CVE-2021-1356

What it is

Multiple vulnerabilities in the web UI of Cisco IOS XE Software could allow an authenticated, remote attacker with read-only privileges to cause the web UI software to become unresponsive and consume vty line instances, resulting in a denial of service (DoS) condition. These vulnerabilities are due to insufficient error handling in the web UI. An attacker could exploit these vulnerabilities by sending crafted HTTP packets to an affected device. A successful exploit could allow the attacker to cause the web UI software to become unresponsive and consume all available vty lines, preventing new session establishment and resulting in a DoS condition. Manual intervention would be required to regain web UI and vty session functionality. Note: These vulnerabilities do not affect the console connection.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account. Limited direct impact.

Exploit odds

1.0% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-03-24 21:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:31 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 1.0% in the next 30 days (EPSS).
• Severity: 4.3 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-03-24 21:15 UTC

Last modified

2026-06-17 03:31 UTC

Exploit probability

1.0% · EPSS, next 30 days

Weakness

CWE-20 · Improper input validation

Record status

Modified

Affected

• Cisco iOS Xe 3.15.1xbs
• Cisco iOS Xe 3.15.2xbs
• Cisco iOS Xe 17.1.1
• Cisco iOS Xe 17.1.1a
• Cisco iOS Xe 17.1.1s
• Cisco iOS Xe 17.1.1t
• Cisco iOS Xe 17.1.2
• Cisco iOS Xe 17.2.1

Attack profile CVSS 3.1 · 4.3

Attack vector

Network

Complexity

Low

Privileges needed

Low

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

Low

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-1356.md and follow it. Check whether this project is affected by CVE-2021-1356. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

Copy as ticket · More options · Status badge