OAUTH

OAuth / OIDC Attack Playbook

OAuth bugs are the single highest-paying ATO vector in modern bug bounty. Every B2B SaaS has an OAuth surface; many implement it wrong.

1. Map the flow

# Discovery
curl -s https://target/.well-known/openid-configuration | jq

# Key endpoints to identify:
# - /authorize  (where redirect_uri/client_id/scope arrive)
# - /token      (where code exchanges for tokens)
# - /userinfo   (claims about user)
# - /jwks.json  (signing keys, see jwt/SKILL.md)
# - /revoke     (sometimes overpermissive)

Capture a normal flow in Burp. Note: client_id, redirect_uri, state, scope, response_type, code_challenge (PKCE).

2. Attack surface

2.1 redirect_uri bypass

The #1 OAuth bug class. Server's allowlist regex is too loose:

Test ALL of these. paramspider + Burp Intruder w/ payload list = systematic.

2.2 State parameter missing / weak (CSRF)

state should bind the auth request to the user's session. Missing or predictable state → attacker initiates OAuth in own browser, sends victim the URL, victim clicks → attacker's account now linked to victim's identity.

GET /authorize?response_type=code
    &client_id=...
    &redirect_uri=https://target.com/cb
    &state=                          ← empty or predictable

2.3 PKCE downgrade

Public clients (mobile, SPA) SHOULD use PKCE. If server accepts code_verifier omission for a flow that should require it:

POST /token
client_id=mobile-app
code=<stolen_code>
# Note: no code_verifier sent

Some servers fall back to non-PKCE flow → stolen code exchanges fine.

2.4 Code reuse / replay

Some servers don't invalidate codes after first exchange. Capture the code (via referer leak, IDOR, log scrape) → exchange in attacker's session for victim's tokens.

2.5 Referer leak of code

Some apps render auth-callback as a regular page that fetches resources from third-party CDNs. The full URL (including ?code=...) is sent in Referer header to those CDNs. Attacker who owns / can compromise a CDN link extracts the code.

Test: visit the callback URL, observe Referer to all third-party hosts.

2.6 Scope creep / mass-assignment in token request

POST /token
grant_type=authorization_code
code=...
scope=read write admin             ← inject elevated scope

Some servers honor a scope parameter at token-exchange time and don't re-validate against original /authorize scope.

2.7 response_type confusion

The hybrid flow (response_type=code id_token) may behave differently:

• Server returns id_token in URL fragment (visible client-side)
• Then code exchange for access_token
• Sometimes the id_token validation is weak/skipped on response_type variations

2.8 client_id confusion

Some servers trust client_id as the only identifier. If two clients share a redirect_uri pattern, you can re-use one's code as another:

client_id=trusted-internal-app
redirect_uri=https://attacker.com/cb (also allowlisted for trusted app!)

2.9 OAuth → Account Takeover chain

The classic ATO via OAuth:

1. Victim signs in to target via OAuth (Google/Microsoft)
2. Attacker finds open-redirect or XSS on target
3. Attacker crafts OAuth init URL w/ redirect_uri pointing to attacker via open-redirect
4. Victim clicks → server issues code → redirected to attacker's host w/ code in URL
5. Attacker exchanges code for victim's tokens → full ATO

2.10 Account-linking vulnerabilities

"Sign in with Google" can link a NEW Google account to an EXISTING email-password account if the server matches solely by email. Attacker registers victim@target-mail.com (a typosquat or sub-add), starts OAuth, links to victim's existing account.

3. Tools

• oauthtoolkit — Tom Hudson's automation
• Burp Pro OAuth flow scanner
• oauth2-test Python lib for fuzz
• ZAP Scripting (OAuth modules)
• Manual w/ Repeater (most flaws need careful semantic work)

4. PoC pattern

1. Capture normal flow
2. For each parameter (redirect_uri, state, scope, response_type, client_id, code_challenge), fuzz w/ Burp Intruder using the bypass list above
3. On any deviation (302 to attacker host, token issued with elevated scope, etc) — capture as evidence
4. Construct ATO chain: combine OAuth bug w/ open-redirect OR XSS for victim-clicks-link → tokens delivered to attacker

5. Severity calibration

6. Defender remediation

• Exact-match redirect_uri (case-sensitive, full URL, no path manipulation)
• Mandatory non-empty unpredictable state bound to session
• Invalidate codes on first use; short TTL (≤ 60s)
• PKCE required for ALL public clients
• Server-side scope validation: reject scope in token request that exceeds the original /authorize scope
• Never link OAuth identity to existing account by email alone — require explicit user confirmation

Cross-references

• Upstream catalog: skills/_corpus/payloads/OAuth Misconfiguration/
• ATO chaining: skills/exploit/web/ato-methodology/SKILL.md
• Open-redirect chains: skills/exploit/web/open-redirect/SKILL.md

Known exemplars

• Microsoft 2020: $50k OAuth ATO chain on Teams via redirect_uri
• Slack 2017: $4-8k bounties on multiple redirect_uri bypasses
• Github 2019: OAuth scope creep bug for $5k
• Frans Rosén's writeup on OAuth bugs (one of the best resources)
• Hackerone disclosure #341876 — Asana account-linking via email