Network Signatures and YARA Rules

From Analysis to Defence

After analysing a malware sample, the analyst produces defensive signatures that security tools can deploy immediately. Two signature types cover the two detection domains:

• YARA rules: match file content on disk or in memory at the endpoint
• Snort/Suricata rules: match network traffic at the sensor

Together they provide defence in depth: YARA stops the file if it reaches a host, Snort/Suricata blocks the C2 communication at the network perimeter.

YARA Rules

YARA is a pattern-matching tool for files and memory. A YARA rule has three sections: metadata, strings, and condition.

Rule structure

rule Detect_Dropper_C2 {
    meta:
        description = "Detects dropper with hardcoded C2"
        author      = "Ashish Revar"
        date        = "2026-05-12"
        hash        = "abc123..."

    strings:
        $mz   = { 4D 5A }                           // PE magic bytes
        $cmd  = "cmd.exe /c" ascii nocase
        $url  = "http://evil.com/gate.php" wide ascii
        $ua   = "Mozilla/5.0 (compatible)" ascii
        $mutex = "Global\\MicrosoftUpdateService3"

    condition:
        $mz at 0 and 2 of ($cmd, $url, $ua, $mutex)
}

String types and modifiers

Type	Syntax	What It Matches
Plain string	$s = "cmd.exe"	Exact byte sequence
ascii	$s = "cmd.exe" ascii	ASCII encoding (default)
wide	$s = "cmd.exe" wide	UTF-16LE encoding (Windows Unicode)
nocase	$s = "cmd.exe" nocase	Case-insensitive
fullword	$s = "cmd" fullword	Only standalone word: not "cmdline"
Hex bytes	$s = { 4D 5A ?? 00 }	Byte pattern with ?? wildcards
Hex with jumps	$s = { 4D 5A [2-4] 50 45 }	Variable-length gap between bytes
Regex	$s = /https?:\/\/[0-9.]{7,15}\/gate/ nocase	Regular expression

Condition logic

condition:
    $mz at 0                     // PE file (MZ at offset 0)
    and filesize < 500KB          // reasonable size
    and 2 of ($cmd, $url, $ua)   // at least 2 of 3 IOC strings
    and not $mutex               // exclude known false positive

Operators: and, or, not, all of, any of, N of, at, in

Writing effective YARA rules

Good rule: specific enough to avoid false positives, broad enough to catch variants

// Too specific — fails if one byte changes
$url = "http://185.220.101.47/gate.php"

// Better — matches the gate endpoint pattern across IP variants
$url = /https?:\/\/[0-9.]{7,15}\/gate\.php/

// Too broad — matches almost any Windows binary
$cmd = "cmd.exe"

// Better — matches the specific abuse pattern
$cmd = "cmd.exe /c vssadmin delete shadows" nocase

Testing YARA rules

# Test against a single file
yara rule.yar suspicious.exe

# Test recursively against a directory
yara -r rule.yar malware_samples/

# Verify no false positives against clean files
yara rule.yar C:\Windows\System32\notepad.exe
# Should produce NO output

# Scan memory of a running process
yara -p 3412 rule.yar

Snort / Suricata Rules

Snort and Suricata inspect network traffic in real time, alerting when C2 patterns cross the sensor.

Rule structure

action proto src_ip src_port direction dst_ip dst_port (options)

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (
    msg:"Malware C2 beacon — custom User-Agent";
    flow:established,to_server;
    content:"User-Agent: Mozilla/5.0 (compatible)";
    content:"POST /gate.php";
    sid:1000042;
    rev:1;
)

Key options

Option	Purpose	Example
flow	Connection direction and state	flow:established,to_server
content	Exact byte match	content:"POST /gate.php"
pcre	Perl-compatible regex	pcre:"/gate\.php\?id=[A-F0-9]{16}/i"
nocase	Case-insensitive match	content:"user-agent"; nocase
distance	Next content N bytes after previous	distance:0
within	Next content within N bytes of previous	within:50
dsize	Match on packet data size	dsize:>100
threshold	Alert rate limiting	threshold:type limit, track by_src, count 1, seconds 60

Example: DGA domain detection

alert udp $HOME_NET any -> any 53 (
    msg:"Possible DGA domain query — high entropy hostname";
    content:"|01 00 00 01 00 00 00 00 00 00|";
    pcre:"/[a-z]{12,20}\.(com|net|org|info)\x00/i";
    sid:1000043;
    rev:1;
)

flow:established,to_server explained

• established: TCP three-way handshake completed
• to_server: traffic flowing from client to server (outbound from $HOME_NET)

This combination matches only genuine C2 POST requests: not SYN packets, not server responses.

Defence in Depth: Combining Both

Network perimeter:
  Snort/Suricata rule → blocks C2 communication
  → Catches malware that reached the host but fires its beacon

Endpoint:
  YARA rule → detects the file on disk or in memory
  → Catches malware before or during execution

Together:
  If YARA misses a new variant → Snort catches the beacon
  If Snort misses encrypted C2 → YARA catches the file

YARA Rule Sources

Source	URL	Content
YARA-Rules project	github.com/Yara-Rules/rules	Community rules for major families
Mandiant/FireEye	github.com/mandiant/red_team_tool_countermeasures	Rules for APT tools
Neo23x0 (Florian Roth)	github.com/Neo23x0/signature-base	High-quality community rules
CAPE output	Local CAPE instance	Auto-generated rules from extracted configs

Always test community rules against a clean file corpus before deployment: false positive rates vary significantly.