Security readout for executives and security teams
Plain-English summary
This Cisco IOS/IOS XE voice issue can let a remote unauthenticated attacker bypass calling restrictions on FXO interfaces and dial arbitrary numbers. The main business risk is toll fraud and unexpected telecom charges, not data theft or outage based on the provided scoring.
Executive priority
Treat as a targeted telecom fraud risk. Prioritize organizations with Cisco voice gateways, FXO lines, and externally reachable SIP or ISDN paths, especially where toll charges are material or hard to detect quickly.
Technical view
The VTSP service insufficiently validates dial strings at FXO interfaces. A malformed dial string delivered over ISDN or SIP may bypass configured destination patterns, allowing unauthorized call placement. CVSS 3.1 is 5.3 with network attack vector, low complexity, no privileges, and low integrity impact.
Likely exposure
Exposure is most likely where Cisco IOS or IOS XE devices provide voice services using FXO interfaces and destination-pattern restrictions, with ISDN or SIP paths reachable by untrusted callers or networks.
Exploitation context
The source bundle does not show CISA KEV listing or cited evidence of active exploitation. The described impact is toll fraud through unauthorized dialing after destination-pattern bypass, causing financial exposure.
Researcher notes
Do not assume broader Cisco platform exposure beyond the cited IOS/IOS XE VTSP and FXO context. The provided bundle does not include fixed versions, workaround details, or active exploitation evidence; validate those directly against Cisco's advisory.
Mitigation direction
- Check Cisco's advisory for affected releases, fixed software, and any documented workarounds.
- Inventory Cisco IOS/IOS XE voice devices using FXO interfaces.
- Restrict untrusted SIP or ISDN access to voice gateways where possible.
- Review destination-pattern and toll-control configuration on exposed gateways.
- Monitor telecom bills and call-detail records for unexpected destinations.
Validation and detection
- Confirm whether deployed Cisco voice gateways use FXO interfaces.
- Identify IOS or IOS XE versions against Cisco advisory guidance.
- Review logs and call-detail records for destination-pattern bypass indicators.
- Verify SIP and ISDN exposure paths to affected devices.
- Confirm compensating controls limit unauthorized call origination.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
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ATT&CK lookup starting points
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Open ATT&CK lookupCVE-2021-34705 mapping review
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Open ATT&CK lookup- Severity
- Medium
- CVSS
- 5.3 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N3.91.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.3MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- 20210922 Cisco IOS and IOS XE Software FXO Interface Destination Pattern Bypass VulnerabilityCVE reference · vendor-advisory, x_refsource_CISCO
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Handling of Undefined Values
Improper Handling of Undefined Values represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.