The VECT Ransomware Encryption Flaw: A Technical Autopsy

Overview

In early 2026, cybersecurity researchers uncovered a critical flaw in the VECT ransomware that transforms it from a conventional encryption-based extortion tool into a data wiper for files larger than 128 KB. This tutorial provides a deep technical analysis of the VECT 2.0 encryption engine, explaining how a seemingly simple nonce management error makes full recovery impossible—even for the attackers themselves. By the end, you will understand the encryption logic, the specific bug in libsodium's ChaCha20 implementation, and why this makes VECT more dangerous than advertised.

Prerequisites

To follow this guide, you should be comfortable with:

• Basic cryptography concepts: encryption, nonces (IVs), symmetric ciphers, AEAD vs. stream ciphers.
• Ransomware fundamentals: how encryption-based ransomware typically works (file enumeration, encryption, key exchange).
• Familiarity with Python or pseudocode: for understanding the code examples that illustrate the flaw.
• Background in binary analysis (optional but helpful) for understanding how the researchers reverse-engineered the VECT binaries.

No prior knowledge of VECT is required; we will cover everything from the ground up.

Step-by-Step Analysis of the VECT Encryption Flaw

1. Understanding the VECT Encryption Design

VECT ransomware uses ChaCha20-IETF (RFC 8439) as its encryption cipher. This is a stream cipher known for high performance. However, unlike the more common ChaCha20-Poly1305 AEAD, VECT uses the cipher without authentication. That means there is no Poly1305 MAC, no integrity verification. The attacker (or victim) cannot detect if encrypted data has been tampered with—but that's not the core flaw.

The ransomware operates on three platforms: Windows, Linux, and ESXi. Despite the different binaries, they all share an identical encryption engine built on libsodium. The file-size threshold for the bug is consistent: any file larger than 131,072 bytes (128 KB) triggers a fatal misstep.

2. The Four-Chunk Encryption Logic

VECT divides the file into up to four chunks, but it only encrypts certain chunks. The logic works as follows (simplified):

1. Read the file size.
2. If the file is ≤ 128 KB, encrypt the entire file using a single nonce.
3. If the file > 128 KB, split the file conceptually into four equal segments (the size of each segment is computed dynamically).
4. Generate four nonces, one for each segment.
5. Encrypt each segment using ChaCha20 with its respective nonce except that the code discards nonces for segments 2, 3, and 4 — it only uses the first nonce for the first segment, then re-uses it (or mishandles the others) due to a bug.

Wait—that seems contradictory. Let's clarify: the intended design was to encrypt each of the four segments independently, each with a unique nonce. But the actual implementation has a critical bug in how it stores and applies the nonces. The researchers at Check Point discovered that for files above 128 KB, only the first nonce is ever used for all segments, because the subsequent nonces are overwritten or never written to the output header. As a result, the encryption of segments 2–4 is effectively useless: the decryption key (the nonce) is missing.

3. Code Example Illustrating the Flaw

Below is a simplified Python representation of the flawed logic (adapted from reverse engineering notes):

def encrypt_file(filename):
    size = get_file_size(filename)
    nonces = [os.urandom(12) for _ in range(4)]  # Generate four nonces
    header = b''
    if size <= 131072:
        # Single encryption
        header += nonces[0]
        ciphertext = encrypt_chunk(filename, 0, size, nonces[0])
        write_output(header + ciphertext)
    else:
        chunk_size = size // 4
        for i in range(4):
            start = i * chunk_size
            end = start + chunk_size
            # BUG: Only first nonce is stored in header
            if i == 0:
                header += nonces[0]
            # Encryption uses the correct nonce[i], but header lacks the others
            ciphertext = encrypt_chunk(filename, start, end, nonces[i])
            write_output(header + ciphertext)  # Overwrites header incorrectly

In reality, the bug is subtler: the code generates a header containing a single nonce plus metadata, but for multi-chunk files, it omits the other three nonces entirely. When the victim (or even the attacker) tries to decrypt, they only have one nonce for the entire file. Since ChaCha20 uses the nonce to initialize the cipher state, the first segment can be decrypted, but the rest cannot, because the nonce used for those segments is unknown.

4. Impact: Wiper by Accident

Because the missing nonces make three-quarters of the file irrecoverable, any file larger than 128 KB is essentially destroyed. For an enterprise, that includes:

• Virtual machine disk images (VMDK/VHDX)
• Databases (.mdf, .ibd, .sqlite)
• Large documents and spreadsheets
• Backup files (.bak, .tar, .zip)

Even if the victim pays the ransom, the attackers cannot provide a working decryption tool because they too lack the necessary nonces. The flaw is baked into the ransomware itself; no amount of key exchange can recover lost nonces. This makes VECT a wiper for meaningful data, not a ransomware wiper—though the operators still ask for payment.

5. Additional Debugging: Misleading Features

VECT advertises speed modes (--fast, --medium, --secure) on Linux and ESXi. Our analysis shows these flags are parsed but silently ignored. All executions use the same hardcoded thresholds. The thread scheduler intended to improve performance actually degrades it, adding overhead without parallelism. Also, the cipher misidentification in public reports (ChaCha20-Poly1305) is widespread; there is no Poly1305 MAC.

Common Mistakes in Understanding VECT

• Thinking the encryption is recoverable: Many assume the flaw only affects some files or that the attacker can fix it. In fact, all files >128 KB are permanently damaged.
• Assuming VECT uses AEAD: Multiple threat intel reports incorrectly state VECT uses ChaCha20-Poly1305. The absence of authentication means no integrity check—but more crucially, the nonce bug is independent.
• Believing speed modes affect behavior: The --fast/--medium/--secure flags do nothing; don't assume operators have any control over encryption granularity.
• Overlooking the cross-platform unity: The same flaw exists in Windows, Linux, and ESXi variants. Any file larger than 128 KB on any platform suffers the same fate.
• Confusing the nonce flaw with key management: The ransomware does have a proper key exchange (RSA or similar) for the master key. But even with the correct key, decryption fails without the nonces. The flaw is in nonce storage, not key encryption.

Summary

The VECT ransomware 2.0 family, despite its professional appearance, contains a catastrophic nonce management error that makes permanent destruction of all files larger than 128 KB inevitable. The encryption engine, shared across Windows, Linux, and ESXi, uses ChaCha20 without authentication and discards three out of four nonces for multi-chunk encryption. Neither victims nor attackers can recover the data. This tutorial has dissected the underlying logic, illustrated the bug with code, and clarified common misconceptions. The key takeaway: VECT is effectively a wiper, not a ransomware, for any meaningful dataset.