AI LOAD INSTRUCTION: Treat race conditions as authorization/state integrity issues: non-atomic read-then-write lets multiple requests observe stale state. Prioritize one-time or balance-like operations. Combine parallel transport (HTTP/1.1 last-byte sync, HTTP/2 single-packet, Turbo Intruder gates) with application evidence (duplicate success responses, inconsistent balances, duplicate ledger rows). Authorized testing only. Routing note: for business workflows, coupons, inventory, or one-time rewards, start with this skill and cross-load
business-logic-vulnerabilities.
0. QUICK START — What to Test First
Target endpoints where check and update are unlikely to be a single atomic database operation:
| Priority | Operation class | Example paths / parameters |
|---|---|---|
| 1 | One-time redeem / coupon / bonus | redeem, apply_coupon, claim_reward, voucher |
| 2 | Balance / quota / stock deduction | transfer, purchase, reserve, inventory |
| 3 | Invite / referral / signup bonus | invite_accept, referral_claim |
| 4 | Password / email / MFA verification | verify_token, confirm_email, reset_password |
| 5 | Idempotent-looking APIs without strong keys | POST that should succeed only once per user |
First moves (conceptual):
- Capture the state-changing request in a proxy.
- Send 20–100 copies as simultaneously as your tooling allows.
- Classify outcome: 0/1 expected successes vs N successes or inconsistent final state.
1. CORE CONCEPT
1.1 TOCTOU (Time-of-check to time-of-use)
Thread A Thread B
| |
+-- CHECK (resource OK) |
| +-- CHECK (resource OK) ← both see "OK"
+-- USE / UPDATE |
| +-- USE / UPDATE ← duplicate effect
TOCTOU means the decision (check) and the mutation (use) are not one indivisible step.
1.2 Non-atomic read-then-write
Typical vulnerable pseudo-flow:
balance = SELECT balance FROM accounts WHERE id = ?
if balance >= amount:
UPDATE accounts SET balance = balance - ? WHERE id = ?
Two concurrent requests can both pass the if before either UPDATE commits.
1.3 Database-level vs application-level locking gaps
| Layer | What goes wrong |
|---|---|
| Application | In-memory flag, cache, or session says "not used yet" while DB already updated — or the reverse. |
| ORM / service | Two instances, no distributed lock; each thinks it owns the decision. |
| DB | Missing SELECT … FOR UPDATE, wrong isolation level, or logic split across multiple statements without transaction. |
| API gateway | Per-IP rate limit is check-then-increment — parallel burst passes duplicate checks. |
Hint: UNIQUE constraints and idempotency keys often eliminate entire bug classes — test whether the app enforces them on the hot path.
7. TOOLS
| Tool | Role |
|---|---|
| PortSwigger/turbo-intruder | High-concurrency replay, gates, scripting in Burp. |
| JavanXD/Raceocat | Race-focused HTTP client patterns (verify compatibility with your stack). |
| nxenon/h2spacex | HTTP/2 low-level / single-packet style experimentation (use responsibly, authorized targets only). |
| Burp Suite — Repeater | Send group (parallel) / single-packet attack for multi-request synchronization. |
8. DECISION TREE
START: state-changing API?
|
NO -----------+---------- YES
| |
stop here one-time / balance / verify?
|
+-------------------------+-------------------------+
| | |
coupon-like rate limit multi-step
| | |
parallel same req parallel vs serial parallel pipelines
| | |
duplicate success? limit exceeded? state mismatch?
/ \ / \ / \
YES NO YES NO YES NO
| | | | | |
report + try HTTP/2 report + try TI report + deepen
evidence single-packet evidence gates per-step
| | | | | |
+----+----+ +----+----+ +----+----+
| | |
tool pick tool pick tool pick
v v v
Burp group / h2spacex TI gates / Raceocat TI + trace IDs
How to confirm (evidence checklist):
- Reproducible duplicate success under parallelism, not flaky single retries.
- Server-side artifact: two rows, two emails, two grants, or wrong final balance.
- Correlate with
x-request(or similar) markers or unique body fields in logs (authorized environments).
Routing summary: if the scenario is more about business rules, pricing, or workflow bypass, load skills/business-logic-vulnerabilities/SKILL.md; this file focuses on concurrency and transport-layer synchronization.
References
Each file is loaded on demand — read one only when the task needs that depth (progressive disclosure).
references/attack-patterns.md— the core attack categories (limit-overrun, rate-limit-bypass, multi-step), the concrete limit-overrun catalogue (coupon, vote, balance, inventory, referral), and the CVE-2022-4037 reference case · read when picking which abuse to attempt and what success evidence to look for.references/transport-attacks.md— HTTP/1.1 last-byte sync, the HTTP/2 single-packet attack + detailed mechanics (Nagle/frame coalescing, server queue, h2spacex), and single-packet multi-endpoint bursts · read when you need to actually synchronize the requests on the wire.references/turbo-intruder-templates.md— ready-to-run Turbo Intruder scripts: same-endpoint gate release and multi-endpoint same-gate, plus thex-requestcorrelation header · read when driving the attack from Burp's Turbo Intruder extension.references/database-isolation.md— the isolation-level exploitation matrix (READ UNCOMMITTED → SERIALIZABLE), canonical vulnerable SQL sequences, and a hot-path audit checklist · read when reasoning about the DB layer or verifying a fix.
Related
- business-logic-vulnerabilities — workflow, coupon abuse, and logic-first checklists (
../business-logic-vulnerabilities/SKILL.md).