This article describes how to resolve telnet login failure using a script

On Junos platform, Junos might not be able to process password string when it comes too fast against "Password:" prompt.

In JTAC test environment, SRX1500 was used for the test and the login failure was sometimes seen when the password string came within 10ms after "Password:" prompt was seen.

This can happen when a script is used to manage the device with telnet protocol.
When you access a device with telnet protocol by hand, the issue will not happen.

In telnet authentication process, it sometimes take a few milli seconds (e.g 10ms) to get ready for processing the password string. 
This delay is due to RE CPU performance and is a product limitation.
To avoid telnet login failure, please add some wait time for sending the password string in a telnet login script.

Example of script flow:

send username

wait "Password:" prompt

wait 100ms

send password

2023-09-26: KB created

2023-10-06: Modified the content to make it more clear