# apernet Hysteria Denial of Service in HTTP sniffing

Source: https://junglewise.ai/threats/apernet-hysteria-denial-of-service-in-http-sniffing-904c3563  
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 popular network proxy tool. A vulnerability in its protocol sniffing feature allows an attacker to crash the server or cause it to run out of memory by sending specially crafted, excessively large HTTP headers. This can lead to a total service outage for all users connected to the proxy.

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: Version 2.9.2 released 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

The vulnerability exists in the 'sniff' implementation of the Hysteria server. When protocol sniffing is enabled, the server attempts to identify the protocol of incoming connections. However, it does not enforce a maximum size limit for HTTP headers. An attacker can establish a connection and continuously stream large amounts of header data without the terminating CRLF sequence. This causes the server to buffer the data indefinitely until it exhausts available system memory (OOM), resulting in a Denial of Service. 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: GitHub Advisory published
- 2026-06-26: patched: Version 2.9.2 released

## 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 server crash via small max_datagram_frame_size](https://junglewise.ai/threats/apernet-hysteria-server-crash-via-small-max-datagram-frame-size-34b2834d)
- [apernet Hysteria UDP ACL bypass and SSRF](https://junglewise.ai/threats/apernet-hysteria-udp-acl-bypass-and-ssrf-b247b4ec)