apernet Hysteria UDP ACL bypass and SSRF

Severity: high · CVSS 7.4 · Published 2026-06-26

Hysteria, a popular network proxy, contains a vulnerability where its security rules for UDP traffic can be bypassed by authenticated users. An attacker can establish a legitimate connection and then reuse that session to send data to restricted internal locations, such as the server's own local services or private corporate network. This could lead to unauthorized access to internal DNS resolvers, management tools, or other private infrastructure.

Technologies: Apernet Hysteria, github.com/apernet/hysteria/core/v2 (Go). Vendors: Apernet, Go.

Quick answers

Is This vulnerability exploited in the wild?
No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
How severe is This vulnerability?
High severity, with a CVSS score of 7.4, published 26 June 2026.
Which products and versions are affected?
Apernet Hysteria >= 2.0.0, <= 2.9.1; github.com/apernet/hysteria/core/v2 (Go) >= 2.0.0, <= 2.9.1.
Is there a fix for This vulnerability?
A patch was published on 2026-06-26: Fixed in version 2.9.2 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is This vulnerability attacked?
It is attacked over the network, in a software library.
Am I affected?
Check whether you run Apernet Hysteria or github.com/apernet/hysteria/core/v2 (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

Hysteria's UDP relay implementation incorrectly caches Access Control List (ACL) and outbound policy evaluations at the session level rather than the packet level. While the Hysteria protocol allows each UDP message within a session to specify a different destination address, the server only performs ACL validation on the first packet of a session. Subsequent packets in the same session are forwarded using 'e.conn.WriteTo' without re-running policy checks. An authenticated attacker can exploit this by initiating a session to a permitted destination and then sending subsequent packets to restricted destinations like 127.0.0.1 or RFC1918 addresses. This results in a Server-Side Request Forgery (SSRF) that bypasses operator-configured egress restrictions. The issue is fixed in version 2.9.2.

Affected products

  • apernet hysteria/core/v2 >= 2.0.0, <= 2.9.1

Timeline

  • 2026-05-23: disclosed
  • 2026-06-26: advisory
  • 2026-06-26: patched: Fixed in version 2.9.2

References

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