Tear or Dear
8 minutes to read
We have a Windows PE file called TearOrDear.exe:
$ file TearOrDear.exe
TearOrDear.exe: PE32 executable (GUI) Intel 80386 Mono/.Net assembly, for MS Windows
If we execute it on a Windows machine, we will see this window form:
C# .NET decompilation
Since it is compiled from C# .NET, we can use tools like JetBrains dotPeek, dnSpy or ILSpy to decompile the executable and get almost the original C# source code.
This time, the code is really ugly (I’ll leave it here, but it’s not necessary to understand everything):
using System;
using System.ComponentModel;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace TearORDear
{
public class LoginForm : Form
{
private int pp;
private int qq;
private int rr;
private int ss;
private int tt;
private int uu;
private int vv;
private int ww;
private int b;
private string o;
private string z;
private string zita;
private string m;
private short[] Src = new short[11]
{
(short) 11, (short) 110, (short) 31, (short) 224, (short) 53, (short) 64, (short) 57, (short) 68, (short) 779, (short) 220, (short) 78
};
private int pepper = 10;
private int pep;
private string aa;
private string oura = "p00f123#";
private string username;
private IContainer components;
private Label Label1;
private Label label2;
private TextBox textBox_user;
private TextBox textBox_pass;
private Button button_Login;
private Label label_Result;
public LoginForm() => this.InitializeComponent();
private void LoginForm_Load(object sender, EventArgs e)
{
}
private bool check2(string[] s1, string s2)
{
string[] s1_1 = new string[26]
{
"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", "s", "d", "f", "g", "h", "j", "k", "l", "z", "x", "c", "v", "b", "n", "m"
};
string s2_1 = s1_1[3] + s1_1[8] + s1_1[7] + s1_1[(int) Math.Sqrt(2.0)];
return this.check3(s1_1, s2_1);
}
private unsafe void kapa(object sender, EventArgs e)
{
int num1 = 0;
this.z = "";
this.o = "";
this.m = "";
int num2;
int* numPtr1 = &num2;
int num3;
int* numPtr2 = &num3;
int num4;
int* numPtr3 = &num4;
int num5;
int* numPtr4 = &num5;
int num6;
int* numPtr5 = &num6;
int num7;
int* numPtr6 = &num7;
int num8;
int* numPtr7 = &num8;
int* numPtr8 = &num1;
*numPtr1 = 79;
*numPtr2 = 128;
*numPtr3 = 128;
*numPtr4 = 105;
*numPtr5 = 112;
*numPtr6 = 112;
*numPtr7 = 129;
*numPtr8 = 130;
this.pp = num2;
this.linear(this.pp);
}
private bool check4(string[] s1, string s2)
{
string[] s1_1 = new string[26]
{
"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", "s", "d", "f", "g", "h", "j", "k", "l", "z", "x", "c", "v", "b", "n", "m"
};
string s2_1 = s1_1[3] + s1_1[8] + s1_1[7] + s1_1[(int) Math.Sqrt(2.0)];
return this.check(s1_1, s2_1);
}
private void linear(int j)
{
int j1 = j;
int num = j;
this.o = char.ToString((char) j);
j = this.pp + 75 + 105 + 108 + 108 + 36;
this.pp = j + num;
this.qq = (this.pp -= j);
this.expo(j1);
}
private bool check3(string[] s1, string s2)
{
string[] s1_1 = new string[26]
{
"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", "s", "d", "f", "g", "h", "j", "k", "l", "z", "x", "c", "v", "b", "n", "m"
};
string s2_1 = s1_1[3] + s1_1[8] + s1_1[7] + s1_1[(int) Math.Sqrt(2.0)];
return this.check4(s1_1, s2_1);
}
private void expo(int j)
{
short[] numArray = new short[12]
{
(short) 104, (short) 108, (short) 100, (short) 101, (short) 163, (short) 100, (short) 122, (short) 111, (short) 112, (short) 122, (short) 101, (short) 270
};
int num = j;
this.b = num - 74;
this.pp += j + num;
this.o = char.ToString((char) j);
j = this.pp + 75 + 105 + 108 + 108 + 36;
this.pp += j + num;
this.pp += j + num + this.via(1, this.qq);
this.qq = this.pp - j + (int) numArray[this.b - 1 + 10 - 9 + 2 - 1 - 1];
this.pp -= j;
this.qq = (int) numArray[this.b - 1 + 10 - 9 + 2 - 1 - 1];
this.o += char.ToString((char) this.qq);
this.via(2, this.qq);
this.via(-2, this.qq);
this.via(-1, this.qq);
this.via(0, this.qq);
}
this.via(2, this.qq);
this.via(-2, this.qq);
this.via(-1, this.qq);
this.via(0, this.qq);
}
private bool check(string[] s1, string s2)
{
string[] strArray = new string[26]
{
"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", "s", "d", "f", "g", "h", "j", "k", "l", "z", "x", "c", "q", "b", "n", "m"
};
string str = strArray[3] + strArray[8] + strArray[7] + strArray[(int) Math.Sqrt(2.0)];
return this.textBox_user.Text == this.aa && strArray[0] == strArray[22];
}
private int via(int type, int j)
{
short[] numArray = new short[10]
{
(short) 115, (short) 112, (short) 115, (short) 114, (short) 115, (short) 116, (short) 117, (short) 118, (short) 119, (short) 110
};
int num1 = j + (int) numArray[1] * (int) numArray[5] ^ 272;
int num2;
if (type == 0)
{
char.ToString((char) (j + 11));
char c = (char) ((int) numArray[2] - 101 + (int) numArray[9] - 9);
this.o += char.ToString(c);
this.o += char.ToString(c);
num2 = num1 + (Encoding.ASCII.GetBytes(this.o).Length + j + (int) numArray[1] * (int) numArray[5] ^ 272);
}
else if (type > 1)
{
char.ToString((char) (j + 11));
this.o += char.ToString((char) ((uint) this.Src[2] + 12U + (uint) this.Src[10]));
num2 = num1 + (Encoding.ASCII.GetBytes(this.o).Length + j + (int) numArray[1] * (int) numArray[5] ^ 272);
}
else
num2 = num1 - (Encoding.ASCII.GetBytes(this.o).Length + j + (int) numArray[1] * (int) numArray[5] ^ 272);
return num2;
}
private void button1_Click(object sender, EventArgs e)
{
this.label_Result.Text = "";
this.kapa(sender, e);
this.pep = 0;
this.aa = this.Multiply(this.encrypted1(this.textBox_user.Text).Substring(0, 5), -1);
this.aa = this.aa.Remove(this.aa.Length - 1);
string s = this.Multiply(this.oura, -9);
if (this.username == this.o && this.check1(s))
{
int num = (int) MessageBox.Show("Correct!");
}
else
this.label_Result.Text = "WRONG CREDENTIALS! Try Again...";
}
private string encrypted1(string s1)
{
string[] strArray = new string[26]
{
"1", "2", "4", "g", "h", "l", "o", "3", "g", "p", "p", "k", "d", "f", "s", "e", "w", "r", "t", "z", "u", "i", "i", "&", "$", "_"
};
if (this.pep == 0)
s1 = strArray[0] + strArray[4] + strArray[10] + strArray[22] + strArray[9];
if (++this.pep == this.pepper)
{
this.o = this.encrypted10(this.textBox_pass.Text);
return s1;
}
string str = strArray[3] + strArray[8] + strArray[8] + strArray[(int) Math.Sqrt(18.0)];
s1 += this.encrypted1(this.textBox_pass.Text);
return s1 == str ? str : s1;
}
private string encrypted10(string s1)
{
string[] strArray = new string[26]
{
"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
};
string str1 = strArray[3] + strArray[8] + strArray[17] + strArray[(int) Math.Sqrt(18.0)];
string str2 = this.encrypted11(this.textBox_pass.Text);
return s1 == str1 ? str1 : str2;
}
public string Multiply(string s, int n)
{
char[] charArray = s.ToCharArray();
Array.Reverse((Array) charArray);
this.username = this.textBox_pass.Text;
return new string(charArray);
}
private string encrypted2(string s1)
{
string[] strArray = new string[26]
{
"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
};
string str1 = strArray[3] + strArray[8] + strArray[17] + strArray[(int) Math.Sqrt(18.0)];
string str2 = this.encrypted10(s1);
return s1 == str1 ? str1 : str2;
}
private string encrypted11(string s1)
{
string[] strArray = new string[26]
{
"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", "s", "d", "f", "g", "h", "j", "k", "l", "z", "x", "c", "v", "b", "n", "m"
};
string str = strArray[3] + strArray[8] + strArray[7] + strArray[(int) Math.Sqrt(2.0)];
return s1 == str ? str : str + "!@#";
}
private bool check1(string s)
{
string[] s1 = new string[26]
{
"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", nameof (s),
"d", "f", "g", "h", "j", "k", "l", "z", "x", "c", "v", "b", "n", "m"
};
string s2 = s1[3] + s1[8] + s1[7] + s1[(int) Math.Sqrt(2.0)];
return this.check2(s1, s2);
}
protected override void Dispose(bool disposing)
{
if (disposing && this.components != null)
this.components.Dispose();
base.Dispose(disposing);
}
private void InitializeComponent()
{
this.Label1 = new Label();
this.label2 = new Label();
this.textBox_user = new TextBox();
this.textBox_pass = new TextBox();
this.button_Login = new Button();
this.label_Result = new Label();
this.SuspendLayout();
this.textBox_pass = new TextBox();
this.button_Login = new Button();
this.label_Result = new Label();
this.SuspendLayout();
this.Label1.AutoSize = true;
this.Label1.Location = new Point(32, 47);
this.Label1.Name = "Label1";
this.Label1.Size = new Size(60, 13);
this.Label1.TabIndex = 0;
this.Label1.Text = "User Name";
this.label2.AutoSize = true;
this.label2.Location = new Point(39, 87);
this.label2.Name = "label2";
this.label2.Size = new Size(53, 13);
this.label2.TabIndex = 1;
this.label2.Text = "Password";
this.textBox_user.Location = new Point(110, 44);
this.textBox_user.Name = "textBox_user";
this.textBox_user.Size = new Size(162, 20);
this.textBox_user.TabIndex = 1;
this.textBox_pass.Location = new Point(110, 84);
this.textBox_pass.Name = "textBox_pass";
this.textBox_pass.Size = new Size(162, 20);
this.textBox_pass.TabIndex = 2;
this.button_Login.Location = new Point(105, 139);
this.button_Login.Name = "button_Login";
this.button_Login.Size = new Size(75, 23);
this.button_Login.TabIndex = 4;
this.button_Login.Text = "Login";
this.button_Login.UseVisualStyleBackColor = true;
this.button_Login.Click += new EventHandler(this.button1_Click);
this.label_Result.AutoSize = true;
this.label_Result.ForeColor = Color.Red;
this.label_Result.Location = new Point(59, 173);
this.label_Result.Name = "label_Result";
this.label_Result.Size = new Size(0, 13);
this.label_Result.TabIndex = 5;
this.AcceptButton = (IButtonControl) this.button_Login;
this.AutoScaleDimensions = new SizeF(6f, 13f);
this.AutoScaleMode = AutoScaleMode.Font;
this.ClientSize = new Size(284, 205);
this.Controls.Add((Control) this.label_Result);
this.Controls.Add((Control) this.button_Login);
this.Controls.Add((Control) this.textBox_pass);
this.Controls.Add((Control) this.textBox_user);
this.Controls.Add((Control) this.label2);
this.Controls.Add((Control) this.Label1);
this.FormBorderStyle = FormBorderStyle.FixedToolWindow;
this.MaximizeBox = false;
this.MinimizeBox = false;
this.Name = nameof (LoginForm);
this.ShowIcon = false;
this.ShowInTaskbar = false;
this.StartPosition = FormStartPosition.CenterScreen;
this.Text = "Login";
this.Load += new EventHandler(this.LoginForm_Load);
this.ResumeLayout(false);
this.PerformLayout();
}
}
}
Dynamic analysis
Instead of trying to do static code analysis, let’s debug the program and use a dynamic analysis approach. Using dnSpy (for 32 bits), we can set a breakpoint at the if statement that tells whether the credentials are correct or not:
Once we run the program (using asdf and fdsa as username and password), we have this:
For some reason, the program overwrites username with the value of the password, but whatever. We are interested in this.o, because we need to have this.username == this.o:
As can be seen, we must use roiw!@# as password.
On the other hand, the check1 method calls other methods (that are useless) and finally calls check, which compares the content of the username textbox with this.aa:
Therefore, using credentials piph:roiw!@# we will have everything correct.
Flag
This time, the flag is just the previous credentials wrapped into flag format:
HTB{piph:roiw!@#}