CVE-2021-1619: Cisco IOS XE Auth Bypass Vulnerability

CVE-2021-1619 Overview

A critical vulnerability exists in the authentication, authorization, and accounting (AAA) function of Cisco IOS XE Software that could allow an unauthenticated, remote attacker to bypass NETCONF or RESTCONF authentication. Successful exploitation enables an attacker to install, manipulate, or delete the configuration of an affected device, or cause memory corruption resulting in a denial of service (DoS) condition. This vulnerability is caused by an uninitialized variable in the AAA processing code.

Critical Impact

Unauthenticated remote attackers can bypass authentication to gain full configuration control over network devices or cause system-wide denial of service through memory corruption.

Affected Products

• Cisco IOS XE Software (versions 16.3.x through 17.2.x)
• Cisco IOS XE SD-WAN Software (multiple versions across 16.9.x through 16.12.x)
• Cisco ASR 1000 Series Aggregation Services Routers, 1000/4000 Series Integrated Services Routers, Cloud Services Router 1000V, and Integrated Services Virtual Routers

Discovery Timeline

• September 23, 2021 - CVE-2021-1619 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-1619

Vulnerability Analysis

This vulnerability affects the AAA subsystem within Cisco IOS XE Software, specifically when processing NETCONF or RESTCONF authentication requests. The flaw allows unauthenticated remote attackers to completely bypass authentication mechanisms designed to protect network device management interfaces.

The impact of this vulnerability is severe. An attacker can leverage the authentication bypass to perform unauthorized administrative actions including installing new configurations, modifying existing network settings, or deleting critical device configurations. Additionally, the uninitialized variable condition can be exploited to corrupt memory, leading to device crashes and denial of service.

Devices are vulnerable when configured to use the AAA feature for NETCONF or RESTCONF access control. The network-accessible nature of these management protocols significantly increases the attack surface for internet-facing or inadequately segmented network infrastructure.

Root Cause

The vulnerability stems from the use of an uninitialized variable (CWE-908) within the AAA processing logic. When authentication requests are received via NETCONF or RESTCONF, the code fails to properly initialize a critical variable before use. This leads to access of an uninitialized pointer (CWE-824), which can result in unpredictable behavior including authentication bypass or memory corruption. The uninitialized state may contain arbitrary data from previous memory operations, allowing attackers to manipulate the authentication decision logic.

Attack Vector

An attacker can exploit this vulnerability remotely over the network without requiring any prior authentication or user interaction. The attack involves sending a series of specially crafted NETCONF or RESTCONF requests to an affected device. These requests trigger the uninitialized variable condition in the AAA processing code.

The exploitation pathway targets the management interfaces:

• NETCONF typically operates on TCP port 830 or via SSH
• RESTCONF operates over HTTPS on TCP port 443

By sending crafted requests that exploit the uninitialized variable state, attackers can either bypass authentication entirely to gain administrative access, or trigger memory corruption that crashes the device. The low complexity of exploitation and network accessibility make this vulnerability particularly dangerous for enterprise network infrastructure.

Detection Methods for CVE-2021-1619

Indicators of Compromise

• Unexpected configuration changes on Cisco IOS XE devices without corresponding administrative activity logs
• Unusual NETCONF or RESTCONF connection attempts from unauthorized source IP addresses
• Device crashes or unexpected reboots potentially indicating memory corruption exploitation attempts
• Authentication logs showing successful access without valid credentials or missing authentication entries

Detection Strategies

• Monitor NETCONF (TCP 830) and RESTCONF (TCP 443) traffic for anomalous connection patterns and high volumes of requests from single sources
• Implement intrusion detection signatures to identify malformed or suspicious AAA authentication bypass attempts
• Review device syslogs for authentication failures followed by unexpected configuration access
• Deploy network traffic analysis to detect unauthorized management protocol communications

Monitoring Recommendations

• Enable detailed AAA accounting and logging on all Cisco IOS XE devices to capture authentication events
• Configure SIEM alerting for configuration changes occurring outside approved change windows
• Implement continuous configuration backup and change monitoring to detect unauthorized modifications
• Monitor device memory utilization and crash events that may indicate exploitation attempts

How to Mitigate CVE-2021-1619

Immediate Actions Required

• Identify all Cisco IOS XE and IOS XE SD-WAN devices within the environment running vulnerable versions (16.3.x through 17.2.x)
• Apply Cisco security patches immediately following the vendor advisory guidance
• Restrict NETCONF and RESTCONF access to trusted management networks only using access control lists
• Implement network segmentation to limit exposure of management interfaces to potential attackers

Patch Information

Cisco has released security patches to address this vulnerability. Organizations should consult the Cisco Security Advisory for cisco-sa-aaa-Yx47ZT8Q for specific fixed software versions and upgrade guidance applicable to their deployment. Given the critical severity, immediate patching is strongly recommended for all affected devices.

Workarounds

• Disable NETCONF and RESTCONF interfaces if they are not required for device management operations
• Implement strict access control lists (ACLs) to limit management interface access to trusted IP addresses only
• Use out-of-band management networks isolated from production traffic to reduce exposure
• Enable TACACS+ or RADIUS server authentication with additional access controls as a defense-in-depth measure

# Example ACL configuration to restrict NETCONF/RESTCONF access
ip access-list extended MGMT-ACCESS
 permit tcp host 10.10.10.5 any eq 830
 permit tcp host 10.10.10.5 any eq 443
 deny tcp any any eq 830 log
 deny tcp any any eq 443 log
!
line vty 0 15
 access-class MGMT-ACCESS in