Reversing & Malware Analysis Training Part 6 - Practical Reversing (I)

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Acknowledgement
 Special thanks to null & Garage4Hackers community for their extended
support and cooperation.
 Thanks to all the Trainers who have devoted their precious time and
countless hours to make it happen.


Reversing & Malware Analysis Training

This presentation is part of our Reverse Engineering & Malware
Analysis Training program. Currently it is delivered only during our local
meet for FREE of cost.

For complete details of this course, visit our Security Training page.


harsimranwalia.info
 b44nz0r
 Research Scientist @ McAfee Labs
 Mechanical Engineer @IIT Delhi
 Independent Security Researcher
 RE, Exploit Analysis/Development, Malware Analysis

Twitter : b44nz0r
Email : walia.harsimran@gmail.com

Outline
 Break Point
 Debug Registers
 Flags
 API Help

Types of Breakpoints
 Software
 Hardware
 Memory

Breakpoint
 Software breakpoints are set by replacing the instruction
at the target address with 0xCC (INT3/ Breakpoint
interrupt)
 Hardware breakpoints are set via debug registers. Only 4
hardware breakpoints can be set
 Debug registers:
 8 debug registers present
 DR0 – DR3 : Address of breakpoint
 DR6 : Debug Status – To determine which breakpoint is active
 DR7 : Debug Control – Flags to control the breakpoints such as
break on read or on-write
 Debug registers are not accessible in Ring 3

Memory
 To access memory, need of permissions
 Lots of permissions
 PAGE_GUARD
 PAGE_READWRITE
 PAGE_EXECUTE
 PAGE_EXECUTE_READ
 To set memory breakpoint,
 the permissions of that memory region is set to
PAGE_GUARD
 whenever an access is made to that memory
STATUS_GUARD_PAGE_VIOLATION exception is raised
 On getting the exception the debugger changes the
permission back to the original
 Notifies the user of the breakpoint

Flags (Eflags Register)
 1 register – 32 bits
 Each bit signifies a flag
 Few important ones are:

Bit # Abbreviation Description

0 CF Carry flag

2 PF Parity flag

4 AF Adjust flag

6 ZF Zero flag

7 SF Sign flag

8 TF Trap flag (single step)

9 IF Interrupt enable flag

11 OF Overflow flag

Flags Demystified
 Carry flag is used to indicate when an arithmetic carry or borrow has
been generated out of the most significant ALU bit position
 Parity flag indicates if the number of set bits is odd or even in the
binary representation of the result of the last operation
 Adjust flag is used to indicate when an arithmetic carry or borrow has
been generated out of the 4 least significant bits
 Zero Flag is used to check the result of an arithmetic operation,
including bitwise logical instructions. It is set if an arithmetic result is
zero, and reset otherwise
 Sign flag is used to indicate whether the result of last mathematic
operation resulted in a value whose most significant bit was set
 A trap flag permits operation of a processor in single-step mode
 Overflow flag is used to indicate when an arithmetic overflow has
occurred in an operation, indicating that the signed two's-
complement result would not fit in the number of bits used for the
operation

Basic Reversing Techniques
 Check for readable strings
 Import table (IAT) for imported Windows
API
 Setting breakpoint on interesting API
 Single stepping

Variables
 Found under Names tab
○ L - library function
○ F - regular function
○ C - instruction
○ A - ascii string
○ D - data
○ I - imported name


Contd..

 Global variables are generally dword_<address>
 dword_402000 – as shown in image

 Local variables are of the form var_<offset>
 var_6C – as shown in image


Loop in IDA
 Red Line
 If condition is false
 (zero flag = 0)
 Green Line
 If condition is true
 (zero flag = 1)


#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main()
{
char a[10],b[10],c[10],d[10];
int i,j,k,l,r,s;
printf("#Crackmenn");
printf("enter username: ");
scanf("%s",a);
printf("enter password: ");
scanf("%s",b);
k = strlen(a);

Crackme Code l = strlen(b);
if (k <5 || k >=10){
printf("nInvalid! Username Lengthn");
printf("nHit Enter to Exitn");
getchar();
} else {
if (l != k){
printf("nInvalid! Password Lengthn");
getchar();
} else {
i = k-1;
j = 0;
while (i >= 0){
c[j] = a[i]+i;
i--;
j++;
}
c[j] = 0;
r = strlen(c);
if (r == l){
i = strcmp(c,b);
if (i == 0){
printf("nCongratulations! You did it..n");
} else {
printf("nAccess Denied! Wrong Passwordn");

References
 Complete Reference Guide for Reversing &
Malware Analysis Training

Thank You !


Reversing & Malware Analysis Training Part 6 - Practical Reversing (I)

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