CVE-2021-1501 Overview
A vulnerability in the SIP inspection engine of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a crash and reload of an affected device, resulting in a denial of service (DoS) condition. The vulnerability is due to a crash that occurs during a hash lookup for a SIP pinhole connection. An attacker could exploit this vulnerability by sending crafted SIP traffic through an affected device.
Critical Impact
Successful exploitation allows remote unauthenticated attackers to crash and reload affected Cisco security appliances, causing network disruption and potential security gaps during device recovery.
Affected Products
- Cisco Adaptive Security Appliance (ASA) Software
- Cisco Firepower Threat Defense (FTD) Software
- Devices with SIP inspection enabled
Discovery Timeline
- April 29, 2021 - CVE-2021-1501 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-1501
Vulnerability Analysis
This vulnerability resides in the SIP (Session Initiation Protocol) inspection engine of Cisco ASA and FTD software. The flaw manifests when the affected device processes specially crafted SIP traffic, triggering an improper hash lookup operation for SIP pinhole connections. The vulnerability allows remote attackers to cause a complete device crash and reload without requiring any authentication or user interaction.
The impact is significant for network security infrastructure, as Cisco ASA and FTD devices typically serve as perimeter firewalls and network security gateways. A successful attack results in temporary loss of network protection and connectivity during the device reload cycle, potentially creating windows of opportunity for additional attacks.
Root Cause
The root cause of CVE-2021-1501 is a crash condition that occurs during hash lookup operations for SIP pinhole connections within the SIP inspection engine. When processing certain malformed or crafted SIP packets, the hash lookup function encounters an unexpected state that leads to an unhandled exception, causing the device to crash and initiate a reload sequence.
SIP inspection is designed to dynamically open pinholes (temporary access rules) for media streams negotiated in SIP signaling. The vulnerability exists in how the inspection engine manages and looks up these pinhole connections in its internal hash table structures.
Attack Vector
The attack can be executed remotely over the network by any unauthenticated attacker who can send SIP traffic through the affected device. The exploitation requires:
- Network access to send SIP traffic to or through the vulnerable device
- SIP inspection enabled on the target ASA or FTD device
- Crafted SIP packets designed to trigger the hash lookup crash condition
The attack does not require any privileges or user interaction, making it highly accessible to potential attackers. Organizations running SIP-based VoIP or unified communications systems through their Cisco firewalls are particularly at risk.
Detection Methods for CVE-2021-1501
Indicators of Compromise
- Unexpected device crashes or reboots on Cisco ASA or FTD appliances
- Crash dumps or core files indicating issues in SIP inspection processes
- Unusual volume or patterns of SIP traffic from external sources
- Multiple consecutive device reloads in a short time period
Detection Strategies
- Monitor device logs for crashinfo and reload events related to SIP inspection
- Implement network monitoring to detect anomalous SIP traffic patterns targeting your security appliances
- Configure SNMP traps or syslog alerts for device reload events
- Analyze SIP traffic for malformed packets or unusual characteristics using network analysis tools
Monitoring Recommendations
- Enable comprehensive logging on ASA and FTD devices for SIP inspection events
- Deploy network intrusion detection systems (IDS) with signatures for SIP-based attacks
- Monitor device availability and uptime metrics for security appliances
- Review Cisco's security advisory for specific indicators and detection guidance
How to Mitigate CVE-2021-1501
Immediate Actions Required
- Review the Cisco Security Advisory to determine if your software version is affected
- Apply the appropriate software update from Cisco to remediate the vulnerability
- If patching is not immediately possible, consider disabling SIP inspection as a temporary workaround
- Implement network access controls to limit SIP traffic to trusted sources only
Patch Information
Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory for specific version information and upgrade paths for affected ASA and FTD software releases. The advisory contains detailed information about fixed software releases and recommended upgrade procedures.
Workarounds
- Disable SIP inspection on the affected device if SIP traffic inspection is not required for your environment
- Implement access control lists (ACLs) to restrict SIP traffic to known, trusted sources only
- Consider deploying a dedicated SIP proxy or session border controller in front of the firewall to filter malicious SIP traffic
- Enable device redundancy (failover) configurations to minimize service disruption during potential attacks
# Example: Disable SIP inspection on Cisco ASA (use with caution)
# Access ASDM or CLI to modify inspection policy
# Note: Consult Cisco documentation before disabling security features
# View current SIP inspection status
show service-policy | include sip
# To disable SIP inspection, modify the policy-map
# This should only be done if SIP inspection is not required
policy-map global_policy
class inspection_default
no inspect sip
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
