CVE-2021-34705Network attack vector

Toll fraud via malformed FXO dial strings

Published Sep 23, 2021 · Updated Sep 16, 2024

Improper dial-string validation in Cisco IOS allows remote attackers to bypass FXO destination patterns and dial arbitrary numbers. The VTSP service accepts a malformed dial string over ISDN or SIP without enforcing configured wildcard destination patterns at an FXO interface. Exploitation requires a vulnerable device with that FXO configuration and incoming-call support, enabling unauthorized calls and toll fraud.

CVSS severity5.3
Medium
EPSS probability1.01%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper dial-string validation in Cisco IOS allows remote attackers to bypass FXO destination patterns and dial arbitrary numbers. The VTSP service accepts a malformed dial string over ISDN or SIP without enforcing configured wildcard destination patterns at an FXO interface. Exploitation requires a vulnerable device with that FXO configuration and incoming-call support, enabling unauthorized calls and toll fraud.

The record

CVE
CVE-2021-34705
Published
Sep 23, 2021
Updated
Sep 16, 2024
Vendor
ClamAV
Product
Cisco IOS
Classifications
CWE-232
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 23, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 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 IOSversion=n/a

Published CVSS scores

5.3CiscoCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N/E:X/RL:X/RC:X

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.

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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

CWE-232Improper Handling of Undefined Values

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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