dbcve.org
iOSOperating system · Cisco

CVE-2021-34705

MEDIUM · 5.3 CVSS v3.1 Published 2021-09-23
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A vulnerability in the Voice Telephony Service Provider (VTSP) service of Cisco IOS/IOS XE Software allows unauthenticated remote attackers to bypass configured destination patterns and dial arbitrary numbers due to insufficient validation of dial strings at Foreign Exchange Office (FXO) interfaces. Attackers exploit this by sending malformed dial strings via ISDN protocol or SIP, enabling toll fraud.

MitigationApply Cisco-provided software patches when available; until then, consider implementing additional call routing controls or restricting FXO interface exposure to reduce attack surface.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
iOSOperating system
Affected:= 12.3\(7\)xm= 12.3\(7\)xr= 12.3\(7\)xr1= 12.3\(7\)xr2= 12.3\(7\)xr3= 12.3\(7\)xr4= 12.3\(7\)xr5= 12.3\(7\)xr6= 12.3\(7\)xr7= 12.3\(8\)ja2= 12.3\(8\)jea1= 12.3\(8\)jea2
Ios XeOperating system
Affected:all versions= 3.7.0bs= 3.7.0s= 3.7.0xas= 3.7.0xbs= 3.7.1as= 3.7.1s= 3.7.2s= 3.7.2ts= 3.7.3s= 3.7.4as= 3.7.4s

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the device OS and version
    Run 'show version' on the Cisco device CLI to obtain the IOS or IOS XE version number
    Affected if The version matches any of the affected versions listed in the CVE (12.3(7)xm, 12.3(7)xr variants, 12.3(8)ja2, 12.3(8)jea1, 12.3(8)jea2 for iOS; any version for IOS XE; or versions 3.7.0bs through 3.7.4s)
  2. Determine if voice telephony features are enabled
    Run 'show telephony-service' or 'show voice service' to check if VTSP and voice features are configured
    Affected if Voice telephony service (VTSP) is enabled on the device
  3. Check for FXO interface configuration
    Run 'show interface' and look for FXO ports (e.g., 'FXO' in interface description or configuration), or use 'show running-config | include fxo'
    Affected if Foreign Exchange Office (FXO) interfaces are present and configured
  4. Verify ISDN or SIP voice configuration
    Run 'show running-config | include isdn' for ISDN or 'show sip' / 'show voip sip' for SIP configuration
    Affected if ISDN PRI/BRI or SIP trunking is configured for voice calls
  5. Check dial-peer configuration
    Run 'show dialplan number' or 'show running-config | include dial-peer' to examine destination patterns
    Affected if Dial peers with destination patterns are configured but may be bypassed by malformed dial strings

The device is affected if it runs a vulnerable IOS/IOS XE version AND has voice telephony (VTSP) with FXO interfaces and ISDN or SIP voice configuration enabled.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply Cisco-provided software patches when available; until then, consider implementing additional call routing controls or restricting FXO interface exposure to reduce attack surface.

Fix this in iOS Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing8.0 h
  • Review / QA4.0 h
18.0 hours of engineering $3,080
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34705 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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