CVE-2021-34699 Overview
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.
Critical Impact
Authenticated attackers can remotely trigger device reloads through improper web UI to CLI parser interaction, causing network infrastructure outages and denial of service conditions affecting network availability.
Affected Products
- Cisco IOS (multiple versions from 12.2 through 15.9)
- Cisco IOS XE (versions 3.3.x through 17.5.x)
- Network devices running TrustSec-enabled configurations
Discovery Timeline
- September 23, 2021 - CVE CVE-2021-34699 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-34699
Vulnerability Analysis
This vulnerability stems from an improper interaction between the device's web-based management interface and the underlying CLI command parser within the TrustSec subsystem. When specific CLI commands are invoked through the web UI rather than directly through the command line interface, the parser fails to properly handle the request context, leading to a condition that triggers a device reload.
The vulnerability affects devices configured with TrustSec functionality, which is Cisco's security architecture for software-defined segmentation. The flaw allows an authenticated user with access to the web management interface to craft requests that bypass normal command validation, ultimately causing the device to enter an unstable state and reload.
Root Cause
The root cause is classified under CWE-435 (Improper Interaction Between Multiple Correctly-Behaving Entities) and CWE-436 (Interpretation Conflict). The vulnerability arises because the web UI and CLI parser components, while functioning correctly in isolation, fail to properly coordinate when the web UI passes certain TrustSec-related commands to the CLI parser. This interaction conflict results in unexpected behavior that causes the device to reload rather than properly processing or rejecting the command.
Attack Vector
The attack requires network access and valid authentication credentials to the device's web management interface. An attacker with low-privilege authenticated access can exploit this vulnerability by:
- Authenticating to the web-based management interface
- Crafting or navigating to specific TrustSec CLI commands through the web UI
- Submitting the request, which triggers the improper interaction between components
- The device reloads, causing a temporary denial of service
The attack can be repeated to cause sustained service disruption. No user interaction beyond the attacker's own actions is required, and the vulnerability has a changed scope, meaning the impact extends beyond the vulnerable component itself to affect the entire network device and connected infrastructure.
Detection Methods for CVE-2021-34699
Indicators of Compromise
- Unexpected device reloads or system crashes in Cisco IOS/IOS XE devices
- Crash dump files indicating TrustSec CLI parser errors
- Repeated authentication events to the web management interface followed by device restarts
- Syslog entries showing abnormal command execution patterns through the HTTP/HTTPS management interface
Detection Strategies
- Monitor system logs for unexpected reload events with crash signatures related to TrustSec or CLI parsing
- Implement alerting on multiple device reloads within short time periods
- Review web server access logs for unusual TrustSec-related command requests
- Deploy network monitoring to detect service interruptions on critical Cisco infrastructure
Monitoring Recommendations
- Enable AAA accounting to track all administrative command execution including web UI actions
- Configure syslog forwarding to centralized SIEM for correlation analysis
- Set up SNMP trap monitoring for device reload events
- Implement automated health checks to detect unexpected device state changes
How to Mitigate CVE-2021-34699
Immediate Actions Required
- Review Cisco's security advisory and determine if your devices are running affected software versions
- Disable the web-based management interface (no ip http server and no ip http secure-server) if not operationally required
- Restrict web UI access to trusted management networks using access control lists
- Monitor affected devices for signs of exploitation while planning upgrade activities
Patch Information
Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory for detailed information about fixed software releases and upgrade paths specific to their deployment. The advisory provides guidance on determining whether devices are vulnerable and identifies the appropriate fixed software versions for each affected release train.
Workarounds
- Disable the HTTP and HTTPS server features on affected devices if web-based management is not required
- Implement infrastructure ACLs to limit management interface access to authorized IP addresses only
- Use SSH/console-only management as an alternative to web-based administration
- Consider implementing control plane policing (CoPP) to rate-limit management traffic
# Disable web server to mitigate vulnerability
configure terminal
no ip http server
no ip http secure-server
exit
write memory
# Alternative: Restrict access to management interfaces
ip access-list extended MGMT-ACCESS
permit tcp 10.0.0.0 0.0.0.255 any eq 443
deny tcp any any eq 443
deny tcp any any eq 80
permit ip any any
interface GigabitEthernet0/0
ip access-group MGMT-ACCESS in
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