---
title: "How we found a bug in the hyper HTTP library"
url: https://daily.dev/posts/how-we-found-a-bug-in-the-hyper-http-library-6s5z0vcux
source_url: https://blog.cloudflare.com/hyper-bug
type: article
source: "Cloudflare"
published: 2026-06-22T18:50:00.975Z
updated: 2026-06-23T06:20:23.332Z
tags: ["rust", "cloudflare"]
reading_time: 17
upvotes: 47
comments: 2
language: en
---

> ## Documentation Index
> Fetch the complete documentation index at: https://daily.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# How we found a bug in the hyper HTTP library

**[Cloudflare](https://daily.dev/sources/cloudflare)** · 17 min read · 47 upvotes · 2 comments

## Summary

Cloudflare's Images team discovered a race condition bug in the open-source hyper HTTP library (affecting versions 0.14 through 1.8) while rearchitecting the Images binding to use Unix sockets instead of FL. The bug caused intermittent response truncation for large images: hyper's dispatch loop discarded the `Poll::Pending` return value from `poll_flush`, allowing the connection to shut down before all buffered data was written to the socket. The issue only surfaced in production with real concurrency and a reader that occasionally paused, never appearing with curl or local tests. The team used `strace` to observe actual syscalls and confirm the premature `SHUT_WR` call. The fix — four lines added to `poll_shutdown` to flush remaining data before closing — was contributed upstream as PR #4018 to hyperium/hyper.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://blog.cloudflare.com/hyper-bug>

## Community discussion

Top comments from developers on daily.dev.

**@ivan0x9** · 1 upvotes

> Good catch. And it always happens that in production a new bug happens

**@jntnlima** · 0 upvotes

> ![GIF](https://static.klipy.com/ii/c3a19a0b747a76e98651f2b9a3cca5ff/99/5a/CSV4BUWG.gif)

## Similar posts on daily.dev

- [Cloudflare Identifies Race Condition in hyper’s HTTP/1 Implementation](https://daily.dev/posts/cloudflare-identifies-race-condition-in-hyper-s-http-1-implementation-znn26hg1e) · InfoQ · 9 upvotes · 0 comments
- [The Case of the Readable Dead Connection: A Ruby Mystery](https://daily.dev/posts/the-case-of-the-readable-dead-connection-a-ruby-mystery-smy8cygd3) · RUBYLAND · 3 upvotes · 1 comments

---

Tags: [#rust](https://daily.dev/tags/rust), [#cloudflare](https://daily.dev/tags/cloudflare)

[View this post on daily.dev](https://daily.dev/posts/how-we-found-a-bug-in-the-hyper-http-library-6s5z0vcux)