# apernet Hysteria server crash via small max_datagram_frame_size

Source: https://junglewise.ai/threats/apernet-hysteria-server-crash-via-small-max-datagram-frame-size-34b2834d  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-26. Updated: 2026-06-26  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/hysteria), [github.com/apernet/hysteria (Go)](https://junglewise.ai/threats/technologies/github-com-apernet-hysteria). Vendors: [Apernet](https://junglewise.ai/threats/vendors/apernet), [Go](https://junglewise.ai/threats/vendors/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

- [https://api.github.com/users/Cherrling](https://api.github.com/users/Cherrling)
- [https://github.com/Cherrling](https://github.com/Cherrling)
- [https://api.github.com/users/Cherrling/gists%7B/gist_id%7D](https://api.github.com/users/Cherrling/gists%7B/gist_id%7D)
- [https://api.github.com/users/Cherrling/repos](https://api.github.com/users/Cherrling/repos)
- [https://avatars.githubusercontent.com/u/37233261?v=4](https://avatars.githubusercontent.com/u/37233261?v=4)
- [https://api.github.com/users/Cherrling/events%7B/privacy%7D](https://api.github.com/users/Cherrling/events%7B/privacy%7D)

## Related threats

- [GO-2026-5813 - Hysteria: http large header with sniff cause server DoS in github.com/apernet/hysteria](https://junglewise.ai/threats/go-2026-5813-hysteria-http-large-header-with-sniff-cause-server-dos-in-28a9ac9e)
- [GO-2026-5807 - Hysteria vulnerable to server crash when max_datagram_frame_size very small in github.com/apernet/hysteria](https://junglewise.ai/threats/go-2026-5807-hysteria-vulnerable-to-server-crash-when-max-datagram-8f887c7f)
- [apernet Hysteria Denial of Service in HTTP sniffing](https://junglewise.ai/threats/apernet-hysteria-denial-of-service-in-http-sniffing-904c3563)
- [apernet Hysteria UDP ACL bypass and SSRF](https://junglewise.ai/threats/apernet-hysteria-udp-acl-bypass-and-ssrf-b247b4ec)