CVE-2021-1220: Cisco IOS XE Software Web UI Denial of Service Vulnerabilities
Telemetry drift only. No threshold crossed and no authority ruling changed.
CVE record, CISA KEV and EPSS as of 2026-10-18.
CVEs like this one computed, by meaning
What to do now held facts and links
- No fix version is held. The CNA's first reference is the place to look: tools.cisco.com
- Record your decision, dated and frozen with today's evidence: patch by a date · mitigated by a compensating control · deferred · accepted the risk · not affected
- Check your version against the ranges this record publishes.
What it is stated by the CNA
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.
- weakness
- CWE-20
- affected
- Cisco IOS XE Software
- published ranges
- cisco ios xe 3.15.1xbs; cisco ios xe 3.15.2xbs; cisco ios xe 16.11.1; cisco ios xe 16.11.1a; and 31 more on the CVE page
- fixed in
- No fix version in held sources.
- severity
- 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
- assigned by
- cisco
- published
- 2021-03-24
check us tools.cisco.com
Exploit code held, dated
- exploit code
- None held in the PoC-in-GitHub feed. A public repository changes this brief.
What moved held, dated
| date | EPSS | event |
|---|---|---|
| 14 Apr 2021 | 0.00692 | |
| 6 Jan 2022 | 0.03031 | |
| 4 Feb 2022 | 0.05463 | |
| 1 Apr 2022 | 0.01055 | |
| 7 Mar 2023 | 0.00128 | scoring model v2023.03.01 |
| 17 Mar 2025 | 0.00186 | scoring model v2025.03.14 |
| 28 Dec 2025 | 0.00205 | |
| 26 Mar 2026 | 0.002 | |
| 15 Jun 2026 | 0.00944 | scoring model v2026.06.15 |
| 13 Oct 2026 | 0.00977 |
22 observations held. A step marked with a scoring model moved because the measurement was replaced, not because the vulnerability changed.
Outside the record researched, cited
This section reads the record's references and adds cited findings from outside it. It needs an account. Everything above is held facts and is complete without it.
What would change this brief held, dated
- CISA KEV
- Not listed in the catalogue we hold. A listing changes this brief.
- exploit probability
- 0.01 on 2026-10-13 (FIRST's EPSS, a probability and not evidence of exploitation). Next band line: 10% (0.10). Crossing a line changes this brief.
- fix
- No fix version is held. One published by the CNA or OSV changes this brief.
How to check this
Every value above is held with two dates: when it was true upstream (as_of) and when we recorded it (tx_from). Re-derive the exploit probability as it stood on any date:
SELECT value, source, source_version, as_of, tx_from
FROM volatile_valid_as_of('CVE-2021-1220', '2026-11-02T23:59:59.999999Z', now());