CVE-2021-3773Public exploitNetwork attack vector

OpenVPN client deanonymization via Netfilter port shadowing

Published Feb 16, 2022 · Updated Mar 28, 2025

Information disclosure in Linux Kernel Netfilter allows remote attackers to infer OpenVPN client endpoint information. Netfilter NAT creates connection-tracking mappings without coordinating translated source ports with sockets listening on the VPN server, allowing a crafted mapping to shadow the server port. The attack requires reachability to the same OpenVPN server and spoofed response traffic; it reveals whether a candidate client is connected and can establish an interception position.

CVSS severity5.9
Medium
EPSS probability5.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Linux Kernel Netfilter allows remote attackers to infer OpenVPN client endpoint information. Netfilter NAT creates connection-tracking mappings without coordinating translated source ports with sockets listening on the VPN server, allowing a crafted mapping to shadow the server port. The attack requires reachability to the same OpenVPN server and spoofed response traffic; it reveals whether a candidate client is connected and can establish an interception position.

The record

CVE
CVE-2021-3773
Published
Feb 16, 2022
Updated
Mar 28, 2025
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-200, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 16, 2022CVE publishedPublication date reported by the CVE source.
  2. Mar 28, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=kernel 5.14.0-49.el9, kernel 5.15.15-100.fc34, kernel 5.15.15-200.fc35

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linux Kernelversion=kernel 5.14.0-49.el9, kernel 5.15.15-100.fc34, kernel 5.15.15-200.fc35

Published CVSS scores

9.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
5.9Red HatCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

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 exploitPublished

Weakness, pattern, technique

CWE-200Exposure of Sensitive Information to an Unauthorized Actor
T1557Adversary-in-the-Middle

Public exploit references

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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