Blockchain security: How blockchain protects data and its remaining vulnerabilities

Blockchain is one of the most difficult databases to tamper with in history, but according to Chainalysis, attackers stole approximately $3.4 billion in cryptocurrency in 2025. The reason why these two facts coexist is that the blockchain security mechanism is based on two independent levels. The core ledger, protected by cryptography, consensus rules, and thousands of independent nodes, is robust in large networks; while financial losses occur mainly at the edge of the system: including theft of private keys, breach of signers, flaws in smart contracts, and the use of cross-chain bridges. Knowing which level of failure occurred in each major accident is the fastest way to assess the real risk. Therefore, this guide will explain how blockchain security works, which attacks actually succeed, and what measures users can take to respond to it.### Core Points * ** Blockchain security is based on four levels **: cryptographic hashes, digital signatures, consensus rules, and a network of independent nodes that verify complete ledgers. * ** Rewriting history is costly **: On large networks such as Bitcoin and Ethereum, the cost of rewriting history is prohibitively high. Successful 51% attacks only target small proof-of-work (PoW) chains such as Bitcoin Gold and Ethereum Classic. * ** Most of the financial losses occurred at the edge of the system **: Chainalysis traced approximately $3.4 billion stolen in 2025, of which approximately $1.5 billion was caused by the Bybit incident alone. * ** Transactions are irreversible **: Transactions are final by design, so stolen or incorrectly signed transactions are usually not reversible. * ** Quantum computing is a long-term risk **: The threat of quantum computing to current signature schemes is long-term, not current reality. The National Institute of Standards and Technology (NIST) finalized its first post-quantum standards in August 2024.## What is blockchain security? Blockchain security is a combination of cryptography, consensus rules, network design and operating habits to ensure the accuracy, tamper-resistance and usability of distributed ledgers. It answers two different questions: Can anyone tamper with the ledger itself? Can anyone steal assets recorded on it? The former is a protocol-level issue, while the latter is mainly about keys, software and people. For basic knowledge of the underlying technology, please refer to "What is Blockchain?".## How blockchain security works: Four major protection layers #### 1. Cryptographic hash Each block stores the hash value of the previous block-a fixed-length digital fingerprint. If you change even one character in an old transaction, its hash value will change, causing all subsequent blocks to become invalid. Bitcoin uses the SHA-256 hash function, and Ethereum uses Kescak-256. This tamper becomes obvious because the tampered history no longer matches copies held by other nodes.### 2. Digital signatures cost coins that require a signature generated by a private key held by the owner. Both Bitcoin and Ethereum rely on elliptic curve cryptography, and since the Taproot upgrade in November 2021, Bitcoin has also supported Schnorr signatures. Nodes check every signature before accepting transactions, so funds cannot be moved without a key.### 3. Consensus rules determine which version of the historical record is valid. Under Proof-of-Work (PoW) mechanisms such as Bitcoin, miners consume real energy to generate blocks, so rewriting the past means re-completing those tasks at a speed that exceeds the sum of the entire network. The Bitcoin White Paper clearly states this assumption: "As long as honest nodes collectively control more CPU computing power than any group of cooperating attacker nodes, the system is secure." Our "Detailed Explanation of the Nakamoto Consensus" provides a more in-depth introduction to its mechanism. Ethereum switched to Proof of Equity (PoS) in September 2022, replacing energy consumption with pledged collateral. According to ethereum.org, an attacker who holds more than half of pledged ETH can dominate fork selection, censor transactions, and cause short-term restructuring, but this requires acquiring a huge amount of ETH. An attacker attempting to finalize two conflicting versions of the chain risks slashing most of its pledge, meaning the protocol will destroy the attacker's own funds.#### 4. Decentralize thousands of independent nodes to store and verify complete ledgers, so there is no centralized database that can be compromised. Even if the blocks are generated by miners or validators, any rule-breaking blocks will be rejected by the node. This redundancy makes it impossible for a single server failure or a single dishonest operator to tamper with records.### Can blockchain be hacked? Yes, in a specific way. The following are the main blockchain security issues, each with documented cases:| attack types| Target| Recorded cases| Today's risks|| :--- | :--- | :--- | :--- ||**51% attack **| Consensus on small PoW chains| Bitcoin Gold, May 2018: More than $18 million in double spending occurred on the exchange| Real for small chains, impractical for Bitcoin|| ** Agreement loopholes **| Basic layer software| Bitcoin, August 2010: Value spillover created 184 billion BTC, patch client followed up in about five hours| Rare, coordinated repair through nodes|| ** Smart contract vulnerability **| Application code at top of chain| The DAO, June 2016: About 3.6 million ETH was removed, accounting for one-third of the funds raised| Persistence in DeFi|| ** Cross-chain bridge key leaked **| Kuanlian Bridge| Ronin Network, March 2022: approximately $625 million| high-risk|| ** Private key theft and signer breach **| People and signature infrastructure| Bybit, February 2025: About $1.5 billion, FBI attributed it to North Korea| Causing the largest amount of damage| Small proof-of-work chains are often victims of 51% attacks because their total mining power is small enough to be overwhelmed by one party. Ethereum Classic lost more than $5 million in a reorganization attack in 2020, one of many such attacks that occurred that year. The only serious protocol flaw in Bitcoin was a 2010 spill, which was a software error rather than a cryptographic breakthrough. Smart contract vulnerabilities and bridge hacks target code and keys built on the chain. In The DAO case, attackers used vulnerabilities in the project code to remove approximately 3.6 million ETH, according to the SEC report on the incident. In the Ronin case, an attacker who obtained a verifier's private key forged a withdrawal request, and U.S. authorities attributed the theft to North Korea's Lazarus Group. Our Guide to Blockchain Bridges and Cross-Chain Security Issues details this risk, and the Smart Contract Glossary Entry explains the code involved. Bybit's leak demonstrates a modern model. In February 2025, attackers withdrew approximately $15 billion from the exchange, which the FBI attributed in a public announcement to North Korea's TraderTraitor organization. According to Safe's forensic review, the attacker hacked into a Safe{Wallet} developer's machine and injected malicious code into the Web application used by Bybit signers, disguising the malicious transaction as a regular transaction. Ethereum implements content approved by the authorized signer. Compromise occurs in interfaces they trust.### Where were the funds actually lost? The latest data points in the same direction. Chainalysis calculates that about $3.4 billion was stolen in 2025, with North Korea-affiliated organizations responsible for at least $2.02 billion, setting a record. The company said this was a situation where "fewer attacks occurred but the returns were much higher than in the past." It also warned that centralized services face growing losses due to attacks on their private key infrastructure and signature processes. Individual users were also affected: About 158,000 personal wallets were breached, affecting at least 80,000 victims, and the amount stolen was $713 million. This trend will continue in 2026. Blockaid described the first half of the year as the largest number of incidents in history, with losses of more than $1 billion, and said two of the four largest incidents started as social engineering attacks that resulted in multiple signers being breached. The common denominator is that the attacker targets people and permissions, not ledgers.## Will quantum computing pose a threat to blockchain security? Not yet, but the issue is no longer purely academic. The signatures of Bitcoin and Ethereum rely on elliptic curve mathematics, and in theory, a large fault-tolerant quantum computer running the Shor algorithm could crack it, allowing attackers to deduce private keys from exposed public keys. There are currently no machines close to this size. Hash functions like SHA-256 are considered less threatened because known quantum attacks can only provide quadratic acceleration. In August 2024, the National Institute of Standards and Technology (NIST) finalized its first three post-quantum standards, pointing out that some experts predict that devices capable of cracking current encryption could emerge within ten years. On Bitcoin, coins stored in addresses where the public key is already disclosed on its chain (including old pay-to-public-key output and reuse addresses) will be the most vulnerable, which is another reason to avoid address reuse.### User blockchain security best practices As most losses occur at the edge, personal security habits are more important than the protocol itself. * ** Keep the key offline **: Keep large balances in your hardware wallet and store the recovered mnemonics offline. Our Cold Wallet vs. Hot Wallet and Cold Storage guides explain the tradeoffs. * ** Verify what you are signing **: Read the full transaction details on the screen of the hardware device itself, not just in the browser or app. Bybit's signer trusted a tampered interface. * ** Treat token authorization as an open portal **: Regularly review and revoke unused smart contract rights, especially after trying new DeFi apps. * ** Use phishing resistant two-factor authentication **: Hardware security keys or Passkeys are more secure than exchanging SMS Captcha on your account. * ** Stay suspicious of non-solicitit contacts **: Fake recruiters, support agents and urgent direct messages are common ways of getting signers and wallets compromised. * ** Scale risks to code **: Bridged and unaudited DeFi protocols carry smart contract risks that audits can reduce but not eliminate. Our Crypto Risk Management Guide shows how to adjust positions accordingly.### Conclusion So, is blockchain safe? ledgers are usually secure, while the systems around them are often not. Hashes, signatures and consensus have remained stable for years on the largest networks, while keys, interfaces, contracts and bridges continue to cause the most damage. Judging by the actual level of project or platform risks, rather than just whether they are called a "blockchain", is the most useful security habit that cryptocurrency users can establish.### Frequently Asked Questions (FAQ)** Is blockchain 100% secure? * No. The ledgers of large networks such as Bitcoin and Ethereum are extremely difficult to tamper with, but nothing around them is risk-free. Private keys can be spooked or stolen, smart contracts can contain vulnerabilities, bridges can be exploited, and confirmed transactions often cannot be revoked. Chainalysis tracks approximately $3.4 billion stolen in 2025, which suggests that the risk lies in how the assets are held and used, rather than the mathematics of the chain itself. ** Can blockchain be hacked? ** Yes, in a specific way. Small proof-of-work chains such as Bitcoin Gold and Ethereum Classic have suffered successful 51% attacks, and software vulnerabilities have occasionally emerged, such as Bitcoin's 2010 overflow bug, which developers patched within hours. Rewriting the history of Bitcoin or Ethereum requires overwhelming thousands of nodes, which has never happened before. However, exchanges, bridges and personal wallets are often hacked because they rely on keys and codes outside of the protocol. ** What are the biggest issues in blockchain security? ** Private key leaks and social engineering caused the biggest losses. Chainalysis highlighted attacks on the private key infrastructure and signature process of centralized services, and Blockaid said that two of the four largest incidents in the first half of 2026 started when attackers breached multi-signature signers. Second is smart contract vulnerabilities and cross-chain bridges. The breach of personal wallets will affect at least 80,000 victims in 2025. ** What is a 51% attack? * A 51% attack occurs when one party controls most of the mining power of the network or pledges tokens. With this control, attackers can reorganize recent blocks and review transactions, and double spend their own coins on the proof-of-work chain. It cannot forge signatures or steal coins from other wallets. Such attacks are mainly feasible on small networks, such as Bitcoin Gold in 2018, when a double spend incident of more than $18 million occurred on the exchange. ** Which cryptocurrency has never been hacked? ** No cryptocurrencies are hack-proof, but no attacker has successfully rewritten the confirmation history of Bitcoin or Ethereum. Bitcoin's only serious protocol flaw (a 2010 spill) was fixed within hours. Coins continue to be stolen from exchanges, bridges, and personal wallets, so a clean record protocol does not make your own positions safe. How you store and sign is more important than what coin you hold. ** How can I protect my cryptocurrency from hackers? ** Keep large balances in your hardware wallet and store recovery mnemonics offline. Confirm each transaction on the device's own screen before signing, revoke token authorizations that are no longer in use, and secure exchange accounts using hardware security keys or Passkeys instead of SMS Captcha. Ignore unsolicitated messages offering work, support or investment, and limit access to bridging and unaudited DeFi applications. ** Can quantum computers crack blockchain security? ** Not today. In theory, large fault-tolerant quantum computers could crack the elliptic curve signatures used by Bitcoin and Ethereum, but no machines close to that size currently exist. Hash functions like SHA-256 face much less of a threat. NIST finalized its first post-quantum standards in August 2024, providing developers with tested migration alternatives during which it is best to avoid reusing Bitcoin addresses.