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 7.5, published 26 June 2026.

Which products and versions are affected?

Apernet Hysteria < 2.9.2; github.com/apernet/hysteria (Go) < 2.9.2.

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.

Check whether you run Apernet Hysteria or github.com/apernet/hysteria (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.

apernet Hysteria server crash via small max_datagram_frame_size

Q: Which products and versions are affected?

Apernet Hysteria < 2.9.2; github.com/apernet/hysteria (Go) < 2.9.2.

Q: 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.

Q: How is This vulnerability attacked?

It is attacked over the network, in a software library.

Q: Am I affected?

Check whether you run Apernet Hysteria or github.com/apernet/hysteria (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.

Severity: high · CVSS 7.5 · Published 2026-06-26

Hysteria is a network proxy tool used to optimize and secure internet connections. A vulnerability allows an authenticated user to crash the Hysteria server by sending specially crafted network settings. This results in a denial-of-service, making the proxy service unavailable to all users until it is manually restarted.

Technologies: Apernet Hysteria, github.com/apernet/hysteria (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 7.5, published 26 June 2026.
Which products and versions are affected?: Apernet Hysteria < 2.9.2; github.com/apernet/hysteria (Go) < 2.9.2.
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 (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

A slice bounds panic exists in the Hysteria server's UDP fragmentation logic. When an authenticated client advertises a very small QUIC 'max_datagram_frame_size', the server's 'sendMessageAutoFrag' function attempts to fragment UDP responses. The 'FragUDPMessage' function in 'core/internal/frag/frag.go' fails to account for cases where the maximum allowed payload size is smaller than the UDP message header itself. This leads to a negative or zero 'maxPayloadSize' calculation, causing a runtime panic and immediate termination of the server process. The issue is fixed in version 2.9.2.

Affected products

apernet hysteria < 2.9.2

Timeline

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

References

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