CVE-2021-34749Public exploitNetwork attack vector

Data exfiltration via inadequate TLS SNI inspection

Published Aug 18, 2021 · Updated Nov 7, 2024

Filter bypass in Cisco Secure Web Appliance allows remote attackers to exfiltrate data from a compromised host through TLS ClientHello SNI. Its web-filtering path inadequately inspects the client-controlled SNI header, allowing data embedded in the TLS handshake to reach blocked servers. Exposure requires transparent mode with HTTPS Proxy and a decryption policy enabled, plus an already compromised protected-network host.

CVSS severity5.8
Medium
EPSS probability1.68%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Filter bypass in Cisco Secure Web Appliance allows remote attackers to exfiltrate data from a compromised host through TLS ClientHello SNI. Its web-filtering path inadequately inspects the client-controlled SNI header, allowing data embedded in the TLS handshake to reach blocked servers. Exposure requires transparent mode with HTTPS Proxy and a decryption policy enabled, plus an already compromised protected-network host.

The record

CVE
CVE-2021-34749
Published
Aug 18, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco Secure Web Appliance
Classifications
CWE-200, T1041
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 18, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 7, 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=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco Secure Web Applianceversion=n/a

Published CVSS scores

5.8Cisco Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N/E:X/RL:X/RC:X
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:P/A:N
8.6NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/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
T1041Exfiltration Over C2 Channel

Public exploit references

  • SNIcatproof of concept · demonstrated

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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