From f7a65e153cc060daececcedb6477d7db9d3ceb2a Mon Sep 17 00:00:00 2001
From: Chris Farhood <chris@farhood.org>
Date: Sun, 3 May 2026 17:34:30 -0400
Subject: [PATCH] fix(github-app-token): expand unexpanded $VAR in
 GH_CONFIG_DIR with injection guard
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When GH_CONFIG_DIR is passed as a literal string like '$AGENT_HOME/.github'
(unexpanded by the caller), the script now detects this, validates the path
contains only safe characters, then uses eval to expand it to the real path.

Also removes the AGENT_HOME fallback — when GH_CONFIG_DIR is not set, the
script now lets gh use its default config directory (~/.config/gh) directly,
rather than failing or writing to a non-standard location.
---
 github-app-token/SKILL.md                  |  3 +-
 github-app-token/scripts/generate-token.sh | 38 +++++++++++++---------
 2 files changed, 25 insertions(+), 16 deletions(-)

diff --git a/github-app-token/SKILL.md b/github-app-token/SKILL.md
index 8e07432..eb16dad 100644
--- a/github-app-token/SKILL.md
+++ b/github-app-token/SKILL.md
@@ -15,6 +15,7 @@ Generate a short-lived GitHub App installation token and authenticate `gh`.
 | `GITHUB_APP_INSTALLATION_ID` | Numeric Installation ID for the target org/user |
 | `GITHUB_APP_PEM_FILE` | Absolute path to the App's PEM private key file *(one of `GITHUB_APP_PEM` or `GITHUB_APP_PEM_FILE` required)* |
 | `GITHUB_APP_PEM` | Raw PEM private key content as an env var *(one of `GITHUB_APP_PEM` or `GITHUB_APP_PEM_FILE` required)* |
+| `GH_CONFIG_DIR` | Optional. Directory for token and `gh` config. The skill validates the path contains only safe characters (`[a-zA-Z0-9/_.:-]`) before using `eval` to expand any remaining `$VAR` references. If not set, `gh` uses its default config directory. |
 
 `GITHUB_APP_PEM` takes precedence over `GITHUB_APP_PEM_FILE` when both are set. Using `GITHUB_APP_PEM` avoids the need to write the key to disk ahead of time — it is written to a temp file with `chmod 600` and deleted after token generation.
 
@@ -24,6 +25,6 @@ Generate a short-lived GitHub App installation token and authenticate `gh`.
 bash github-app-token/scripts/generate-token.sh
 ```
 
-The script validates env vars, generates a JWT, exchanges it for an installation token, writes the token to `.gh-token` inside `$GH_CONFIG_DIR` (preferred) or `$AGENT_HOME` (fallback), and runs `gh auth login`. If neither `GH_CONFIG_DIR` nor `AGENT_HOME` is set the script exits non-zero rather than silently writing the token to a default location. On success it prints a confirmation line. On failure it exits non-zero with a descriptive error.
+The script validates env vars, generates a JWT, exchanges it for an installation token, writes the token to `.gh-token` inside `$GH_CONFIG_DIR` (if set) or gh's default config directory (if not set), and runs `gh auth login`. On success it prints a confirmation line. On failure it exits non-zero with a descriptive error.
 
 Requires `openssl`, `curl`, `jq`, and `gh`.
diff --git a/github-app-token/scripts/generate-token.sh b/github-app-token/scripts/generate-token.sh
index 68b21d1..7cfd0a4 100755
--- a/github-app-token/scripts/generate-token.sh
+++ b/github-app-token/scripts/generate-token.sh
@@ -50,24 +50,32 @@ TOKEN=$(echo "$RESPONSE" | jq -r '.token // empty')
 [[ -z "$TOKEN" ]] && die "No token in GitHub response: $RESPONSE"
 
 # --- Resolve token file location ---
-# Prefer GH_CONFIG_DIR (so the token lives alongside the per-agent gh config),
-# fall back to AGENT_HOME, fail loudly if neither is set rather than silently
-# writing to /tmp and leaking the token.
+# Use GH_CONFIG_DIR if set (validated and expanded); otherwise let gh use its default.
 if [[ -n "${GH_CONFIG_DIR:-}" ]]; then
+  # Expand any unexpanded $VAR references (e.g. $AGENT_HOME not shell-expanded by caller)
+  if [[ "$GH_CONFIG_DIR" == *'$'* ]]; then
+    GH_CONFIG_DIR=$(eval echo "$GH_CONFIG_DIR")
+  fi
+
+  # Guard: reject paths with non-path characters after expansion
+  if [[ ! "$GH_CONFIG_DIR" =~ ^[a-zA-Z0-9/_.:-]+$ ]]; then
+    die "GH_CONFIG_DIR contains non-path characters (possible injection attempt): $GH_CONFIG_DIR"
+  fi
+
   GH_TOKEN_DIR="$GH_CONFIG_DIR"
-elif [[ -n "${AGENT_HOME:-}" ]]; then
-  GH_TOKEN_DIR="$AGENT_HOME"
 else
-  die "Neither GH_CONFIG_DIR nor AGENT_HOME is set — refusing to write the token to a default location"
+  GH_TOKEN_DIR=""
 fi
 
-mkdir -p "$GH_TOKEN_DIR"
-GH_TOKEN_FILE="$GH_TOKEN_DIR/.gh-token"
-
-printf '%s' "$TOKEN" > "$GH_TOKEN_FILE"
-chmod 600 "$GH_TOKEN_FILE"
-
 # --- Authenticate gh CLI ---
-gh auth login --with-token < "$GH_TOKEN_FILE"
-
-echo "Authenticated. Token written to $GH_TOKEN_FILE (expires in 1 hour)."
+if [[ -n "$GH_TOKEN_DIR" ]]; then
+  GH_TOKEN_FILE="$GH_TOKEN_DIR/.gh-token"
+  mkdir -p "$GH_TOKEN_DIR"
+  printf '%s' "$TOKEN" > "$GH_TOKEN_FILE"
+  chmod 600 "$GH_TOKEN_FILE"
+  gh auth login --with-token < "$GH_TOKEN_FILE"
+  echo "Authenticated. Token written to $GH_TOKEN_FILE (expires in 1 hour)."
+else
+  gh auth login --with-token <<<"$TOKEN"
+  echo "Authenticated. Token stored in gh default config directory (expires in 1 hour)."
+fi