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

> Remote attackers can bypass destination patterns on Cisco IOS FXO interfaces to dial arbitrary numbers, enabling toll fraud and financial loss.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 23 Sep 2021
- **Updated:** 16 Sep 2024
- **Also known as:** GHSA-xxj5-mhw2-jgp8

## What is CVE-2021-34705?

CVE-2021-34705 is a medium-severity vulnerability in Cisco Ios, published on 23 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-232 (Improper Handling of Undefined Values)

## Which versions are affected by CVE-2021-34705?

It affects 2 products.


Products: Cisco Ios, Cisco Ios Xe.

## How do I fix CVE-2021-34705?

No fix has been published for CVE-2021-34705 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-34705 being exploited?

There is no known exploitation of CVE-2021-34705 in the wild and no public exploit code.

## Other vulnerabilities in Cisco Ios

- [CVE-2017-12240](https://insights.sbomify.com/CVE-2017-12240/): Cisco IOS and IOS XE Software DHCP Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2017-3881](https://insights.sbomify.com/CVE-2017-3881/): Cisco IOS and IOS XE Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2018-0167](https://insights.sbomify.com/CVE-2018-0167/): Cisco IOS, XR, and XE Software Buffer Overflow (high 8.8, exploited in the wild)
- [CVE-2017-6736](https://insights.sbomify.com/CVE-2017-6736/): Cisco IOS and IOS XE Software SNMP Remote Code Execution (high 8.8, exploited in the wild)
- [CVE-2017-6737](https://insights.sbomify.com/CVE-2017-6737/): Cisco IOS and IOS XE Software SNMP Remote Code Execution (high 8.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-34705 and fix it if it is affected.

What it is: CVE-2021-34705 is a medium-severity vulnerability in Cisco Ios, published on 23 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-34705 in the wild and no public exploit code.

Affected product: Cisco Ios. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-34705/ (Markdown: https://insights.sbomify.com/CVE-2021-34705/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34705
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34705
- OSV: https://osv.dev/vulnerability/CVE-2021-34705

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-34705/
Summaries are generated and may contain mistakes; the official description above is the record.