dbcve.org
iOSOperating system · Cisco

CVE-2021-34699

HIGH · 7.7 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 →
83/100
Remediation priority · High
Remotely reachable 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 TrustSec CLI parser of Cisco IOS and Cisco IOS XE Software could allow an authenticated, remote attacker to cause an affected device to reload. This vulnerability is due to an improper interaction between the web UI and the CLI parser. An attacker could exploit this vulnerability by requesting a particular CLI command to be run through the web UI. A successful exploit could allow the attacker to cause the device to reload, resulting in a denial of service (DoS) condition.

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 TrustSec CLI parser of Cisco IOS and Cisco IOS XE Software allows an authenticated, remote attacker to cause device reload by requesting a specific CLI command through the web UI. The root cause is improper interaction between the web UI and CLI parser, leading to a denial of service condition.

MitigationApply the relevant Cisco security update for CVE-2021-34699 to affected IOS/IOS XE devices. If patching is not immediately possible, consider disabling the web UI or restricting web UI access to trusted sources only.

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:= 12.2\(6\)i1= 15.0\(1\)sy= 15.0\(1\)sy1= 15.0\(1\)sy2= 15.0\(1\)sy3= 15.0\(1\)sy4= 15.0\(1\)sy5= 15.0\(1\)sy6= 15.0\(1\)sy7= 15.0\(1\)sy7a= 15.0\(1\)sy8= 15.0\(1\)sy9
Ios XeOperating system
Affected:= 3.3.0se= 3.3.0xo= 3.3.1se= 3.3.1xo= 3.3.2se= 3.3.2xo= 3.3.3se= 3.3.4se= 3.3.5se= 3.5.0e= 3.5.1e= 3.5.2e

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
Low
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
None
Availability
High

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

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 device OS and version
    Execute `show version` on the device CLI to display the running IOS or IOS XE version
    Affected if The displayed version matches one of the vulnerable versions: Cisco iOS 12.2(6)i1, 15.0(1)sy through 15.0(1)sy9; Cisco IOS XE 3.3.0se through 3.5.2e
  2. Verify web UI is enabled
    Run `show running-config | include ip http server|secure-server` or check configuration for `ip http server` or `ip secure http server`
    Affected if HTTP or HTTPS server is enabled (the attack exploits the web UI interface to trigger the vulnerable CLI parser)
  3. Confirm TrustSec is licensed or available
    Check `show version | include TrustSec` or verify the device model supports TrustSec features
    Affected if The device has TrustSec capability (the vulnerability exists in the TrustSec CLI parser component)
  4. Check for recent reload events
    Review `show reload` history and `show log` for unexpected reloads, particularly those without clear cause
    Affected if Device has experienced unexplained reloads that could indicate exploitation attempts

Device is affected if it runs a vulnerable IOS or IOS XE version AND has the web UI enabled, as the attacker exploits the web UI to trigger the TrustSec CLI parser vulnerability causing device reload.

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.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the relevant Cisco security update for CVE-2021-34699 to affected IOS/IOS XE devices. If patching is not immediately possible, consider disabling the web UI or restricting web UI access to trusted sources only.

Recommended fix Moderate confidence

Cisco IOS 15.5(3)M or later / Cisco IOS XE 16.9.1 or later (specific to your platform - consult Cisco advisory)

  1. 1. Identify the exact Cisco IOS or IOS XE version currently running on the affected device using the 'show version' command.
  2. 2. Access Cisco's official security advisory for CVE-2021-34699 at tools.cisco.com to obtain the specific fixed release for your product line.
  3. 3. Download the appropriate fixed IOS or IOS XE software release from Cisco's software download center.
  4. 4. Plan an upgrade window as the device will need to be reloaded during the upgrade process.
  5. 5. Back up the current device configuration before proceeding with the upgrade.
  6. 6. Upload the new software image to the device using TFTP, FTP, or USB transfer methods.
  7. 7. Configure the device to boot from the new image using the 'boot system' command.
  8. 8. Reload the device with the 'reload' command to apply the new software version.
Caveat Review Cisco's release notes for your specific platform for potential command syntax or feature changes between your current and target version

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in iOS Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-34699 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2021-34699 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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
  • No spam, self-promotion, credentials, or personal data