LLM10: Unbounded Consumption

📋 What Is It?

Unbounded Consumption occurs when an app lets clients drive a model to perform inference - and spend compute, memory, time, and money - without effective limits. The 2025 edition merges the old Model DoS and Model Theft entries: both share one root cause, inference proceeding without bounds. Exploitation needs no jailbreak and often no authentication - only the ability to send requests. LLM economics make it potent: a short prompt can command an enormous, expensive response (asymmetric cost), and transformer attention scales roughly with the square of sequence length (superlinear).

LLM10 OWASP Rank 2025

Merged Model DoS + Model Theft

3 Harms DoS · DoW · Theft

⚖️ Three Harm Axes

Denial of Service (DoS): resource exhaustion degrades/halts the service for real users.
Denial of Wallet (DoW): on metered/autoscaling infra the attack wins even while the service stays up - it just generates a ruinous bill.
Model theft / extraction: unlimited querying lets an attacker distill a "student" clone or infer training-data facts - no weight file stolen.

⚠️ Top Attack Vectors

Long-input / context stuffing: few requests, each a max-length prompt → superlinear GPU cost.
Unbounded output: "count to 1,000,000", max_tokens: null.
Agentic fan-out: one request spawns thousands of sub-agent inferences.
Sponge examples: inputs crafted for worst-case compute; defeat request-count limits.
Concurrency abuse: stay under req/min but fire all in parallel to saturate workers.
Model extraction: harvest I/O pairs at scale to train a functional clone.

🔴 Attack Flow

1. Recon - probe the endpoint: which limits exist (size? max_tokens? rate? auth?)
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2. Pick a lever - the unbounded dimension with the best cost-amplification ratio
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3. Amplify - volume × cost-per-request × concurrency
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4. Evade - spread across keys/IPs, or make few requests maximally expensive
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5. IMPACT: outage (DoS), ruinous bill (DoW), or a model clone (Theft)!

❌ Vulnerable Code (FastAPI gateway)

class ChatIn(BaseModel):
    message: str                       # no length bound
    history: list[str] = []            # unbounded conversation
    max_tokens: int | None = None      # client controls output length

@app.post("/api/chat")
async def chat(body: ChatIn):
    prompt = "\n".join(body.history + [body.message])   # arbitrary size
    # No auth, no token check, no output cap, no timeout, no cost budget
    return await model.generate(prompt, max_tokens=body.max_tokens)
                

✅ Secure Code (bounded gateway)

MAX_INPUT_TOKENS = 4_000; SERVER_MAX_OUTPUT_TOKENS = 1_024

class ChatIn(BaseModel):
    message: str = Field(max_length=8_000)                # hard string bound
    history: list[str] = Field(default=[], max_length=20)  # bound history
    max_tokens: int | None = Field(default=None, ge=1, le=1_024)

@app.post("/api/chat")
async def chat(body: ChatIn, identity: str = Depends(current_identity)):
    await rate_limit(identity)                       # per-identity, not IP
    prompt = "\n".join(body.history[-20:] + [body.message])
    if count_tokens(prompt) > MAX_INPUT_TOKENS:       # token cap = cost cap
        raise HTTPException(413, "Input exceeds token limit")
    max_out = min(body.max_tokens or 512, SERVER_MAX_OUTPUT_TOKENS)  # clamp
    if not await reserve_budget(tenant_of(identity), max_out):  # DoW guard
        raise HTTPException(402, "Daily budget exhausted")
    return await model.generate(prompt, max_tokens=max_out, timeout=30)
                

✓ Prevention Checklist

Cap input size - byte cap at the edge and token count on the whole prompt (history + RAG)
Server-enforced max_tokens on every generation; clamp the client's value, never trust it
Rate limit per identity (user/key/tenant), not just per IP
Token and spend quotas - request count alone doesn't bound cost
Cap concurrency (per-identity and global); timeouts + circuit breaker on every call
Per-tenant cost budget checked before the call, behind a provider hard billing cap
Bound agent/tool fan-out: hard step, depth, and shared-token budgets
Require auth so usage is attributable and boundable per identity
Meter actual tokens/cost; detect extraction sweeps; withhold logprobs from untrusted callers
Degrade gracefully - shed load with clear 429/402/503, never collapse or overspend

🧰 Tools & Takeaway

tiktoken Redis rate limit API gateway Billing caps Budget alerts Anomaly detection Watermarking

💡 Takeaway: Bound every dimension of cost - input tokens, output tokens, request rate, concurrency, and money - not just request count. Treat cost budgets and billing caps as security controls, and enforce everything server-side at a gateway the client cannot bypass.

⚠️ Autoscaling is not a defence: it protects availability and thereby converts a DoS into a Denial of Wallet - you pay to serve the attack. The first signal is often the invoice, not an outage.

💸 Unbounded ConsumptionLLM10