CVE-2021-41530Network attack vector

Traffic amplification via pre-handshake HTTP response

Published Oct 4, 2021 · Updated Aug 4, 2024

TCP reflection in Forcepoint NGFW Engine 6.5.11 and earlier, 6.8.6 and earlier, and 6.10.0 allows remote attackers to amplify traffic. The engine sends a comparatively large static HTML response before completing the TCP handshake when an initial SYN, or an immediate PSH+ACK, carries a request for a blocked URL. Exploitation requires HTTP User Response configuration; spoofed requests can redirect at least 100 times their traffic volume toward another address.

CVSS severity7.5
High
EPSS probability0.93%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

TCP reflection in Forcepoint NGFW Engine 6.5.11 and earlier, 6.8.6 and earlier, and 6.10.0 allows remote attackers to amplify traffic. The engine sends a comparatively large static HTML response before completing the TCP handshake when an initial SYN, or an immediate PSH+ACK, carries a request for a blocked URL. Exploitation requires HTTP User Response configuration; spoofed requests can redirect at least 100 times their traffic volume toward another address.

The record

CVE
CVE-2021-41530
Published
Oct 4, 2021
Updated
Aug 4, 2024
Vendor
Forcepoint
Product
Forcepoint NGFW Engine
Classifications
T1498.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 4, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=NGFW Engine versions 6.5.11 and earlier, 6.8.6 and earlier, and 6.10.0 are all vulnerable, if HTTP User Response has been configured.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Forcepoint · Forcepoint NGFW Engineversion=NGFW Engine versions 6.5.11 and earlier, 6.8.6 and earlier, and 6.10.0 are all vulnerable, if HTTP User Response has been configured.

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1498.002Reflection Amplification

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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