Understanding the `~s` Break Signal Issue in Cisco Passwords: Risks an

[Farshad Qhayati]

Introduction 

When working with Cisco devices (especially Nexus switches or routers with console access), you may encounter unexpected behavior if your password contains the sequence `~s`. This happens because Cisco interprets `~s` as a break signal —a special key combination used for debugging or accessing ROMMON. 

Maybe you will see this message: 'Please press ~a to return to the active SUP console.'

In This article I explain: 

- Why `~s` in passwords causes issues. 

- How to avoid or mitigate the problem. 

Why `~s` in Passwords Is Problematic

1. `~s` as a Break Signal

- In Cisco console sessions, `~s` is a keyboard escape sequence that sends a hardware break signal.  When typed at the start of a line, it can: 

• Interrupt the login process. 

• Force the device into **ROMMON mode (for password recovery, or ..... ). 

• Terminate an ongoing command. 

 

1. When Does This Happen?

• Mostly Console sessions (less in SSH/Telnet sessions). 

• If `~s` appears at the beginning or middel of a password input line  

• If typed too quickly (Cisco may misinterpret it as a break request). 

 

How to Fix or Avoid the Issue

1. Escape the `~` Character , But If your password must contain or includes `~s`: 

• Add a delay between `~` and `s` (e.g., type `~`, pause for 1 second or more, then `s`). 

• Enclose the password in quotes (if supported by the device): 

                username admin

                password "abc~s123" 

 

So, if you are locked out due to `~s`, as happened to me a few days ago, and you can't log in because `~s` causes an interruption:

1. Re-enter the password slowly, ensuring that `~s` is not typed consecutively or in quick succession."

2. But the previous method did not work for me. So, I divided the password into two parts: from the beginning of the password up to the character `~`, and from the character `s` to the end of the password. I then copied and pasted each part separately into the CLI in two steps. By doing this, I was able to log in to the system, and as my first action, I changed the password. Like this:

Password `Pass~sWord`

Part 1: `Pass~`

Part 2: `sWord`

 

If you’ve encountered this issue, share your experience below!