/*
	Author: DosX
	GitHub: https://github.com/DosX-dev
    Website: https://dosx.su
	Telegram: @DosX_dev
*/

// This module was written specifically for the Detect It Easy project.
// Please retain the copyright information when distributing it.

import "pe"
import "math"

// ============================================================================
//  Base private rules (reusable predicates)
// ============================================================================

private rule IsPE {
	condition:
		uint16(0) == 0x5A4D and uint32(uint32(0x3C)) == 0x00004550
}

private rule IsNative {
	condition:
		IsPE and pe.data_directories[14].virtual_address == 0
}

private rule Is64 {
	condition:
		IsPE and pe.machine == 0x8664
}

// ============================================================================
//  PE Header anomalies
// ============================================================================

rule Anomaly__ZeroSizedOptionalHeader {
	// OptionalHeader size must be non-zero for any real PE
	condition:
		IsPE and pe.size_of_optional_header == 0
}

rule Anomaly__SuspiciousImageBase {
	// Standard bases: 0x00400000 (EXE), 0x10000000 (DLL), 0x00010000 (CE).
	// Anything else is unusual and may indicate manual crafting or packing.
	condition:
		IsPE and
		IsNative and
		pe.image_base != 0x00400000 and
		pe.image_base != 0x10000000 and
		pe.image_base != 0x00010000 and
		pe.image_base != 0x01000000 and  // some Delphi/MSVC
		pe.image_base != 0x0000000140000000 and // x64 default
		pe.image_base % 0x10000 != 0  // must be 64K-aligned at minimum
}

rule Anomaly__ZeroEntryPoint {
	// EntryPoint == 0 is only valid for DLLs with no init code.
	// For executables it's an anomaly (packed/corrupt).
	condition:
		IsPE and
		pe.entry_point_raw == 0 and
		pe.characteristics & 0x2000 == 0  // not a DLL
}

rule Anomaly__EntryPointBeyondImage {
	// Entry point RVA exceeds SizeOfImage — broken or tampered header
	condition:
		IsPE and
		pe.entry_point >= pe.size_of_image
}

rule Anomaly__InvalidSectionAlignment {
	// SectionAlignment must be >= FileAlignment.
	// SectionAlignment < 0x200 is almost always hand-crafted.
	condition:
		IsPE and
		pe.section_alignment < pe.file_alignment
}

rule Anomaly__TinyFileAlignment {
	// FileAlignment below 0x200 (512) is legal for small alignment PEs,
	// but anything below 0x1 is invalid, and <0x200 is highly abnormal
	// for anything except special-purpose or packed binaries.
	condition:
		IsPE and (
			pe.file_alignment < 0x200 and
			pe.file_alignment != pe.section_alignment  // exclude valid same-alignment PEs
		)
}

rule Anomaly__ChecksumMismatch {
	// pe.checksum is the value from the header.
	// pe.calculate_checksum() is the actual computed one.
	// Mismatch = modified after linking or no checksum written.
	// Only flag when the header claims a non-zero checksum but it's wrong.
	condition:
		IsPE and
		pe.checksum != 0 and
		pe.checksum != pe.calculate_checksum()
}

rule Anomaly__ZeroSizeOfImage {
	condition:
		IsPE and pe.size_of_image == 0
}

rule Anomaly__LargeNumberOfSections {
	// Windows loader limits sections to 96 in practice.
	// >16 sections is already unusual; >40 is highly suspicious.
	condition:
		IsPE and pe.number_of_sections > 40
}

rule Anomaly__SuspiciousSubsystem {
	// Only WINDOWS_GUI (2), WINDOWS_CUI (3), NATIVE (1),
	// EFI_* (10-13) and WINDOWS_CE_GUI (9) are common.
	// Value 0 or >14 is almost certainly tampered.
	condition:
		IsPE and (
			pe.subsystem == 0 or pe.subsystem > 14
		)
}

rule Anomaly__DllCharacteristicsForcedIntegrity {
	// IMAGE_DLLCHARACTERISTICS_FORCE_INTEGRITY (0x0080)
	// Extremely rare outside of kernel drivers and signed system components.
	condition:
		IsPE and
		pe.dll_characteristics & 0x0080 != 0 and
		pe.subsystem != 1  // not NATIVE (driver)
}

// ============================================================================
//  Section anomalies
// ============================================================================

rule Anomaly__WritableCodeSection {
	// A section that is both executable AND writable is a classic packer indicator.
	// .text should normally be RX, not RWX.
	condition:
		IsPE and
		IsNative and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].characteristics & 0x20000000 != 0 and  // MEM_EXECUTE
			pe.sections[i].characteristics & 0x80000000 != 0 and  // MEM_WRITE
			pe.sections[i].characteristics & 0x20 != 0			// CNT_CODE
		)
}

rule Anomaly__ExecutableDataSection {
	// .data, .rdata, .bss being executable is suspicious
	condition:
		IsPE and
		IsNative and
		for any i in (0..pe.number_of_sections - 1) : (
			(pe.sections[i].name == ".data" or
			 pe.sections[i].name == ".rdata" or
			 pe.sections[i].name == ".bss") and
			pe.sections[i].characteristics & 0x20000000 != 0  // MEM_EXECUTE
		)
}

rule Anomaly__SectionNameEmpty {
	// Sections without names are suspicious (packers strip section names)
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].name == ""
		)
}

rule Anomaly__SectionNameNonPrintable {
	// Non-printable characters in section names indicate hand-crafting
	strings:
		$np = /[\x01-\x1f\x7f-\xff]{1}/ // at least one non-printable
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].name matches /[^\x20-\x7e]/
		)
}

rule Anomaly__SectionZeroRawSize {
	// A section with zero raw size but non-zero virtual size
	// that also has executable permissions — shellcode/packer technique
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].raw_data_size == 0 and
			pe.sections[i].virtual_size > 0 and
			pe.sections[i].characteristics & 0x20000000 != 0  // MEM_EXECUTE
		)
}

rule Anomaly__SectionRawSizeExceedsFile {
	// Raw data extends beyond physical file — corrupt or tampered
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].raw_data_offset + pe.sections[i].raw_data_size > filesize and
			pe.sections[i].raw_data_size > 0
		)
}

rule Anomaly__SectionVirtualSizeMuchLarger {
	// VirtualSize >> RawDataSize (>10x) — unpacking stub pattern.
	// Excludes .bss (legitimately has zero raw, large virtual).
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].raw_data_size > 0 and
			pe.sections[i].virtual_size > pe.sections[i].raw_data_size * 10 and
			pe.sections[i].name != ".bss"
		)
}

rule Anomaly__SectionHighEntropy {
	// Entropy > 7.2 for any section indicates compressed or encrypted data.
	// .rsrc can legitimately be high-entropy (PNG/JPEG), so we exclude it.
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].name != ".rsrc" and
			pe.sections[i].raw_data_size > 256 and
			math.entropy(pe.sections[i].raw_data_offset, pe.sections[i].raw_data_size) > 7.2
		)
}

rule Anomaly__DuplicateSectionNames {
	// Two or more sections sharing the same name — hand-crafted PE
	condition:
		IsPE and
		pe.number_of_sections >= 2 and
		for any i in (0..pe.number_of_sections - 2) : (
			for any j in (i + 1..pe.number_of_sections - 1) : (
				pe.sections[i].name == pe.sections[j].name and
				pe.sections[i].name != ""
			)
		)
}

// ============================================================================
//  Entry point anomalies
// ============================================================================

rule Anomaly__EntryPointInLastSection {
	// EP in the last section is a classic indicator of appended code/packing.
	// Legitimate binaries almost always have EP in the first code section.
	// NOTE: pe.entry_point is a raw file offset (not RVA) when scanning files,
	// so we compare with raw_data_offset/raw_data_size, not virtual_address/virtual_size.
	condition:
		IsPE and
		IsNative and
		pe.number_of_sections >= 2 and
		pe.sections[pe.number_of_sections - 1].raw_data_size > 0 and
		pe.entry_point >= pe.sections[pe.number_of_sections - 1].raw_data_offset and
		pe.entry_point < pe.sections[pe.number_of_sections - 1].raw_data_offset +
						 pe.sections[pe.number_of_sections - 1].raw_data_size
}

rule Anomaly__EntryPointInNonCodeSection {
	// EP within a section that lacks the CODE flag.
	// Uses raw file offsets — pe.entry_point is a file offset, not RVA.
	condition:
		IsPE and
		IsNative and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].raw_data_size > 0 and
			pe.entry_point >= pe.sections[i].raw_data_offset and
			pe.entry_point < pe.sections[i].raw_data_offset + pe.sections[i].raw_data_size and
			pe.sections[i].characteristics & 0x20 == 0  // no CNT_CODE
		)
}

rule Anomaly__EntryPointOutsideAnySections {
	// EP doesn't fall within any defined section — header trick or packer stub.
	// Exclusions to prevent false positives:
	//   - EP == 0  → valid for DLLs without DllMain (caught separately by Anomaly__ZeroEntryPoint for EXEs)
	//   - DLLs	 → AddressOfEntryPoint == 0 is fully legitimate, skip entirely
	//   - sections with VirtualSize == 0 are skipped to avoid miscalculated boundaries
	// NOTE: pe.entry_point is a raw file offset; virtual_address is RVA — different spaces.
	// We check raw file offset ranges. Sections with no raw data (e.g. .bss) are skipped.
	condition:
		IsPE and
		IsNative and
		pe.characteristics & 0x2000 == 0 and  // not DLL
		pe.entry_point != 0 and
		pe.number_of_sections > 0 and
		not for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].raw_data_size > 0 and
			pe.entry_point >= pe.sections[i].raw_data_offset and
			pe.entry_point < pe.sections[i].raw_data_offset + pe.sections[i].raw_data_size
		)
}

rule Anomaly__EPStartsWithNops {
	// EP begins with a NOP sled (4+ NOPs) — shellcode/packer trick
	strings:
		$nopsled = { 90 90 90 90 }
	condition:
		IsPE and
		IsNative and
		$nopsled at pe.entry_point
}

rule Anomaly__EPStartsWithInt3 {
	// EP begins with INT 3 (0xCC) — debug trap or anti-debug trick
	strings:
		$int3 = { CC }
	condition:
		IsPE and
		IsNative and
		$int3 at pe.entry_point
}

// ============================================================================
//  Import anomalies
// ============================================================================

rule Anomaly__NoImports {
	// Native PE with zero imports — almost impossible for legit PE, typical packer trait.
	// Exclude DLLs (they can be pure export-only).
	condition:
		IsPE and
		IsNative and
		pe.characteristics & 0x2000 == 0 and  // not DLL
		pe.number_of_imports == 0
}

rule Anomaly__SingleImportDll {
	// Only one import DLL (usually kernel32) — packer manually resolves everything else
	condition:
		IsPE and
		IsNative and
		pe.characteristics & 0x2000 == 0 and
		pe.number_of_imports == 1
}

rule Anomaly__SuspiciousMinimalImports {
	// Only LoadLibrary + GetProcAddress imported — classic manual resolve setup.
	// Often seen in packed/crypted executables.
	strings:
		$loadlib  = "LoadLibraryA"
		$loadlibW = "LoadLibraryW"
		$getproc  = "GetProcAddress"
	condition:
		IsPE and
		IsNative and
		pe.number_of_imports <= 2 and
		($loadlib or $loadlibW) and
		$getproc
}

// ============================================================================
//  Overlay / data appending anomalies
// ============================================================================

rule Anomaly__LargeOverlay {
	// Overlay is data appended after the last section's raw data.
	// A large overlay (>50% of file) often means embedded payloads.
	condition:
		IsPE and
		pe.number_of_sections > 0 and
		pe.overlay.offset > 0 and
		pe.overlay.size > filesize / 2
}

rule Anomaly__OverlayPresent {
	// Overlay presence alone isn't an anomaly, but it's of interest when
	// combined with high entropy (embedded encrypted/compressed data).
	condition:
		IsPE and
		pe.overlay.offset > 0 and
		pe.overlay.size > 1024 and
		math.entropy(pe.overlay.offset, pe.overlay.size) > 7.0
}

// ============================================================================
//  Timestamp anomalies
// ============================================================================

rule Anomaly__FutureTimestamp {
	// TimeDateStamp in the future (>2026) — likely forged
	condition:
		IsPE and
		pe.timestamp > 1767225600  // Jan 1, 2026 UTC
}

rule Anomaly__AncientTimestamp {
	// TimeDateStamp before 1990 — PE format didn't exist before ~1993.
	// Also catches zero timestamps (Jan 1, 1970).
	condition:
		IsPE and
		pe.timestamp < 631152000 and  // Jan 1, 1990 UTC
		pe.timestamp != 0
}

rule Anomaly__ZeroTimestamp {
	// Zero timestamp — intentionally stripped (privacy or evasion)
	condition:
		IsPE and
		pe.timestamp == 0
}

// ============================================================================
//  Data directory anomalies
// ============================================================================

rule Anomaly__TLSCallbackPresent {
	// TLS callbacks execute before the entry point — used for anti-debug/evasion.
	// Not inherently malicious, but uncommon in benign software.
	condition:
		IsPE and
		IsNative and
		pe.data_directories[9].virtual_address != 0 and  // IMAGE_DIRECTORY_ENTRY_TLS
		pe.data_directories[9].size > 0
}

rule Anomaly__DebugDirectoryStripped {
	// Native PE with Rich header but no debug directory — info was stripped
	strings:
		$rich = { 52 69 63 68 }  // "Rich"
	condition:
		IsPE and
		IsNative and
		$rich in (0x40..0x400) and
		pe.data_directories[6].virtual_address == 0  // IMAGE_DIRECTORY_ENTRY_DEBUG
}

rule Anomaly__CLRHeaderInNativePE {
	// .NET metadata directory (index 14) present but PE also imports
	// heavy native API — confused tooling or .NET loader with native stubs
	condition:
		IsPE and
		pe.data_directories[14].virtual_address != 0 and
		pe.number_of_imports > 5
}

// ============================================================================
//  Resource anomalies
// ============================================================================

rule Anomaly__ResourceHighEntropy {
	// High-entropy resource section often indicates encrypted payload.
	// Legitimate high-entropy resources: PNG, JPEG, compressed icons.
	// We look for suspiciously high entropy (>7.4) which exceeds even PNG.
	condition:
		IsPE and
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].name == ".rsrc" and
			pe.sections[i].raw_data_size > 4096 and
			math.entropy(pe.sections[i].raw_data_offset, pe.sections[i].raw_data_size) > 7.4
		)
}

rule Anomaly__ResourceDominatedBinary {
	// Resources make up >90% of the file — possible embedded payload binary.
	// Legit cases: icon editors, resource-only DLLs.
	// We exclude DLLs to reduce false positives.
	condition:
		IsPE and
		pe.characteristics & 0x2000 == 0 and  // not DLL
		for any i in (0..pe.number_of_sections - 1) : (
			pe.sections[i].name == ".rsrc" and
			pe.sections[i].raw_data_size > filesize * 9 / 10
		)
}

// ============================================================================
//  Structural / whole-file anomalies
// ============================================================================

rule Anomaly__TinyPE {
	// Legitimate PE minimum is typically >4KB. Under 1KB is deeply abnormal.
	condition:
		IsPE and filesize < 1024
}

rule Anomaly__DOSStubMissing {
	// PE offset (e_lfanew) points right after the DOS header with no stub.
	// Normal MSVC/MinGW binaries include the "This program cannot be run in DOS mode" stub.
	// Missing stub = hand-crafted or size-optimized PE.
	condition:
		IsPE and
		uint32(0x3C) < 0x50  // PE header at less than 80 bytes — no room for DOS stub
}

rule Anomaly__DOSStubCustom {
	// Standard DOS stub contains "This program cannot be run in DOS mode".
	// A different or missing message = custom tooling.
	strings:
		$std_stub = "This program cannot be run in DOS mode"
		$std_stub2 = "This program must be run under Win32"
	condition:
		IsPE and
		uint32(0x3C) >= 0x80 and  // there is space for a stub
		not $std_stub in (0..uint32(0x3C)) and
		not $std_stub2 in (0..uint32(0x3C))
}

rule Anomaly__SelfModifyingHeaders {
	// PE header in a writable section — allows runtime header modification
	condition:
		IsPE and
		pe.number_of_sections > 0 and
		pe.sections[0].virtual_address <= pe.entry_point and
		pe.sections[0].characteristics & 0x80000000 != 0 and  // MEM_WRITE
		pe.sections[0].virtual_address == 0x1000
}

rule Anomaly__WholeFileHighEntropy {
	// Overall file entropy >7.0 strongly suggests compression/encryption.
	// pe.is_pe already ensures valid structure.
	condition:
		IsPE and
		filesize > 4096 and
		math.entropy(0, filesize) > 7.0
}

rule Anomaly__VersionInfoMissing {
	// Native PE without any version info resource — common for packers,
	// less common for production software.
	condition:
		IsPE and
		IsNative and
		pe.characteristics & 0x2000 == 0 and  // not DLL
		pe.number_of_resources == 0
}

// ============================================================================
//  Signature / trust anomalies
// ============================================================================

rule Anomaly__AuthenticodeCorrupt {
	// Security directory is present (claims to be signed) but has
	// size smaller than minimal PKCS#7 structure (~8 bytes minimum header).
	condition:
		IsPE and
		pe.data_directories[4].virtual_address != 0 and
		pe.data_directories[4].size < 8
}

rule Anomaly__SecurityDirPointsBeyondFile {
	// Security dir offset + size exceeds actual file size — appended or corrupt
	condition:
		IsPE and
		pe.data_directories[4].virtual_address != 0 and
		pe.data_directories[4].virtual_address + pe.data_directories[4].size > filesize
}