CVE-2026-47857: In Reactor Core, applications that use the Flux.windowTimeout operator with fairBackpressure enabled are vulne

Routine

MEDIUM 5.9REMOTEUPGRADE

CVE-2026-47857 is a medium-severity flaw (CVSS 5.9) affecting reactor core. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 70%)
CISA exploited list	No
Weakness	CWE-190
Published	2026-08-27
Last modified	2026-09-04

Publisher (CNA): VMware · Vendor: broadcom

Last updated 2026-09-04 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-27 01:17 UTCPublished CVE record published
2026-09-04 20:10 UTCUpdated Record last modified

broadcom reactor core UPGRADE

Affected: < 3.4.42 · >= 3.5.0 and < 3.7.20 · >= 3.8.0 and < 3.8.6.1

Fixed in: 3.4.42, 3.7.20, 3.8.6.1

Safe to downgrade to: a release before 3.5.0

1. Check installed version

apt-cache policy reactor-core || rpm -q reactor-core || reactor-core --version

2. UPGRADE to 3.4.42 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade reactor-core

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade reactor-core

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.5.0 (Debian / Ubuntu)

apt-cache madison reactor-core sudo apt install reactor-core=<VERSION_BEFORE_3.5.0> sudo apt-mark hold reactor-core

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list reactor-core sudo dnf downgrade reactor-core-<VERSION_BEFORE_3.5.0> sudo dnf versionlock add reactor-core

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Advisories and coverage

Official advisories

Spring Security Advisories: cve-2026-47857 - MEDIUM - Reactor Core windowTimeout fair-backpressure stream hang due to 20-bit index wrap-around 2026-08-20

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-47857?

reactor core: upgrade to 3.4.42, 3.7.20, 3.8.6.1. Exact commands for each OS are on this page.

Is CVE-2026-47857 being exploited?

CVE-2026-47857 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-47857 affect?

CVE-2026-47857 affects reactor core. Severity is medium (CVSS 5.9). 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 earli

How urgent is CVE-2026-47857?

Priority 33/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-47863 MEDIUM 5.9 UPGRADE In Reactor Core, applications that use the Flux.bufferTimeout operator with fairBackpressure enabled are vulne

References

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