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SmarterTools SmarterMail versions prior to build 9610 contain a critical cryptographic flaw in their file and email sharing endpoints. These endpoints rely on DES-CBC encryption to secure sharing tokens and protect attachments. The vulnerability arises from how the cryptographic keys and initialization vectors (IVs) are generated. Instead of using a cryptographically secure random number generator, SmarterMail uses the `System.Random` class in .NET.
`System.Random` is a pseudo-random number generator (PRNG) designed for speed, not security. Its output is deterministic and based on a mathematical formula that starts from an initial numeric value called a “seed.” If an attacker can determine the seed, they can replicate the entire sequence of “random” numbers the application uses. In this case, the seed for `System.Random` is derived from the system time with very poor entropy. The effective seed space is reduced to approximately only 19,000 possible values, which is trivial for a modern computer to brute-force.
The most dangerous aspect of this vulnerability is the presence of an “oracle.” The attachment download endpoint inadvertently acts as this oracle. By interacting with this endpoint, an unauthenticated attacker can observe the application’s behavior and deduce the specific seed being used on the live server. This oracle effectively confirms the attacker’s guesses about the seed.
Once the correct seed is discovered, an attacker can derive the exact encryption keys and IVs used for all current sharing tokens. This allows them to forge their own valid tokens for any email, attachment, or file storage content on the server. The attack requires no prior access to the targeted data and can be performed remotely over the network.
The vendor has addressed this issue in SmarterMail build 9610 by replacing the weak `System.Random` PRNG with a cryptographically secure random number generator (CSPRNG) and moving away from the outdated DES-CBC cipher. Administrators are strongly urged to update immediately to prevent unauthorized data breaches.
DailyCVE Form:
Platform: SmarterTools SmarterMail
Version: < 9610
Vulnerability : Cryptographic weakness
Severity: Medium/High
date: 2026-04-27
Prediction: 2026-04-27
What Undercode Say:
Seed Space Brute Force Script
Simulating enumeration of the ~19,000 seed space
for seed in {1..19000}; do
echo "Testing seed: $seed"
Command to derive token using seed
./derive_token.sh --seed $seed --target /api/download?id=123
done
Python Proof-of-Concept Snippet for Token Forgery
import random
from Crypto.Cipher import DES
def forge_token(seed, target_id):
Initialize PRNG with guessed seed
rng = random.Random(seed)
key = bytes([rng.randint(0, 255) for _ in range(8)])
iv = bytes([rng.randint(0, 255) for _ in range(8)])
cipher = DES.new(key, DES.MODE_CBC, iv)
Craft malicious payload
payload = f"id={target_id}&access=full".encode()
padded = payload + b"\x00" (8 - len(payload) % 8)
token = cipher.encrypt(padded)
return token.hex()
Example: Forge token for email ID 1337
print(forge_token(1337, 1337))
Exploit:
- Identify Target: Discover a vulnerable SmarterMail instance (Build < 9610).
- Oracle Attack: Use the attachment download endpoint to brute-force the seed. Send crafted requests and analyze responses to pinpoint the exact `System.Random` seed from the ~19,000 possibilities.
- Derive Keys: Once the seed is confirmed, generate the identical DES-CBC keys and IVs as the server.
- Forge Tokens: Encrypt a malicious payload with the derived keys to create a valid sharing token for any arbitrary file or email.
- Exfiltrate Data: Present the forged token to the server to gain unauthorized access and download sensitive content.
Protection:
- Immediate Action: Upgrade SmarterTools SmarterMail to build 9610 or newer. This version replaces `System.Random` with a CSPRNG and addresses the weak encryption algorithm.
- Hardening: If an immediate upgrade is impossible, isolate the SmarterMail server from untrusted networks and restrict access to the attachment download endpoint via Web Application Firewall (WAF) rules.
- Validation: Implement strong, cryptographically secure token generation and avoid using DES or any other weak block ciphers for new applications.
Impact:
Successful exploitation allows an unauthenticated attacker to read any email, download any attachment, and access any file stored on the SmarterMail server. This constitutes a complete breach of the confidentiality of all hosted data. Since the attack requires no user interaction and can be fully automated, it poses a severe risk to any organization using an affected version of SmarterMail.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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