Credential stuffing turns somebody else’s password breach into a problem for your login service. Attackers try previously stolen username-password pairs against other sites, relying on password reuse. A correct password can therefore be evidence of a stolen credential rather than the legitimate account holder.
This guide expands the Credential Stuffing branch of the Cyber Threat Tree. It complements our password spraying guide: spraying distributes a small set of guesses across accounts, while stuffing reuses known pairs. The distinction matters when analysts interpret failures and successful sessions.
Map every credential stuffing entry point
Inventory the browser login, mobile endpoints, APIs, legacy authentication paths, and account-recovery workflows. Record where MFA is enforced and where an alternative path changes the requirement. A protected front page cannot compensate for an overlooked endpoint accepting the same credentials.
Use approved test accounts to check those paths. Document what the system considers a successful authentication, a completed second factor, and an issued session. These events are related but should not be collapsed into one ambiguous success field.
Correlate activity beyond an IP address
OWASP’s credential-stuffing guidance recommends layered defenses and distinguishes stuffing from spraying and brute-force attacks. MFA is a central control, while rate limits, bot signals, and other measures provide additional resistance.
Build a detection view that relates account, application, time, request outcome, second-factor outcome, and session issuance. Source networks, device signals, and request patterns can help, but no single attribute establishes identity. Shared networks can carry many legitimate customers, and distributed attackers can change addresses.
Investigate unusual successful sessions alongside repeated failures. An attack can have a low overall success rate while still taking over valuable accounts. Keep passwords, one-time codes, and session secrets out of the logs used for this analysis.
Make a stolen password insufficient
Enforce MFA on the access paths that matter, using phishing-resistant methods where supported. Verify enrollment and recovery rules as carefully as the main sign-in. The OWASP MFA guidance is a useful reference for those lifecycle decisions.
Review policy exceptions with an owner and expiry condition. A long-standing exemption for a legacy integration can quietly become the easiest route into an account. Decide whether that integration should use a more suitable authentication mechanism or be retired.
Help users avoid password reuse through appropriate password controls and password-manager support. Do not describe MFA as a guarantee against every takeover method; session theft, phishing, recovery abuse, and endpoint compromise need their own controls.
Reduce abuse without locking out the audience
Tune throttling and challenges against observed traffic, legitimate customer behavior, and the service’s availability requirements. Use a staged rollout and measure false positives. A blanket lockout can become an attacker-triggered outage, while an inaccessible challenge can exclude real customers.
Track attempted accounts, challenged requests, issued sessions, confirmed takeovers, support complaints, and recovery outcomes. These measures explain the tradeoff more clearly than a single count of blocked IP addresses.
Respond to the account, not just the request
For suspected takeover, preserve the event timeline and review unfamiliar sessions, recovery changes, enrolled factors, and sensitive actions. Revoke access and reset affected credentials through the platform’s supported procedures from a trusted administrative context.
Do not assume a password reset terminates every session or reverses attacker changes. Confirm the outcome, notify the account holder through an established channel, and investigate whether the same credential was used elsewhere in the organization.
Credential stuffing defense works when the application can distinguish a password match from trusted access, detect account-level consequences, and recover control without introducing a weaker emergency path.
