File structures (malcat.struct)

malcat.struct: FileStructure: The malcat.struct object is a bindings.FileStructure instance that gives you access to all the structures identified by the File parsers.

Note that in addition to this documentation, you can find usage examples in the sample script which is loaded when you hit F8.

Table of Contents

Structures, fields and values
Accessing structures
Inspecting fields
Editing

Structures, fields and values 

In Malcat, structures are named objects (the names you see in the Structure/text view for instance) and are composed of a single field (aka the root field) which can be:

an atomic field : an integer, a string, a timestamp, etc
a record field : like a typical C structure, a sequence of named fields
an array field : like a typical C array, a sequence of identical (unnamed) fields
a bitfield : a sequence of Bit fields

A record or array can itself be composed of other sub-records or sub-arrays, there is no limit on the depth of fields. Most of the time, a structure’s level-0 field is a record or an array, but it can be an atomic field too in some cases. The root object containing all level-0 fields is the malcat.struct variable of type FileStructure.

Fields are python objects of type StructAccess, which have among other properties a value. Atomic fields have a pythonic value, e.g. a datetime.datetime will be used for the value of a Timestamp field, or a str for fields of type String. Records, Arrays and Bitfields have no value per se, accessing their value field returns back their StructAccess instance.

Accessing structures 

class bindings.FileStructure

This class contains all structures identified by the File parsers. Note that all addresses used in this class are effective addresses. See Addressing in Malcat for more details.

__iter__()

Iterate over the file’s identified structures

for s in malcat.struct:
    print("#{:x}: {}".format(malcat.map.a2p(s.address), s.name))

Return type: iterator over the list of structure (StructAccess)

__getitem__(interval)

Iterate over all the structures contained in the interval (effective address):

for s in malcat.struct[0x100:]:
    print("#{:x}: {}".format(malcat.map.a2p(s.address), s.name))

Parameters: interval (slice) – effective address interval
Return type: iterator over the list of structure (StructAccess)

__getitem__(name)

return the value of the root field for the first structure named name

ep_rva = malcat.struct['OptionalHeader']['AddressOfEntryPoint']
print(f"EntryPoint: 0x{malcat.map.imagebase + ep_rva:x}")

Parameters: name (str) – name of the structure
Return type: StructAccess for aggregate fields, the python type for atomic fields
Raises: KeyError if no structure can be found

__getattr__(name)

return the root field for the first structure named name. Example:

optional_header = malcat.struct.OptionalHeader

Parameters: name (str) – name of the structure
Return type: StructAccess
Raises: KeyError if no structure can be found

at(name)

return the root field for the first structure named name. Useful if name contains non-alphanumerical characters and __getattr__() can’t be used.

Parameters: name (str) – name of the structure
Return type: StructAccess
Raises: KeyError if no structure can be found

__contains__(name)

return True iff a structure named name exists

if not "#US" in malcat.struct:
    raise ValueError("No user strings!")

Parameters: name (str) – name of the structure
Return type: bool

find(ea)

return the structure’s root field which starts at or contains the effective address ea, or None if no one can be found.

Parameters: ea (int) – effective address for the query
Return type: StructAccess or None

find_forward(ea)

return the structure’s root field which starts at or contains the effective address ea or starts directly after ea, or None if no structure is defined beyond ea.

Parameters: ea (int) – effective address for the query
Return type: StructAccess or None

find_backward(ea)

return the structure’s root field which starts at or contains the effective address ea or the first one that start before ea, or None if no structure is defined before ea.

Parameters: ea (int) – effective address for the query
Return type: StructAccess or None

__len__()

return the number of identified structures

if len(malcat.struct) == 0:
    raise ValueError("No structure found!")

Return type: bool

Inspecting fields 

The FileStructure object has many methods which gives access to a structure’s root field. Fields are python objects of class StructAccess and have the following methods and attributes:

class bindings.StructAccess

This class represents a field, which can be the root field of a FileStructure, or a child of an aggregate* field, e.g. a record field or an array field.

Attributes

All types of fields have the following attributes and methods:

value: depends on the field type: the value of the field. For aggregate fields, this would return itself (aka a StructAccess instance) since aggregate fields have no value per se. For atomic fields, the returned type depends on the field type: int, str, datetime, etc.

name: str

the name of the field. Example:

print(malcat.struct["Directories"][0].name)
>> Directories[0]
print(malcat.struct["Directories"][0].StreamSize.name)
>> StreamSize

address: int: the effective address of the field.

offset: int: the physical address of the field. Fields can only be defined on file-backed memory, so they always have a valid physical address.

size: int: how many bytes does the field takes on disk

__len__()

return the field’s size

Return type: int

bytes: bytes: raw bytes of field on disk. Returned bytes have a size of size

has_enum()

some atomic fields have a fixed set of values. If so, has_enum will be true

Return type: bool

enum: str

the textual representation of the field’s value if the field has an enum defined (i.e. has_enum() is True). Example:

print(malcat.struct.PE.Machine.value)
>> 332
print(malcat.struct.PE.Machine.enum)
>> IMAGE_FILE_MACHINE_I386

Aggregate fields

For records, arrays and bitfields, you have access to the following additional methods:

count: int: number of sub-fields/members of the aggregate. For atomic fields this attribute is still defined, but it will be always 1

__iter__()

Iterate over all the aggregate’s members, i.e. all the bits of a bitfield, all the rows of an array or all members of a record field

sections = malcat.struct["Sections"]
for s in sections:
    print("{}: #{:x}".format(s["Name"], malcat.map.a2p(s["PointerToRawData"])))

Returns: iterator over the list of field members
Return type: iterator over StructAccess instances
Raises: Error for atomic fields

__getitem__(interval)

Iterate through from the ith to the jth sub-elements of the array/record/bitfield

for s in malcat.struct.Sections[1:]:
    print("#{:x}: {}".format(malcat.map.a2p(s.address), s.name))

Parameters: interval (slice) – index interval
Return type: iterator over the list of members (StructAccess)

__getitem__(name)

return the value of the the aggregate member named name. Note that this method is not valid for arrays, since array’s elements don’t have names.

is_executable = malcat.struct.PE.Characteristics['ExecutableImage']

Parameters: name (str) – name of the member
Return type: StructAccess for aggregate fields, the python type for atomic fields
Raises: KeyError if no member named name can be found

at(name)

return the first aggregate member named name. Note that this method is not valid for arrays, since array’s elements don’t have names.

is_executable = malcat.struct.PE.Characteristics.at('ExecutableImage').value

Parameters: name (str) – name of the member
Return type: StructAccess
Raises: KeyError if no member named name can be found

__getattr__(name)

return the first aggregate member named name. Note that this method is not valid for arrays, since array’s elements don’t have names.

is_executable = malcat.struct.PE.Characteristics.ExecutableImage.value
# equivalent
is_executable = malcat.struct.PE.Characteristics.at('ExecutableImage').value
# equivalent
is_executable = malcat.struct.PE.Characteristics['ExecutableImage']

Parameters: name (str) – name of the member
Return type: StructAccess
Raises: KeyError if no member named name can be found

__getitem__(i)

return the value of the the ith aggregate member of the field

is_executable = malcat.struct.PE.Characteristics[1]

Parameters: i (int) – position of the aggregate member to query
Return type: StructAccess for aggregate fields, the python type for atomic fields
Raises: KeyError if the aggregate field has less than i members

at(i)

return the ith aggregate member

is_executable = malcat.struct.PE.Characteristics.at(1).value

Parameters: i (int) – position of the aggregate member to query
Return type: StructAccess
Raises: KeyError if the aggregate field has less than i members

Editing 

The same way you access an aggregate field’s value, you can also edit it. See the examples below:

# all of these lines do the same thing: set the ExecutableImage bit of the PE characteritics bitfield to True:
malcat.struct.PE.Characteristics['ExecutableImage'] = True
malcat.struct.PE.Characteristics.at('ExecutableImage').value = True
malcat.struct.PE.Characteristics.ExecutableImage.value = True
malcat.struct.PE.Characteristics[1] = True
malcat.struct.PE.Characteristics.at(1).value = True

# setting an int value
malcat.struct.MZ.InitialSS.value = 5

# changing a time / date value
malcat.struct["PE"]["TimeDateStamp"] = datetime.datetime.now()

# changing a string value. Make sure that the string is not bigger than 8 chars!
malcat.struct["Sections"][0]["Name"] = "newname"

Note

When editing a field, you’ll have to make sure that the new values does not take more place on disk than the new one. Otherwise, an error will be thrown. You’ll also have to make sure to assign the correct python type and value, like a positive int less 65536 than for an UInt16 field.

File structures (malcat.struct)

Structures, fields and values

Accessing structures

Inspecting fields