Instagram uses expiring certificates as single day TLS certificates

TLDR: Instagram changes their TLS certificates daily and they use certificates that are just about to expire in a week.

Intro

Since I occasionally help debug issues with office networks, I have built a simple cross-platform app for myself. One part of the app is a tool that gets the site certificate and displays various needed information. To test edge cases and verify that it works as needed I was testing it with different sites and different certificates. While that testing I stumbled upon a strange anomaly that I have never seen before. A certificate for Instagram was issued for 53 days. Usually, I expect a certificate to be issued for 90, 180, 365, or close to those values and this seemed very odd.

Apart from that certificate has about a week left to expire and I thought I would come in a few days to see how many days of buffer Instagram uses to trigger certificate update. But a few days later when I came back it still showed a certificate that had 8 days to expire. After a week it still had 8 days to expire.

So this led me to a hypothesis that Instagram changes its certificate once a week and they use certificates that are about to expire to reduce certificate lifetime. At the time I was a bit busy and couldn’t exactly verify that apart from checking certificate lifetime once in a while. I returned to it after server weeks.

Setup

After I got a little more time to test my hypothesis I allocated a separate machine for testing and wrote a little script to download a certificate from Instagram, extract the needed information from it, and store it on the disk for later analysis if needed. The certificate was saved in a custom format with extracted information and the certificate itself was stored in it. The name of the file was just a sha1 hash of the certificate and this name was later used to check if the certificate has changed.

Script downloaded certificate, got its sha1, and checked if there is already a file with that hash in the data directory. If the file is missing then it will do certificate processing to extract some information and store it in a file with a new hash, extract information, and the certificate itself. Then the script slept for 5 minutes and repeated the process. 5 minutes is an arbitrary delta that gives a 10-minute window for when the certificate changed. The task was not to know the exact minute and second timing of certificate change and a single request once in 5 minutes is polite enough to not cause any trouble.

The final stored result of the certificate and extracted data looks something like this:

Current Time: 2025-06-08 22:15:16
Serial: 13214245945560377533494110836988353923
sha-1: 0f4040bb4d2387253c1ccb1a9ba61ba5abce03ee
Valid from: 2025-03-20 00:00:00+00:00
Valid until: 2025-06-16 23:59:59+00:00
Issuer: <Name(C=US,O=DigiCert Inc,OU=www.digicert.com,CN=DigiCert SHA2 High Assurance Server CA)>
Subject: <Name(C=US,ST=California,L=Menlo Park,O=Meta Platforms\, Inc.,CN=*.instagram.com)>
Signature algorithm: sha256
Time left: 8 days or 697482.528974 seconds


-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----

Results

Apart from the allocated machine rebooting once or twice the test went surprisingly very well compared to the little time I spent on writing and setting things up. In total, it collected 20 certificates per domain. There were 1 or 2 days that I missed due to a machine reboot. But if we don’t consider those days then it is clear that Instagram changes certificates daily and rarely twice a day.

In general, Instagram starts using a certificate that has a lifetime of slightly more than 8 days and replaces it one day later when it has left a little more than 7 days before expiration. We can assume that they use “1 certificate per day”. Apart from that ‘instagram.com’ and ‘www.instagram.com’ use different certificates even though the certificate for instagram.com could sign www subdomain because it uses a wildcard certificate.

Let’s have a look a collected certificates in general.

Above we have 3 graphs for each certificate valid_from, valid_until, and the approximate time for when the certificate was swapped. Apart from a couple of anomalies (due to machine reboots), we can see that certificates are changed daily. Certificate validity start and end times are increasing by a single day. The only unusual thing here is that for “instagram.com” the first 3 certificates had the same “valid_from” field. Which might indicate that I accidentally stumbled upon Instagram is changing its certificate time allocation range. Remember initially I came across when the total time for the certificate was 53 days. It could be that they used to get certificates with the same starting day but different ending days which could have resulted in odd date ranges for the certificate.

From the above graphs, we can see that certs are changed somewhere between 16:00 and 17:00 in UTC. When we look into the minutes graph it happens somewhere around 25-30 minutes. Since checking for certificates happens once every 5 minutes we have a “radius” of 5 minutes around each minute datapoint. But also since it was a machine with a weak wifi connection it could be that the internet might not have always been available and that could have added a bit more jitter to the data.

For these two graphs we have a measurement of how much time left for the certificate to expire at the time when we queried it. First graph show how much time left for current active certificate and second graph shows how much time left for the previous one. As we can clearly see (if we remove outliers) that when a certificate is introduced it has about 8 days left for it to expire and removed certificate still had 7 days.

Conclusion

In this short period of about a month it was clear that instagram changes it’s certificates on daily basis. It’s main domain ‘instagram.com’ and subdomain ‘www.instagram.com’ each get separate certificates even though main domain has wildcard certificate and could could secure it’s subdomains. Changing certificates daily could be an attempt from instagram reduce “certificate life” to just one day instead of currently used mininum of about 90 days. It is hard for me to agree that it actually improves performance as the main attack vector of stealing private keys is still there if they are stored in close proximity and when somebody gets access to one they probably would have gotten access to all of them. But maybe they have some clever mechanism on the backend which stores them separately or somehow rotates access to them for the group of different people.