An Edge Case of Time in AWS PHP SDK
When Amazon Web Services rolled out their version 4 signature we started seeing sporadic errors on a few projects when we created pre-authenticated link to S3 resources with a relative timestamp. Trying to track down the errors wasn’t easy. It seemed that it would occur rarely while executing the same exact code. Our code was simply to get a pre-authenticated URL that would expire in 7 days, the max duration V4 signatures are allowed to be valid. The error we’d get was “The expiration date of a signature version 4 presigned URL must be less than one week”. Weird, we kept passing in “7 days” as the expiration time. After the error occurred a couple of times over a few weeks I decided to look into it.
The code throwing the error was located right in the SignatureV4 class. The error is thrown when the end timestamp minus the start timestamp for the signature was greater than a week. Looking through the way the timestamps were generated it went something like this:
- Generate the start timestamp as current time for the signature assuming one is not passed.
- Do a few other quick things not related to this problem.
- Do a check to insure that the end minus start timestamp is less than a week in seconds.
So a rough example with straight PHP could of the above steps for a ‘7 days’ expiration would be as follows:
<?php
// Generate the start timestamp
$start = time();
// Do other stuff....
// If the end timestamp is over 7 weeks in seconds throw an error
if( strtotime('+7 days') - $start > 86400) {
throw new \Exception('must be less than 7 weeks');
}Straight forward enough, right? the problem lies when a second “rolls” between generating the $start and the end timestamp check. For example, if you generate the $start at 2017-08-20 12:01:01.999999. Let’s say this gets assigned the timestamp of 2017-08-20 12:01:01. Then the check for the 7 weeks occurs at 2017-08-27 12:01:02.0000 it’ll throw an exception as duration between the start and end it’s actually now for 86,401 seconds total. It turns outs triggering this error is easier than you’d think. Run this script locally:
<?php
while(true){
$start = time();
// Sleeps below are not even necessary.
// time_nanosleep(0,1);
//or can use usleep(1);
$end = strtotime('1 week');
if($end-$start > 604800)
{
echo "Failed!! ".date('Y-m-d H:i:s',$start).' vs '.date('Y-m-d H:i:s',$end).' start: '.$start.' end: '.$end;
exit;
}
}That will throw an exception within a few seconds of running most likely.
After I figured out the error, the next step was to submit a issue to make sure I’m not misunderstanding how the library should be used. The simplest fix for me was to generate the end expiration timestamp before generating the start timestamp. After I made the PR, Kevin S. from AWS pointed out that while this fixed the problem, the duration still wasn’t guaranteed to always be the same for the same relative time period. For example, if you created 1000 presigned URLs all with ‘+7 days’ as the valid period, some may be 86400 in duration others may be 86399. This isn’t a huge problem, but Kevin made a great point that we could solve the problem by locking the relative timestamp for the end based on the start timestamp. After adding that to the PR it was accepted. As of release 3.32.4 the fix is now included in the SDK.