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`: ```typescript 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`: ```typescript 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