flawopen.com/Teardowns/cve-2021-22960-nodejs-llhttp-request-smuggling

● CVE-2021-22960 · CVSS 6.5 · Moyenne
Recherche · FlawOpen

CVE-2021-22960: Node.js llhttp HTTP Request Smuggling Teardown

Analyse technique détaillée du code source et mesures de durcissement pour vulnerabilidade : How incomplete Transfer-Encoding chunk extension parsing in deps/llhttp caused HTTP request desynchronization, enabling cache poisoning and credential theft.

💡 Explication en Langage Simple (ELI5)

Analogie concrète : 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.

Concepts Clés et Termes

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).

Analyse de Cause Racine

La cause fondamentale provient de paramètres de limites non validés dans les systèmes open source, permettant une désynchronisation d'état et le contournement des contrôles de sécurité.

Déroulement de l'Attaque Étape par Étape

Step 1

Étape d'attaque : Send Ambiguous Request

Mécanisme technique d'exploitation : The attacker sends an HTTP request with chunk extensions containing unescaped control characters.

Step 2

Étape d'attaque : Frontend Interpretation

Mécanisme technique d'exploitation : The frontend proxy treats the entire payload as a single continuous request body.

Step 3

Étape d'attaque : Node.js Parser Desync

Mécanisme technique d'exploitation : Node.js's llhttp stops parsing prematurely, leaving the smuggled second request in the TCP buffer.

Step 4

Détournement : Victim Session Hijacking

Mécanisme technique d'exploitation : The next innocent user's request is prepended to the smuggled request, exfiltrating their session tokens.

Code Source : Vulnérable vs Sécurisé

IMPLÉMENTATION VULNÉRABLE
// 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;
}
PATCH SÉCURISÉ ET ROBUSTE
// 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;
}

Liste de Contrôle de Sécurité pour l'Ingénierie

Sources