Critical Vulnerability Alert: CVE-2025-30208 in AetherProxy

August 12, 2025
Scott Wu

A high-severity HTTP/2 Request Smuggling vulnerability, nicknamed “Phantom Tunnel,” has been discovered in the AetherProxy gateway. Tracked as CVE-2025-30208, this flaw has a CVSS score of 8.6 (High) and could allow attackers to bypass security controls, hijack user sessions, and access sensitive data.

If you’re using AetherProxy, you need to act now.

TL;DR:

  • What: A critical HTTP/2 Request Smuggling vulnerability in AetherProxy.
  • Affected Versions: All versions prior to 5.1.3.
  • Impact: Allows an attacker to bypass web application firewalls (WAFs), poison web caches, and potentially take over user accounts.
  • Action: Update to version 5.1.3 or newer immediately.

What is AetherProxy?

AetherProxy is a popular, high-performance API gateway and reverse proxy used extensively in modern cloud-native and microservices architectures. It sits between clients and backend services, handling traffic routing, load balancing, authentication, and security.

The “Phantom Tunnel” Vulnerability Explained

HTTP Request Smuggling is an attack technique that exploits discrepancies in how a chain of HTTP servers (like a proxy and a backend server) process ambiguous requests. CVE-2025-30208 is a new variant that specifically targets the HTTP/2 protocol.

Here’s the problem: AetherProxy’s HTTP/2 frame processing logic can be desynchronized from the backend server’s logic. An attacker can send a carefully crafted sequence of HTTP/2 HEADERS and DATA frames. AetherProxy misinterprets the request boundaries but forwards the raw stream to the backend, which interprets it differently.

This allows the attacker to “smuggle” a second, malicious request inside the first one. Your proxy sees one safe request, but the backend server sees two separate requests: the safe one, followed by the attacker’s smuggled one.

Because the smuggled request appears to originate from the proxy itself, it often bypasses security rules and is processed with a higher level of trust. For example, an attacker could smuggle a request to an internal admin endpoint like /admin/delete_user?id=123 that would normally be blocked if sent directly.

Impact and Are You Affected?

The consequences of this vulnerability are serious. A successful exploit can lead to:

  • Security Bypass: Evading WAF and other rule-based security systems.
  • Session Hijacking: Stealing session cookies of other users whose requests are processed immediately after the attacker’s.
  • Cache Poisoning: Tricking the proxy into caching malicious content and serving it to legitimate users.
  • Account Takeover: Using smuggled requests to perform actions on behalf of other users.

You are vulnerable if your infrastructure uses an AetherProxy instance with a version prior to 5.1.3 and has HTTP/2 enabled on its public-facing listeners.

🛡️ Remediation: How to Secure Your Systems

The AetherProxy development team has already released a patch that correctly normalizes and validates HTTP/2 streams.

  1. Update AetherProxy: The most important step is to upgrade all instances of AetherProxy to version 5.1.3 or newer. This version contains the patched code that resolves the desynchronization issue.
  2. Monitor Your Systems: After updating, closely monitor your access logs for any unusual or malformed requests that could indicate attempted exploitation. Look for requests to sensitive endpoints that don’t align with expected application behavior.

Resources and Citations

This vulnerability was discovered by a team of independent security researchers, who have provided a detailed technical analysis.

  • Official AetherProxy Website: https://aetherproxy.io
  • Official GitHub Repository: https://github.com/AetherProxy/AetherProxy
  • Official Security Advisory: https://github.com/AetherProxy/AetherProxy/security/advisories/GHSA-fictional-wxyz-9876
  • NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2025-30208
  • Discoverer’s Technical Blog: A fantastic technical deep-dive can be found here: https://portswigger.net/blog/http2-request-smuggling-reborn

Don’t wait for this vulnerability to be exploited in the wild. Patch your systems today.

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