VLESS Vulnerability: bypassing split tunneling in clients

Critical VLESS Client Flaw: Bypassing Split Tunneling and IP Leaks

All popular mobile VLESS clients based on xray/sing-box run an unauthenticated local SOCKS5 proxy. This lets spyware bypass per-app split tunneling and private spaces (like Knox, Shelter, or Island) by directly scanning localhost ports and pulling the proxy's exit IP.

The standard traffic flow is:

VpnService -> tun2socks -> xray socks5 -> VPN server -> freedom

Spyware connects to SOCKS5 while skipping VpnService. Private spaces isolate VpnService but not the loopback interface, leaving the proxy exposed.

Regulators' playbooks list common ports: SOCKS (1080, 9000, 5555, 16000-16100), HTTP (80, 443, 3128). This technique has been used in the wild.

Tested Clients

Tested March 10, 2026; developers notified. As of April 7, 2026, no fixes:

• Happ: Critically vulnerable, dumps configs via unauthenticated xray API (HandlerService). Traffic decryption possible when combined with another xray flaw.
• v2RayTun: Vulnerable.
• V2BOX: Vulnerable.
• v2rayNG: Vulnerable.
• Hiddify: Vulnerable.
• Exclave: Vulnerable.
• Npv Tunnel: Vulnerable.
• Neko Box: Vulnerable.

Clash and sing-box in standard setups are also at risk. The fix requires SOCKS5 authentication, but UDP stays unprotected.

Happ Vulnerability Details

Happ enables unauthenticated xray API via HandlerService, exposing configs: keys, inbound IP, SNI. Reverse engineering found no justification—xray restarts on config changes anyway. Devs cited stats needs (LogService would suffice) but refused fixes. Updates blocked after App Store removal.

Recommendation: Server-side blocks via UserAgent Happ/*. One compromised Happ client can leak your entire server to censors.

Protection Steps

No client is fully secure. iOS woes worsened by app store removals.

• Split inbound and outbound IPs. Alternative: Wrap outbound traffic in CloudFlare WARP.
• Selective routing: geoip:ru -> direct, everything else -> proxy.
• Server-side block of geoip:ru to dodge traffic pattern analysis from spy apps (Yandex, Wildberries, Ozon).

Windows example: v2rayN with "Everything except Russia" preset.

Key Takeaways

• The flaw lets spyware scan localhost, grab proxy IPs, and dodge VpnService plus private spaces.
• Happ also leaks configs via unsecured xray API, risking traffic decryption.
• No clients patched; fix needs SOCKS5 auth + dropping UDP.
• Protect with IP splitting, geoip routing, Happ/RU blocks on servers.
• Technique's been exploited before; regulators know about it.

— Editorial Team