CVE-2021-34705: Cisco IOS and IOS XE Software FXO Interface Destination Pattern Bypass Vulnerability

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.0% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-09-23 · updated 2026-06-17
Weakness: CWE-232

Description

A vulnerability in the Voice Telephony Service Provider (VTSP) service of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to bypass configured destination patterns and dial arbitrary numbers. This vulnerability is due to insufficient validation of dial strings at Foreign Exchange Office (FXO) interfaces. An attacker could exploit this vulnerability by sending a malformed dial string to an affected device via either the ISDN protocol or SIP. A successful exploit could allow the attacker to conduct toll fraud, resulting in unexpected financial impact to affected customers.

Affected products

Cisco IOS: 12.3(7)xm, 12.3(7)xr, 12.3(7)xr1, 12.3(7)xr2, 12.3(7)xr3, 12.3(7)xr4 (+1433)
Cisco IOS XE: 3.7.0bs, 3.7.0s, 3.7.0xas, 3.7.0xbs, 3.7.1as, 3.7.1s (+275)

References

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