	Mandatory Label\System Mandatory Level

Risk Assessment

Failure to follow the principle of least privilege may allow exploits to execute with elevated privileges.

Recommendation	Severity	Likelihood	Remediation Cost	Priority	Level
WIN02-C	High	Likely	High	P9	L2

Automated Detection

Tool	Version	Checker	Description
CodeSonar		BADFUNC.CREATEPROCESS	Use of CreateProcess
PC-lint Plus		586	Fully supported

Related Guidelines

ISO/IEC TR 24772	Adherence to Least Privilege [XYN]
MITRE CWE	CWE-250, Execution with unnecessary privileges CWE-272, Least privilege violation

Bibliography

[Saltzer 1974]
[Saltzer 1975]
[DHS 2006]	Least Privilege