CVE-2026-22019 – libcurl HTTP/2 CONNECT Tunnel Mixup

Program Code (C simulation)

// curl_http2_tunnel_mixup.c - Simulated libcurl handling CONNECT over HTTP/2
#include <stdio.h>
void handle_http2_stream(int stream_id) {
    // Vulnerability: after CONNECT, the proxy mixes streams with the tunneled data
    if (stream_id == 0) {
        printf("CONNECT to target\n");
    } else {
        // Data from another stream may leak into the tunnel
        printf("Stream %d data crosses tunnel boundary\n", stream_id);
    }
}
int main() {
    handle_http2_stream(0);
    handle_http2_stream(1);  // should be isolated
    return 0;
}

CVE-2026-22019 – libcurl HTTP/2 CONNECT Tunnel Stream Mixup

Overview

A vulnerability in libcurl’s HTTP/2 proxy CONNECT implementation fails to properly isolate streams after the tunnel is established. An attacker with control over one HTTP/2 stream can inject data into a separate tunnel, leading to response smuggling or credential interception.

Vulnerability Details

• Type: Stream Isolation Failure
• Impact: Data injection, man‑in‑the‑middle.
• Root Cause: After a CONNECT request completes, the proxy incorrectly demultiplexes streams, allowing cross‑stream data contamination.

Exploit Demonstration

Compile and run the simulation:

gcc -o curl_http2_tunnel curl_http2_tunnel_mixup.c
./curl_http2_tunnel

Output shows stream crossing the tunnel boundary.