In our previous post, we saw how to detect and protect against Direct Poisoned Pipeline Execution (D-PPE). We also saw how to detect that vulnerability using Pamindai Xygeni, as well as some protection mechanisms.
Diracun Pipeline Palaksanaan (PPE) is produced when the attacker can modify the pipeline logic in either of two ways:
- By modifying the CI config file (the pipeline) -> APD Langsung (D-APD)
- By modifying files referenced by the pipeline (contona: skrip anu dirujuk ti jero pipeline configuration file) -> APD teu langsung (I-PPE)
In this post, we will deep dive into Indirect PPE . But, before that, and as a complement to my previous post, let’s see first how GitHub manages the execution of pipelines and what are the protection mechanisms against D-PPE.
How does GitHub protect the execution of pipelines coming from PRs?
How does GitHub work regarding the execution of modified pipelines?
Dirobah pipelines can come from Pushes or Pull Requests (PR). As a major best practice, it’s strongly recommended to avoid any direct “push” to a protected branch and use Pull Requests as a mechanism to enforce some review before accepting any contributed code.
Pull Requests may arrive from two different sources:
- PRs coming from forks
- PRs coming from dahan
PRs from forks can come either from masarakat or pribados repositori.
As we are dealing with PPE (Poisoned Pipeline Execution), our main point is not the “acceptance” of a PR but the execution of a modified pipeline during the PR’s acceptance/approval process. At the core of a PPE attack, there is an unintended execution of a “malicious” modified pipeline.
Dina sababaraha kecap, Karacunan Pipeline Palaksanaan (APD) dihasilkeun nalika panyerang tiasa ngarobih pipeline nurut akal.
Aya dua varian:
- APD langsung (D-PPE): Dina skénario D-PPE, panyerang ngarobih file konfigurasi CI dina hiji repositori anu aranjeunna gaduh aksés, boh ku cara ngadorong parobihan langsung ka cabang jauh anu teu dijaga dina repo, atanapi ku cara ngalebetkeun PR kalayan parobihan tina cabang atanapi garpu. Kusabab CI pipeline palaksanaan dihartikeun ku paréntah dina file konfigurasi CI anu dimodifikasi, paréntah jahat panyerang pamustunganana dijalankeun dina simpul build sakali build pipeline dipicu.
- APD teu langsung (I-PPE): Dina kasus-kasus tertentu, kamungkinan D-PPE henteu sayogi pikeun musuh anu gaduh aksés ka SCM gudang (contona upami pipeline dikonfigurasi pikeun narik file konfigurasi CI tina cabang anu misah sareng dijaga dina repositori anu sami). Dina skenario sapertos kitu, tinimbang ngaracuni pipeline sorangan, panyerang nyuntikkeun kode jahat kana file anu dirujuk ku pipeline (contona: skrip anu dirujuk ti jero pipeline file konfigurasi)
Dina duanana kasus, GitHub bakal ngalaksanakeun anu dirobah pipeline tanpa peryogi ulasan atanapi persetujuan sateuacanna.
PRs from forks on masarakat ngaso
GitHub allows configuring the behaviour when processing PRs coming from forks in public repos.
When a PR is coming from a fork, GitHub always forces some level of “approval” before executing the pipeline associated with the PR. This level of approval trades off from a weak to a strict approval.
At Org level (Org>>Settings>>Actions>>General), you can decide among several “approval” options:
The strictest is the last one (“Require approval from all outside collaborators”) because GitHub will always require approval when the PR is coming from forks from outside collaborators.
But even in this strict case, there are differences between collaborators with read and write permissions.
- When the PR comes from a maca user, the palaksanaan tina pipeline is STOPPED until there is an approval of changes. If the approval is ok, then the modified pipeline dieksekusi.
- When the PR comes from a nulis user, the approval is not needed and the modified pipeline is always executed !!
As a conclusion, PRs coming from forks on public repositories are lightly protected against PPE. There is some protection against external (read) users, but nothing related to internal (write) users.
kumaha upami PRs coming from forks from private repos?
PRs from forks on pribados ngaso
In this scenario, GitHub provides some useful configuration settings.
Above settings can be configured either at organ atawa di Repo tingkat.
Iraha no option is checked, GitHub will ask for approval jeung it will not execute the modified pipeline. This is the safest configuration!!
nu unsafest configuration nyaéta iraha "Run workflows from fork pull request” is checked. In this case, same for both read and write users, Github will automatically execute the modified pipeline!! And this situation can be even leuwih goreng lamun"Kirimkeun token tulis ka alur kerja ti fork pull requests"Jeung"Send secrets and variables to workflows from fork pull requests” are checked. Do not do this unless clearly justified!!
Upami "Require approval for fork pull request workflows” is checked, the above situation is somewhat enhanced: GitHub will ask for approval and not execute the modified pipeline for the read user, but it will still execute it for a write user.
Forks seen, what about PRs coming from branches?
PRs from dahan
To protect this scenario you must rely on Branch Protection Rules.
At repo level, you can create branch protection rules for any branch. These rules add some constraints to modification of protected branches.
Although you configure a rule to “Merlukeun a pull request sateuacan ngahiji"Jeung"Require approvals" the modified pipeline will be automatically executed upon PR creation.The “approval” will only apply to the merge action.
What about Indirect Poisoned Pipeline eksekusi
As we saw above, D-PPE can be mitigated by using pull_request_target, tapi éta does not apply to I-PPE.
If you use pull_request_target, the default checkout will be the base code. But if you want to validate some checks on the contributed code (PR code) you need to explicitly checkout the PR code. Therefore, if the PR code has modified any shell script invoked by the pipeline, the “base” (safe) pipeline will invoke the “modified” shell script → Indirect PPE!!
The solution to this is a bit more complicated (there is not a magic bullet like pull_request_target).
urang pipeline is now safe to D-PPE because we are using pull_request_target. But it is still vulnerable to I-PPE.
In our test example, we need to checkout the PR code basically to make the build, but the tests are executed on the artifact generated by the build.
Janten .. why don’t check out both codebases?
- Checkout PR code because is the contributed code what we want to build and test
- Checkout Base code to run the original version of the pipeline and the build/tests scripts
This might be done by checking out those codebases to different folders: the base code might be checked out to the root folder, and the PR to a different folder. In this case we would execute the build and the test script from the root folder against the code placed into the new folder.
This is an easy solution, of course!! But, for learning purposes I would like to introduce a quite interesting variant (…)
GitHub alur_jalan kajadian pemicu
sajaba ti pull_request_target, GitHub provides another trigger event: alur_jalan. This event allows execution of a pipeline conditioned to another pipeline’s execution.
alur_jalan jeung pull_request_target triggers are similar in one aspect : both will be executed in privileged mode and, despite the PR modifications, the base pipeline will be executed !!
Let’s see our current pipeline:
name: PR TARGET CI on: pull_request_target: branches: [ main ] env: MY_SECRET: ${{ secrets.MY_SECRET }} jobs: prt_build_test_and_merge: runs-on: ubuntu-latest steps: # checkout PR code - name: Checkout repository uses: actions/checkout@v4 with: # This is to get the PR code instead of the repo code ref: ${{ github.event.pull_request.head.sha }} # Simulation of a compilation - name: Building ... run: | mkdir ./bin touch ./bin/mybin.exe ls -lR # Simulation of running tests - name: Running tests ... id : run_tests run: | echo Running tests.. chmod +x runtests.sh ./runtests.sh echo Tests executed. # # Let’s omit the check conditions at this moment … # - name: pr_check_conditions_to_merge [...]The build section is safe to D-PPE, but the test section is still vulnerable to I-PPE.
nu pipeline itself is safe to D-PPE due to the pull_request_target trigger. But the test step is still vulnerable to I-PPE due to invoking an external shell script.
Avoiding I-PPE
The purpose of the above pipeline is to build and test the contributed code, being safe to PPE.
Janten .. Why don’t split the pipeline into two ? One for building and another for testing..
- Anu 1st pipeline (Ngawangun CI) bakal pariksa kode PR (pikeun ngawangunna), jieun wangunanana sareng hasilkeun artefak.
- Anu ka-2 pipeline (Tés CI) bakal pariksa kodeu Dasar (pikeun nyingkahan modifikasi skrip shell) sareng ngajalankeun skrip asli ngalawan artefak éta.
- Pikeun nyingkronkeun Test CI pipeline pikeun ngajalankeun SAANGGEUS CI Build pipeline, urang bakal ngagunakeun alur_jalan pemicu.
In this way:
- pipeline Ngawangun CI is tengtrem ka duanana D-PPE (Alatan pull_request_target) jeung I-PPE (sabab éta henteu deui ngajalankeun skrip shell).
- pipeline Tés CI ogé tengtrem ka duanana D-PPE (Alatan alur_jalan) jeung I-PPE (sabab éta mariksa kode dasar pikeun kéngingkeun skrip shell asli)
Let’s see the code of both pipelines according to these modifications …
1st pipeline (Ngawangun CI):
name: Build CI on: pull_request_target: branches: [ main ] env: MY_SECRET: ${{ secrets.MY_SECRET }} GITHUB_PAT: ${{ secrets.GH_PAT }} jobs: prt_build_and_upload: runs-on: ubuntu-latest steps: - name: Checking out PR code uses: actions/checkout@v4 if: ${{ github.event_name == 'pull_request_target' }} with: # This is to get the PR code instead of the repo code ref: ${{ github.event.pull_request.head.sha }} - name: Building ... run: | mkdir ./bin touch ./bin/mybin.exe # Save some PR info for later use by the 2nd pipeline echo "${{github.event.pull_request.title}}" > ./bin/PR_TITLE.txt echo "${{github.event.number}}" > ./bin/PR_ID.txt # Upload the binary as a pipeline artifact - name: Archive building artifacts uses: actions/upload-artifact@v3 with: name: archive-bin path: | bin2nd pipeline (Tés CI):
ame: Test CI on: workflow_run: workflows: [ 'PR TARGET CI' ] types: [completed] env: MY_SECRET: ${{ secrets.MY_SECRET }} GITHUB_PAT: ${{ secrets.GH_PAT }} jobs: deploy: runs-on: ubuntu-latest if: ${{ github.event.workflow_run.conclusion == 'success' }} steps: # By default, checks out base code (not PR code) - name: Checkout repository uses: actions/checkout@v4 # Download the artifact - name: 'Download artifact' uses: actions/github-script@v6 with: script: | let allArtifacts = await github.rest.actions.listWorkflowRunArtifacts({ owner: context.repo.owner, repo: context.repo.repo, run_id: context.payload.workflow_run.id, }); let matchArtifact = allArtifacts.data.artifacts.filter((artifact) => { return artifact.name == "archive-bin" })[0]; let download = await github.rest.actions.downloadArtifact({ owner: context.repo.owner, repo: context.repo.repo, artifact_id: matchArtifact.id, archive_format: 'zip', }); let fs = require('fs'); fs.writeFileSync(`${process.env.GITHUB_WORKSPACE}/myartifact.zip`, Buffer.from(download.data)); # Unzip the artifact - name: 'Unzip artifact' run: | unzip -o myartifact.zip # Runs tests - name: Running tests ... id : run_tests run: | echo Running tests.. chmod +x runtests.sh ./runtests.sh echo Tests executed. # # Let’s omit the check conditions at this moment … # - name: pr_check_conditions_to_merge [...] Wow… nice solution!! But ….. Are we safe? I’m afraid that no 😭
Indeed, we have introduced a new vulnerability!! Which one? This will be the subject of our next post 🙂 … Stay tuned!!
PS: Sorry, I can’t keep quiet 🤐 ..Have you heard about Karacunan Artefak ? 😂
