Signature expired: is now earlier than error : InvalidSignatureException

Question

I am trying a small example with AWS API Gateway and IAM authorization. The AWS API Gateway generated the below Endpoint :

https://xyz1234.execute-api.us-east-2.amazonaws.com/Users/users 

with POST action and no parameters.

Initially I had turned off the IAM for this POST Method and I verified results using Postman it works. Then I created a new IAM User and attached AmazonAPIGatewayInvokeFullAccess Policy to the user thereby giving permission to invoke any API's. Enabled the IAM for the POST Method.

I then went to Postman - and added Authorization with AccessKey, Secret Key, AWS Region as us-east-2 and Service Name as execute-api and tried to execute the Request but I got InvalidSignatureException Error with 403 as return code.

The body contains following message :

Signature expired: 20170517T062414Z is now earlier than 20170517T062840Z (20170517T063340Z - 5 min.)"  

What am I missing ?

Answer 1

A request signed with AWS sigV4 includes a timestamp for when the signature was created. Signatures are only valid for a short amount of time after they are created. (This limits the amount of time that a replay attack can be attempted.)

When the signature is validated the timestamp is compared to the current time. If this indicates that the signature was not created recently, then signature validation fails with the error message you mentioned.

If you get this on in a Docker container on Windows that uses WSL, then it may help to fix the WSL time with by running wsl -d docker-desktop -e /sbin/hwclock -s in a Powershell. You can verify this is the case beforehand by logging into the container and typing date in the terminal and comparing it with your host machine time.

A common cause of this is when the local clock on the host generating the signature is off by more than a couple of minutes.

Answer 2

You need to synchronize your machines local clock with NTP.

for eg. on an ubuntu machine:

sudo ntpdate pool.ntp.org 

System time goes out of sync quite often. You need to keep them in sync periodically.

You can run a daily CRON job to keep your system time in sync as mentioned at this link: Periodically synchronize time in Linux

Create a bash script to sync time called ntpdate and put the below into it

#!/bin/sh # sync server time /usr/sbin/ntpdate pool.ntp.org >> /tmp/ntpdate.log 

You can place this script anywhere you like and then set up a cron I will be putting it into the daily cron directory so that it runs once every day So my ntpdate script is now in /etc/cron.daily/ntpdate and it will run every day

Make this script executable

chmod +x /etc/cron.daily/ntpdate 

Test it by running the script once and look for some output in /tmp/ntpdate.log

/etc/cron.daily/ntpdate 

In your log file you should see something like

26 Aug 12:19:06 ntpdate[2191]: adjust time server 206.108.0.131 offset 0.272120 sec