An online/offline password attack is defined by where an attacker checks guesses. Online, candidates are sent to a live login service, where its defenses can limit attempts. Offline, candidates are tested against stolen password hashes without contacting that service, so login throttling cannot stop the work. The distinction determines which defenses can still help.
What is the difference between online and offline password attacks?
| Factor | Online guessing | Offline cracking |
|---|---|---|
| Where guesses are checked | At a live login service or other authentication endpoint. | Locally against stolen password hashes or equivalent verifier material. |
| What the attacker needs | Access to the service’s login endpoint. | A copy of the password hashes or equivalent material, commonly obtained in a database breach. |
| Does the service’s rate limit apply? | Yes. The service can slow, block, or otherwise constrain attempts. | No. Guesses are not passing through the login service. |
| Defender’s main leverage | Rate limiting, password blocklists, and other login protections. | Suitable salted password hashing with an appropriately high work factor, resistant passwords, and response to the compromise. |
Neither attack necessarily tests every possible character combination. Attackers can prioritize likely passwords, common patterns, or credentials exposed in other breaches. “Brute force” describes repeated guessing broadly; the practical guesses may be selected rather than exhaustive.
How do online brute-force defenses work?
With online guessing, every attempt has to reach the verifier. That gives the service an opportunity to count failures and constrain further requests. Rate limiting can slow attempts, while blocklists can reject passwords known to be common or compromised. OWASP describes these as relevant authentication measures in its Authentication Cheat Sheet.
NIST SP 800-63B-4 requires verifiers to implement controls against online guessing when applicable. For specified authenticator cases, it sets 100 consecutive failed attempts as an upper bound; agencies may set lower limits. This is not a universal recommendation that every consumer service allow 100 tries. The applicable requirement depends on the authenticator and context. See NIST SP 800-63B-4.
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Related attack names describe different guess patterns, not interchangeable defenses:
- Password guessing: repeated guesses directed at an account.
- Password spraying: a small set of common passwords tried across many accounts.
- Credential stuffing: usernames and passwords exposed elsewhere tried at another service.
Distinct passwords matter especially for credential stuffing: a password compromised at one site should not unlock an account at another. A password manager can help people generate and keep unique passwords, but it does not repair a service’s password storage.
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Why can attackers crack password hashes offline?
A password hash is a stored verifier derived from a password. If an attacker obtains the hash file, they can calculate candidate passwords and compare the resulting values with the stolen hashes without asking the website to authenticate. The website’s account lockout or rate limit cannot count those local calculations.
NIST SP 800-63B-4 (2025) describes current offline hash-computation capability as “many billions of hashes per second” in the absence of rate limiting. That is a broad qualitative statement, not a benchmark for every algorithm, configuration, or attacker. It should not be used to claim a fixed time to crack a particular password. Actual results depend on the password, hashing scheme and cost factor, and available computing resources.
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How do salts and password hashing make offline cracking harder?
NIST’s password-storage requirement states: “Passwords SHALL be salted and hashed using a suitable password hashing scheme.” A salt is a unique value stored with the hash; it is not a secret key. NIST says salts should be at least 32 bits and selected to minimize collisions among stored hashes. OWASP explains in its Password Storage Cheat Sheet that unique salts prevent an attacker from reusing one calculation across multiple accounts.
Salting does not make a weak password uncrackable. It prevents precomputed hash lookup from applying directly and makes each account’s candidate calculations distinct. The hashing scheme’s cost factor adds work to each guess. NIST’s recommendation is: “The chosen cost factor SHOULD be as high as practical without negatively impacting verifier performance.” That balances resistance to offline guessing with acceptable authentication performance.
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NIST also recommends recording the scheme and cost-factor reference so the verifier can migrate to a newer scheme or raise the work factor over time. It recommends an additional keyed hashing or encryption iteration using a secret key stored separately; when deployed, that added layer can make brute-force attacks impractical while the key remains secret. This is an additional control, not something every service necessarily uses.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should users and service operators do?
If you use the account
- Use a distinct password for each service so a breach at one site does not directly expose another account.
- If a service reports a breach or you have evidence your password was exposed, change it there and anywhere else you reused it.
If you operate the authentication service
- Apply controls against online guessing, including appropriate attempt limiting and checks against common or compromised passwords.
- Store passwords using a suitable salted password-hashing scheme, with a cost factor high enough for the service’s performance constraints.
- Plan for hash-scheme and cost-factor migration; keep any additional secret key separate from the hash database if using a keyed iteration.
- After a hash compromise, treat the event as a storage and credential incident: assess exposure and require affected users to change credentials as appropriate. Online rate limits do not contain cracking of an already stolen hash file.
NIST SP 800-63B-4 also advises against mandatory periodic password changes absent evidence of compromise and against composition rules. Its guidance favors measures such as blocklists, secure storage, machine-generated passwords, and rate limiting. See NIST’s Strength of Passwords guidance. NIST notes that a hash’s size is independent of password length, while implementations still need reasonable processing limits.
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Why does the online/offline distinction matter?
Online defenses control the path into a live verifier; offline defenses make stolen verification data expensive to test and reduce the likelihood that guesses will succeed. Neither replaces the other: throttling cannot protect a copied hash file, and strong hash storage does not stop unlimited attempts against a poorly protected login.
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