# CVE-2026-47857: Reactor Core windowTimeout fair-backpressure stream hang due to 20-bit index wrap-around

> Attackers can cause a stream hang and denial of service in Reactor Core applications using windowTimeout with fairBackpressure due to an index wrap-around.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Aug 2026
- **Updated:** 27 Aug 2026
- **Also known as:** CLEANSTART-2026-AM40974, CLEANSTART-2026-OE47351, CLSA-2026-1791442790, CLSA-2026-1791455070, CLSA-2026-1791457895, CLSA-2026-1791972789, CLSA-2026-1792078057, CLSA-2026-1792164225, CLSA-2026-1792395134, CLSA-2026-1792395135, CLSA-2026-1792395162, CLSA-2026-1792395647, ECHO-7436-73b1-3ab0, GHSA-p9xf-3946-vr35

## What is CVE-2026-47857?

CVE-2026-47857 is a medium-severity vulnerability in Spring Reactor Core, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In Reactor Core, applications that use the Flux.windowTimeout operator with fairBackpressure enabled are vulnerable to a Denial of Service (DoS) condition.
Reactor Core 3.8.0 - 3.8.6
Reactor Core 3.5.0 - 3.7.19
Reactor Core 3.4.41 and earlier

Weakness: CWE-190 (Integer Overflow or Wraparound)

## Which versions are affected by CVE-2026-47857?

It affects 1 product.


Products: Broadcom Reactor Core.

## How do I fix CVE-2026-47857?

Upgrade to a fixed version.

## Is CVE-2026-47857 being exploited?

There is no known exploitation of CVE-2026-47857 in the wild and no public exploit code.

## Other vulnerabilities in Spring Reactor Core

- [CVE-2026-47863](https://insights.sbomify.com/CVE-2026-47863/): Denial of service in Reactor Core (medium 5.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-47857 and fix it if it is affected.

What it is: CVE-2026-47857 is a medium-severity vulnerability in Spring Reactor Core, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-47857 in the wild and no public exploit code.

Affected product: Spring Reactor Core. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-47857/ (Markdown: https://insights.sbomify.com/CVE-2026-47857/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-47857
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-47857
- OSV: https://osv.dev/vulnerability/CVE-2026-47857

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-47857/
Summaries are generated and may contain mistakes; the official description above is the record.