Modify System Image: Downgrade System Image
Adversaries may install an older version of the operating system of a network device to weaken security. Older operating system versions on network devices often have weaker encryption ciphers and, in general, fewer/less updated defensive features. 
On embedded devices, downgrading the version typically only requires replacing the operating system file in storage. With most embedded devices, this can be achieved by downloading a copy of the desired version of the operating system file and reconfiguring the device to boot from that file on next system restart. The adversary could then restart the device to implement the change immediately or they could wait until the next time the system restarts.
Downgrading the system image to an older versions may allow an adversary to evade defenses by enabling behaviors such as Weaken Encryption. Downgrading of a system image can be done on its own, or it can be used in conjunction with Patch System Image.
Some vendors of embedded network devices provide cryptographic signing to ensure the integrity of operating system images at boot time. Implement where available, following vendor guidelines. 
Many vendors provide digitally signed operating system images to validate the integrity of the software used on their platform. Make use of this feature where possible in order to prevent and/or detect attempts by adversaries to compromise the system image. 
|Credential Access Protection||
Some embedded network devices are capable of storing passwords for local accounts in either plain-text or encrypted formats. Ensure that, where available, local passwords are always encrypted, per vendor recommendations. 
Use multi-factor authentication for user and privileged accounts. Most embedded network devices support TACACS+ and/or RADIUS. Follow vendor prescribed best practices for hardening access control.
Refer to NIST guidelines when creating password policies. 
|Privileged Account Management||
Restrict administrator accounts to as few individuals as possible, following least privilege principles. Prevent credential overlap across systems of administrator and privileged accounts, particularly between network and non-network platforms, such as servers or endpoints.
Many embedded network devices provide a command to print the version of the currently running operating system. Use this command to query the operating system for its version number and compare it to what is expected for the device in question. Because image downgrade may be used in conjunction with Patch System Image, it may be appropriate to also verify the integrity of the vendor provided operating system image file.
- Graham Holmes. (2015, October 8). Evolution of attacks on Cisco IOS devices. Retrieved October 19, 2020.
- Cisco. (n.d.). Cisco IOS Software Integrity Assurance - Secure Boot. Retrieved October 19, 2020.
- Cisco. (n.d.). Cisco IOS Software Integrity Assurance - Deploy Signed IOS. Retrieved October 21, 2020.
- Cisco. (n.d.). Cisco IOS Software Integrity Assurance - Credentials Management. Retrieved October 19, 2020.
- Cisco. (n.d.). Cisco IOS Software Integrity Assurance - TACACS. Retrieved October 19, 2020.
- Grassi, P., et al. (2017, December 1). SP 800-63-3, Digital Identity Guidelines. Retrieved January 16, 2019.