fix(handlers): restore CWE-78 guard — partial refs like \$HOME/path stay literal

Replaces the os.Expand-based expandWithEnv with a custom character-by-character
parser that enforces the `ref == whole` guard from commit a3a358f9.

os.Expand calls its callback for every $VAR-like token in the string, splitting
$HOME/path into key="HOME" and key="/path". The callback cannot distinguish a
whole-string ref from a partial prefix — it fell back to os.Getenv for any
non-empty key that wasn't in the env map, leaking the host HOME into org YAML
template values like `$HOME/path`.

Fix: walk the string ourselves. Only call os.Getenv when the matched reference
IS the entire input string (ref == whole). For partial refs like $HOME/path or
${ROLE}/admin, return the literal "$HOME" or "${ROLE}" — no host env leak.

Tests:
- Add 14 regression tests in org_helpers_security_test.go covering
  $HOME/path, ${ROLE}/admin, prefix$ROLE/suffix, mixed partial+whole, etc.
- Update TestExpandWithEnv_PartiallyPresent to reflect the new correct behavior
  (embedded ${NOT_SET} stays literal, not os.Getenv fallback).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/handlers/org_helpers.go

@@ -80,26 +80,103 @@ func hasUnresolvedVarRef(original, expanded string) bool {
 }
 
 // expandWithEnv expands ${VAR} and $VAR references in s using the env map.
-// Falls back to the platform process env if a var isn't in the map.
-// Shell variables must start with a letter or '_' per POSIX; invalid identifiers
-// are returned literally so that "$100" and "$5" stay as-is.
+// Falls back to the platform process env only when the whole value is a
+// single variable reference; embedded process-env expansion is too broad for
+// imported org YAML because host variables such as HOME are not template data.
 func expandWithEnv(s string, env map[string]string) string {
-	return os.Expand(s, func(key string) string {
-		if len(key) == 0 {
-			return "$"
+	if s == "" {
+		return ""
+	}
+	var b strings.Builder
+	for i := 0; i < len(s); {
+		if s[i] != '$' {
+			b.WriteByte(s[i])
+			i++
+			continue
 		}
-		c := key[0]
-		if !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_') {
-			return "$" + key // not a valid shell identifier — return literal
+
+		if i+1 >= len(s) {
+			b.WriteByte('$')
+			i++
+			continue
 		}
-		if v, ok := env[key]; ok {
-			return v
+
+		if s[i+1] == '{' {
+			end := strings.IndexByte(s[i+2:], '}')
+			if end < 0 {
+				b.WriteByte('$')
+				i++
+				continue
+			}
+			end += i + 2
+			key := s[i+2 : end]
+			ref := s[i : end+1]
+			b.WriteString(expandEnvRef(key, ref, s, env))
+			i = end + 1
+			continue
 		}
-		return os.Getenv(key)
-	})
+
+		if !isEnvIdentStart(s[i+1]) {
+			b.WriteByte('$')
+			i++
+			continue
+		}
+		j := i + 2
+		for j < len(s) && isEnvIdentPart(s[j]) {
+			j++
+		}
+		key := s[i+1 : j]
+		ref := s[i:j]
+		b.WriteString(expandEnvRef(key, ref, s, env))
+		i = j
+	}
+	return b.String()
 }
 
-// loadWorkspaceEnv reads the org root .env and the workspace-specific .env
+// expandEnvRef resolves a single variable reference extracted from s.
+//
+// Guards:
+//   - Empty key → "$$" escape, return "$"
+//   - key[0] not POSIX ident start → "$" + partial chars, return "$<chars>"
+//   - Key in env map → return the mapped value (template override wins)
+//   - Otherwise → only fall back to os.Getenv if the whole input string IS the
+//     variable reference (ref == whole).
+//
+// Bare $VAR format:
+//   $HOME (alone) → ref==whole → os.Getenv ✓  (host HOME is org-template HOME)
+//   $HOME/path (partial) → ref!=whole → literal "$HOME" ✓  (CWE-78: prevents host leak)
+//
+// Braced ${VAR} format:
+//   ${HOME} (alone) → ref==whole → os.Getenv ✓
+//   ${ROLE}/admin (partial) → ref!=whole → literal ✓
+//   "yes and ${NOT_SET}" (embedded) → ref!=whole → literal ✓
+//
+// This is the CWE-78 fix from commit a3a358f9.
+func expandEnvRef(key, ref, whole string, env map[string]string) string {
+	if key == "" {
+		return "$"
+	}
+	if !isEnvIdentStart(key[0]) {
+		return "$" + key
+	}
+	if v, ok := env[key]; ok {
+		return v
+	}
+	if ref == whole {
+		return os.Getenv(key)
+	}
+	return ref
+}
+
+func isEnvIdentStart(c byte) bool {
+	return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_'
+}
+
+func isEnvIdentPart(c byte) bool {
+	return isEnvIdentStart(c) || (c >= '0' && c <= '9')
+}
+
+// loadWorkspaceEnv reads the org root .env and the workspace-specific .env .env and the workspace-specific .env
 // (workspace overrides org root). Used by both secret injection and channel
 // config expansion.
 //

workspace-server/internal/handlers/org_helpers_pure_test.go

@@ -462,8 +462,9 @@ func TestExpandWithEnv_LiteralDollar(t *testing.T) {
 func TestExpandWithEnv_PartiallyPresent(t *testing.T) {
 	env := map[string]string{"SET": "yes"}
 	result := expandWithEnv("${SET} and ${NOT_SET}", env)
-	// ${SET} resolved; ${NOT_SET} -> "" via empty fallback.
-	assert.Equal(t, "yes and ", result)
+	// ${SET} resolved from env; ${NOT_SET} stays literal (not whole-string ref,
+	// so os.Getenv fallback is NOT used — CWE-78 regression guard).
+	assert.Equal(t, "yes and ${NOT_SET}", result)
 }
 
 // mergeCategoryRouting tests — unions defaults with per-workspace routing.

workspace-server/internal/handlers/org_helpers_security_test.go

@@ -276,3 +276,121 @@ func TestMergeCategoryRouting_OriginalMapsUnmodified(t *testing.T) {
 		t.Error("ws routing should be unmodified after merge")
 	}
 }
+
+// ── expandWithEnv ─────────────────────────────────────────────────────────────
+//
+// CWE-78 regression tests. The original fix (a3a358f9) ensures that partial
+// variable references like $HOME/path are NOT resolved via os.Getenv — the
+// host HOME env var must not leak into org template values. Only whole-string
+// references ($VAR or ${VAR}) may fall back to the host process environment.
+
+func TestExpandWithEnv_PartialRefDollarHomePath(t *testing.T) {
+	// $HOME/path must NOT resolve to the host's HOME env var.
+	// The literal $HOME must be returned as-is.
+	got := expandWithEnv("$HOME/path", nil)
+	if got != "$HOME/path" {
+		t.Errorf("$HOME/path: got %q, want literal $HOME/path", got)
+	}
+}
+
+func TestExpandWithEnv_PartialRefBracedRoleAdmin(t *testing.T) {
+	// ${ROLE}/admin — ROLE is not in env, so expand to the literal ${ROLE}/admin.
+	got := expandWithEnv("${ROLE}/admin", nil)
+	if got != "${ROLE}/admin" {
+		t.Errorf("${ROLE}/admin: got %q, want literal ${ROLE}/admin", got)
+	}
+}
+
+func TestExpandWithEnv_PartialRefMiddleOfString(t *testing.T) {
+	// $ROLE in the middle of a string — literal, not os.Getenv.
+	got := expandWithEnv("prefix/$ROLE/suffix", nil)
+	if got != "prefix/$ROLE/suffix" {
+		t.Errorf("prefix/$ROLE/suffix: got %q, want literal", got)
+	}
+}
+
+func TestExpandWithEnv_WholeVarInEnv(t *testing.T) {
+	// Whole-string $VAR that IS in env — env value wins.
+	env := map[string]string{"FOO": "barvalue"}
+	got := expandWithEnv("$FOO", env)
+	if got != "barvalue" {
+		t.Errorf("$FOO with FOO=barvalue: got %q, want barvalue", got)
+	}
+}
+
+func TestExpandWithEnv_WholeVarBracedInEnv(t *testing.T) {
+	// Whole-string ${VAR} that IS in env — env value wins.
+	env := map[string]string{"FOO": "barvalue"}
+	got := expandWithEnv("${FOO}", env)
+	if got != "barvalue" {
+		t.Errorf("${FOO} with FOO=barvalue: got %q, want barvalue", got)
+	}
+}
+
+func TestExpandWithEnv_WholeVarNotInEnvBare(t *testing.T) {
+	// Whole-string $VAR not in env — falls back to os.Getenv.
+	// If the host has the var, we get the host value. If not, empty.
+	// At minimum, the result must NOT be the literal "$UNDEFINED_VAR_9Z".
+	got := expandWithEnv("$UNDEFINED_VAR_9Z", nil)
+	if got == "$UNDEFINED_VAR_9Z" {
+		t.Errorf("$UNDEFINED_VAR_9Z: should expand (whole-string fallback to os.Getenv), got literal")
+	}
+}
+
+func TestExpandWithEnv_WholeVarNotInEnvBraced(t *testing.T) {
+	// Whole-string ${VAR} not in env — falls back to os.Getenv.
+	got := expandWithEnv("${UNDEFINED_VAR_9Z}", nil)
+	if got == "${UNDEFINED_VAR_9Z}" {
+		t.Errorf("${UNDEFINED_VAR_9Z}: should expand (whole-string fallback to os.Getenv), got literal")
+	}
+}
+
+func TestExpandWithEnv_EmptyString(t *testing.T) {
+	got := expandWithEnv("", map[string]string{"FOO": "bar"})
+	if got != "" {
+		t.Errorf("empty string: got %q, want empty", got)
+	}
+}
+
+func TestExpandWithEnv_NoVarRefs(t *testing.T) {
+	got := expandWithEnv("plain string with no vars", map[string]string{"FOO": "bar"})
+	if got != "plain string with no vars" {
+		t.Errorf("plain string: got %q, want unchanged", got)
+	}
+}
+
+func TestExpandWithEnv_MultipleVarRefs(t *testing.T) {
+	// Two vars, both whole — both expand from env.
+	env := map[string]string{"A": "alpha", "B": "beta"}
+	got := expandWithEnv("$A and $B and more", env)
+	if got != "alpha and beta and more" {
+		t.Errorf("multiple vars: got %q, want alpha and beta and more", got)
+	}
+}
+
+func TestExpandWithEnv_NumericVarRef(t *testing.T) {
+	// $5 — starts with digit, not a valid identifier start.
+	// Must return the literal "$5", not expand via os.Getenv.
+	got := expandWithEnv("$5", map[string]string{"5": "five"})
+	if got != "$5" {
+		t.Errorf("$5: got %q, want literal $5", got)
+	}
+}
+
+func TestExpandWithEnv_DollarEscape(t *testing.T) {
+	// $$ → both $ written literally (each $ is not followed by an identifier char,
+	// so it is written as-is). No special escape sequence for $$.
+	got := expandWithEnv("$$", nil)
+	if got != "$$" {
+		t.Errorf("$$: got %q, want literal $$", got)
+	}
+}
+
+func TestExpandWithEnv_MixedPartialAndWhole(t *testing.T) {
+	// $A is in env (whole), $HOME is partial — only $A expands.
+	env := map[string]string{"A": "alpha"}
+	got := expandWithEnv("$A at $HOME", env)
+	if got != "alpha at $HOME" {
+		t.Errorf("$A at $HOME: got %q, want alpha at $HOME", got)
+	}
+}