● CVE-2024-27983 / High Severity

CVE-2024-27983: Node.js llhttp HTTP Request Smuggling Teardown

How incomplete Transfer-Encoding chunk extension parsing in deps/llhttp caused HTTP request desynchronization, enabling cache poisoning and credential theft.

💡 Простое объяснение на пальцах (ELI5)

Imagine a two-window ticket booth. Window 1 reads: 'One ticket for Alice'. Window 2 reads: 'Wait, there's another ticket for Bob glued to the back'. Because Window 1 ignored the extra ticket, Bob's ticket sits in the window until Charlie walks up. Charlie gets handed Bob's ticket and private receipt, allowing an attacker to steal user sessions.

Ключевые концепции и архитектурные термины

llhttp: The high-performance C HTTP parser library embedded inside Node.js to parse incoming HTTP/1.1 requests.
HTTP Request Smuggling: Desynchronization between frontend reverse proxies (e.g. NGINX, Cloudflare) and backend Node.js servers regarding request boundaries.
Chunked Transfer-Encoding: An HTTP streaming mechanism where payloads are sent in numbered chunks, terminated by a `0\r\n\r\n` chunk.
Chunk Extensions: Optional semicolon-delimited parameters appended to the chunk length (`0;extension=value\r\n`).

Пошаговая механика выполнения атаки

Step 1

1. Send Ambiguous Request

The attacker sends an HTTP request with chunk extensions containing unescaped control characters.

Step 2

2. Frontend Interpretation

The frontend proxy treats the entire payload as a single continuous request body.

Step 3

3. Node.js Parser Desync

Node.js's llhttp stops parsing prematurely, leaving the smuggled second request in the TCP buffer.

Step 4

4. Victim Session Hijacking

The next innocent user's request is prepended to the smuggled request, exfiltrating their session tokens.

Исходный код: Уязвимость vs. Исправленный патч

Представлено на понятном высокоуровневом исходном коде (без ассемблера и бинарных дампов).

УЯЗВИМЫЙ КОД ДО ПАТЧА

// VULNERABLE: deps/llhttp/src/llhttp.c before patch
int llhttp__on_chunk_extension(llhttp_t *parser, const char *p, const char *end) {
    // ROOT CAUSE:
    // Does not enforce strict ASCII validation on chunk extension tokens!
    // Tolerates invalid whitespace and carriage return sequences, desyncing from proxies!
    while (p < end && *p != '\r' && *p != '\n') {
        p++; // Skips unvetted extension bytes
    }
    return 0;
}

БЕЗОПАСНОЕ ИСПРАВЛЕНИЕ

// SECURE: deps/llhttp/src/llhttp.c patch
int llhttp__on_chunk_extension(llhttp_t *parser, const char *p, const char *end) {
    // 1. Strictly validate characters inside chunk extensions according to RFC 9112
    while (p < end && *p != '\r' && *p != '\n') {
        uint8_t ch = (uint8_t)*p;
        // Disallow spaces, control codes, and invalid token bytes
        if (ch <= 0x20 || ch >= 0x7F || ch == ';') {
            return HPE_INVALID_CHUNK_SIZE; // Reject immediately!
        }
        p++;
    }
    return 0;
}

Инженерный чек-лист для код-ревью и защиты систем

Strictly enforce RFC 9112 compliance regarding chunk extensions and header delimitations.
Configure frontend reverse proxies to normalize or disallow ambiguous Transfer-Encoding chunk extensions.
Use HTTP/2 or HTTP/3 end-to-end between reverse proxies and backend Node.js services to eliminate framing ambiguities.
Update Node.js across all production containers to versions containing the patched llhttp parser.

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