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Relock - Virus for Windows by roy g biv

Virus for Windows

roy g biv
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Released in EOF#2 magazine

What is virtual code?

This is an idea that I had after I read about the Locreate in Uninformed #6 journal. There, the author describes about relocation data being used to alter the in-memory image. We have seen this before, where a virus uses relocation data to decrypt itself. The author talks about packer but it is really cryptor. There are some interesting things in the article, but it was not very special for me. So, my idea is to remove all code from a section, and use relocation data to restore it. Since the section is now only in virtual memory, I call it virtual code. It seems that IDA does not support multiple relocations being applied to the same location, so it cannot handle my files. :)

This technique was also almost ready for many months, but I work on too many projects at the same time and finish none of them. Now I have.

How does it work?

The idea is that the section is virtual, so we know that it is full of 00s. If we set our ImageBase value to 0, then we will be relocated to 0x10000. It means that if we use relocation types that work with the top 16 bits, then we can perform many +1 operations on our 00s to reach any value that we want. I also use relocation types other than the IMAGE_REL_BASED_HIGHLOW that we all know, to make fewer operations, and it might challenge some emulators. :) We can apply relocations in any order on each page, so we can even be polymorphic by ordering the relocations randomly.

Tricky relocations ;)

Some info about relocation data now:

Windows NT does not relocate EXEs, only DLLs.

Windows 2000/XP load to 0x10000 for invalid base, but requested base must be 64kb aligned, else error occurs.

Windows 9x/Me load to 0x400000 for invalid base, and requested base can be any value.

Windows NT supports types 0-5.

Windows 2000 supports types 0-B.

Windows XP supports types 0-A.

Windows 9x/Me supports types 0-4 only.

type    name                            function
0       IMAGE_REL_BASED_ABSOLUTE        ignored

1       IMAGE_REL_BASED_HIGH            adds top 16 bits of delta to word

2       IMAGE_REL_BASED_LOW             adds low 16 bits of delta to word
                                        but since Windows requires 64kb aligned images, so these bits are always 0

3       IMAGE_REL_BASED_HIGHLOW         adds all 32 bits of delta to dword

4       IMAGE_REL_BASED_HIGHADJ         occupies 2 slots
                                        - forms 28-bit big-endian address
                                        - adds delta, aligns to 64kb
                                        - stores top 16 bits
                                        result is always same as type 1 because 64kb alignment

5       IMAGE_REL_BASED_MIPS_JMPADDR    weird 32-bit encoding
                                        formula follows:


                                        useful for quickly reaching mutiple of 40 at offset mem+1
                                        maintains all 8 bits at mem
                                        maintains low 6 bits at mem+1
                                        maintains high 6 bits at mem+3

6       same as type 0

7       same as type 0

8       invalid                         error occurs if used

9       IMAGE_REL_BASED_MIPS_JMPADDR16  weird 64-bit encoding
                                        formula follows:

            mem=oword align ptr

                                        useful for quickly reaching mutiple of 20 at offset mem+d
                                        maintains all 8 bits at mem
                                        maintains all 8 bits at mem+1
                                        maintains all 8 bits at mem+2
                                        maintains all 8 bits at mem+3
                                        maintains all 8 bits at mem+4
                                        maintains low 6 bits at mem+5
                                        maintains high 1 bit at mem+a
                                        maintains all 8 bits at mem+b
                                        maintains low 4 bits at mem+c
                                        maintains bit 4 at mem+d
                                        maintains high 4 bits at mem+f

A       IMAGE_REL_BASED_DIR64           same as type 3 on Window 2000
                                        fully 64-bit on Windows XP

B       IMAGE_REL_BASED_HIGHADJ64(?)    undocumented
                                        occupies 3 slots
                                        - forms 44-bit big-endian address
                                        - adds delta
                                        - aligns to 64kb
                                        - stores top 16 bits

Other info

Windows processes import table before relocations (because imports are RVAs not VAs), so relocation data cannot be used to construct import table at runtime, only to alter it afterwards for obfuscation purposes.

Virtual code section must be writable else file will not load.

Relocation data can be used to alter file header if header is write-enabled by using < 4kb section alignment, but some fields must not be altered (eg "MZ", "PE", Machine, NumberOfSections, SizeOfOptionalHeader, Magic).

Changing AddressOfEntrypoint has no effect (value is cached before relocation data are processed), however relocation data can be used to set ImageBase to impossible values (non-64kb aligned) and AddressOfEntryPoint can be adjusted indirectly using that. For example, if you add three bytes to ImageBase, then it is like subtracting three bytes from AddressOfEntryPoint.

IMAGE_FILE_RELOCS_STRIPPED value in Characteristics field is ignored if image must be relocated.

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