Linux/WSL2: Cloudflare 403 blocks all chatgpt.com API requests — rustls TLS fingerprint detected as bot while macOS native-tls works fine on same network

Description

Codex CLI on Linux (WSL2) is completely unusable with ChatGPT login — every request to chatgpt.com/backend-api/ returns HTTP 403 with a Cloudflare JavaScript challenge page. The same account, same proxy node, same network works perfectly on macOS.

Root Cause Analysis

After binary analysis, the difference is the TLS implementation:

| | macOS binary | Linux binary |
|---|---|---|
| TLS library | native-tls (SecureTransport) | rustls 0.23.36 |
| JA3 fingerprint | Browser-like (Safari-similar) | Automated client signature |
| Cloudflare result | ✅ Pass | ❌ 403 challenge |

The Linux binary (x86_64-unknown-linux-musl) is statically compiled with rustls + rama-tls-rustls, producing a TLS Client Hello that Cloudflare's bot detection flags as non-browser traffic. The macOS binary uses the system TLS stack (SecureTransport), whose fingerprint resembles Safari and passes Cloudflare without challenge.

Evidence from binary strings:

# 97 references to rustls vs 3 to native-tls in Linux binary
tokio-rustls-0.26.4
rama-tls-rustls-0.3.0-alpha.4
rustls-0.23.36
utils/rustls-provider/src/lib.rs

Impact

This affects all chatgpt.com endpoints on Linux, not just login:

• ❌ chatgpt.com/backend-api/codex/responses — can't send/receive messages
• ❌ chatgpt.com/backend-api/plugins/featured — startup hangs loading plugins
• ❌ Plugin marketplace — can't refresh plugin list
• ❌ OAuth token refresh — sessions can't renew

Codex launches, MCP servers start, but the agent never responds to any input.

Environment

• Codex version: 0.120.0
• OS: Ubuntu 24.04 on WSL2 (Windows 11, kernel 6.6.87.2)
• Install: npm global (@openai/codex)
• Proxy: Clash Verge mixed mode on port 7890 (same proxy used on macOS where it works)
• Exit IP: Zenlayer US (Los Angeles) — same node on both platforms

Reproduction

# On Linux/WSL2:
curl -I --proxy http://127.0.0.1:7890 https://chatgpt.com/backend-api/plugins/featured
# → HTTP 403, cf-mitigated: challenge

# On macOS (same proxy, same node):
# → Works fine

Logs

WARN codex_core::plugins::manager: failed to warm featured plugin ids cache
  error=remote plugin sync request to https://chatgpt.com/backend-api/plugins/featured
  failed with status 403 Forbidden: <html>...<span id="challenge-error-text">
  Enable JavaScript and cookies to continue</span>...

WARN codex_tui::chatwidget: failed to load full apps list;
  falling back to installed apps snapshot: Failed to load apps:
  Request failed with status 403 Forbidden

Suggested Fix

1. Short-term: Allowlist the Codex CLI's User-Agent / request pattern in Cloudflare WAF rules for chatgpt.com
2. Long-term: Use native-tls (OpenSSL) on Linux instead of rustls, or implement JA3 fingerprint randomization via rama-tls-rustls (which is already bundled but apparently not configured for fingerprint emulation)

Workaround

Using OPENAI_API_KEY instead of ChatGPT login bypasses chatgpt.com entirely (routes through api.openai.com which is not affected).

Related Issues

• #16052 — Cloudflare challenge blocks CLI token exchange (Linux)
• #16808 — Plugin marketplace unreachable due to Cloudflare 403
• #16543 — Startup hangs when plugins/featured returns Cloudflare challenge
• #16618 — Windows desktop app gets Cloudflare 403 on plugin/connectors discovery
• #14215 — Third-party OAuth client 403 due to proxy (different root cause but same symptom)