CVE-2023-52137: Improper Neutralization of Special Elements used in a Command ('Command Injection') (CWE-77)
value: The [`tj-actions/verify-changed-files`](https://github.com/tj-actions/verify-changed-files) action allows for command injection in changed filenames, allowing an attacker to execute arbitrary code and potentially leak secrets. The [`verify-changed-files`](https://github.com/tj-actions/verify-changed-files) workflow returns the list of files changed within a workflow execution. This could potentially allow filenames that contain special characters such as `;` which can be used by an attacker to take over the [GitHub Runner](https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners) if the output value is used in a raw fashion (thus being directly replaced before execution) inside a `run` block. By running custom commands, an attacker may be able to steal secrets such as `GITHUB_TOKEN` if triggered on other events than `pull_request`.

This has been patched in versions [17](https://github.com/tj-actions/verify-changed-files/releases/tag/v17) and [17.0.0](https://github.com/tj-actions/verify-changed-files/releases/tag/v17.0.0) by enabling `safe_output` by default and returning filename paths escaping special characters for bash environments.
CWE description: The product constructs all or part of a command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended command when it is sent to a downstream component.
CVSS v3 base score: 8.8
Published: 2023-12-29T17:16Z
Repository: https://github.com/tj-actions/verify-changed-files
Language: YAML
Files changed: README.md, action.yml, entrypoint.sh
Commit date: 2023-12-28 06:39:29 -0700
Commit message: Merge pull request from GHSA-ghm2-rq8q-wrhc

* feat: add `safe_output` input enabled by default

* fix: migrate README to safe uses of interpolation

* fix: also sanitize `)`

* fix: remove sanitization of `'`

* fix: also sanitize `|`

* fix: also sanitize `&`

* fix: also sanitize `;`
Diff: diff --git a/README.md b/README.md
index d475d61..1f7f312 100644
--- a/README.md
+++ b/README.md
@@ -58,21 +58,25 @@ Verify that certain files or directories did or did not change during the workfl
 
       - name: Verify Changed files
         uses: tj-actions/verify-changed-files@v16
         id: verify-changed-files
         with:
+          safe_output: false # true by default, set to false because we are using an environment variable to store the output and avoid command injection.
           files: |
              *.txt
              test_directory
              action.yml
              **/*.{jpeg,py}
              !*.sql
 
       - name: Run step only when any of the above files change.
         if: steps.verify-changed-files.outputs.files_changed == 'true'
+        env:
+          FILES_CHANGED: |-
+            ${{ steps.verify-changed-files.outputs.changed_files }}
         run: |
-          echo "Changed files: ${{ steps.verify-changed-files.outputs.changed_files }}"
+          echo "Changed files: $FILES_CHANGED"
         # Outputs: "Changed files: new.txt test_directory/new.txt"
 ```
 
 ### Using the [`contains`](https://docs.github.com/en/actions/learn-github-actions/expressions#contains) function.
 
@@ -80,10 +84,11 @@ Verify that certain files or directories did or did not change during the workfl
 ...
       - name: Verify Changed files
         uses: tj-actions/verify-changed-files@v16
         id: verify-changed-files
         with:
+          safe_output: false
           files: |
              new.txt
              test_directory
 
       - name: Perform action when test_directory changes
@@ -97,14 +102,19 @@ Verify that certain files or directories did or did not change during the workfl
 ```yaml
 ...
       - name: Verify Changed files
         uses: tj-actions/verify-changed-files@v16
         id: verify-changed-files
+        with:
+          safe_output: false
       
       - name: List all changed files tracked and untracked files
+        env:
+          FILES_CHANGED: |-
+            ${{ steps.verify-changed-files.outputs.changed_files }}
         run: |
-          echo "Changed files: ${{ steps.verify-changed-files.outputs.changed_files }}"
+          echo "Changed files: $FILES_CHANGED"
 ```
 
 If you feel generous and want to show some extra appreciation:
 
 Support this project with a :star:
diff --git a/action.yml b/action.yml
index d4ff418..4e81f99 100644
--- a/action.yml
+++ b/action.yml
@@ -23,10 +23,14 @@ inputs:
     required: false
   fail-message:
     description: 'Message to display when files have changed and the `fail-if-changed` input is set to `true`.'
     default: "Files have changed."
     required: false
+  safe_output:
+    description: "Apply sanitization to output filenames before being set as output."
+    required: false
+    default: "true"
 
 outputs:
   files_changed:
     description: Boolean indicating that files have changed.
     value: ${{ steps.verify-changed-files.outputs.files_changed }}
@@ -59,9 +63,10 @@ runs:
         INPUT_FILES_PATTERN_FILE: ${{ steps.glob.outputs.paths-output-file }}
         INPUT_SEPARATOR: ${{ inputs.separator }}
         INPUT_MATCH_GITIGNORE_FILES: ${{ inputs.match-gitignore-files }}
         INPUT_FAIL_IF_CHANGED: ${{ inputs.fail-if-changed }}
         INPUT_FAIL_MSG: ${{ inputs.fail-message }}
+        INPUT_SAFE_OUTPUT: ${{ inputs.safe_output }}
 
 branding:
   icon: file-text
   color: white
diff --git a/entrypoint.sh b/entrypoint.sh
index 937980e..6d15487 100755
--- a/entrypoint.sh
+++ b/entrypoint.sh
@@ -64,10 +64,20 @@ CHANGED_FILES=$(echo "$CHANGED_FILES"  | awk '{gsub(/\|/,"\n"); print $0;}' | so
 if [[ -n "$CHANGED_FILES" ]]; then
   echo "Found uncommited changes"
 
   CHANGED_FILES=$(echo "$CHANGED_FILES" | awk '{gsub(/\|/,"\n"); print $0;}' | awk -v d="$INPUT_SEPARATOR" '{s=(NR==1?s:s d)$0}END{print s}')
 
+  if [[ "$INPUT_SAFE_OUTPUT" == "true" ]]; then
+    CHANGED_FILES=${CHANGED_FILES//$/\\$} # Replace $ with \$
+    CHANGED_FILES=${CHANGED_FILES//\(/\\\(}} # Replace ( with \(
+    CHANGED_FILES=${CHANGED_FILES//\)/\\\)}} # Replace ) with \)
+    CHANGED_FILES=${CHANGED_FILES//\`/\\\`} # Replace ` with \`
+    CHANGED_FILES=${CHANGED_FILES//|/\\|} # Replace | with \|
+    CHANGED_FILES=${CHANGED_FILES//&/\\&} # Replace & with \&
+    CHANGED_FILES=${CHANGED_FILES//;/\\;} # Replace ; with \;
+  fi
+
   echo "files_changed=true" >> "$GITHUB_OUTPUT"
   echo "changed_files=$CHANGED_FILES" >> "$GITHUB_OUTPUT"
 
   if [[ "$INPUT_FAIL_IF_CHANGED" == "true" ]]; then
     if [[ -n "$INPUT_FAIL_MSG" ]]; then

Vulnerable code:           echo "Changed files: ${{ steps.verify-changed-files.outputs.changed_files }}"
          echo "Changed files: ${{ steps.verify-changed-files.outputs.changed_files }}"
Fixed code:           safe_output: false # true by default, set to false because we are using an environment variable to store the output and avoid command injection.
        env:
          FILES_CHANGED: |-
            ${{ steps.verify-changed-files.outputs.changed_files }}
          echo "Changed files: $FILES_CHANGED"
          safe_output: false
        with:
          safe_output: false
        env:
          FILES_CHANGED: |-
            ${{ steps.verify-changed-files.outputs.changed_files }}
          echo "Changed files: $FILES_CHANGED"
  safe_output:
    description: "Apply sanitization to output filenames before being set as output."
    required: false
    default: "true"
        INPUT_SAFE_OUTPUT: ${{ inputs.safe_output }}
  if [[ "$INPUT_SAFE_OUTPUT" == "true" ]]; then
    CHANGED_FILES=${CHANGED_FILES//$/\\$} # Replace $ with \$
    CHANGED_FILES=${CHANGED_FILES//\(/\\\(}} # Replace ( with \(
    CHANGED_FILES=${CHANGED_FILES//\)/\\\)}} # Replace ) with \)
    CHANGED_FILES=${CHANGED_FILES//\`/\\\`} # Replace ` with \`
    CHANGED_FILES=${CHANGED_FILES//|/\\|} # Replace | with \|
    CHANGED_FILES=${CHANGED_FILES//&/\\&} # Replace & with \&
    CHANGED_FILES=${CHANGED_FILES//;/\\;} # Replace ; with \;
  fi