diff --git a/install-scripts/install-safe-chain.ps1 b/install-scripts/install-safe-chain.ps1
index f95fdfd..fac897b 100644
--- a/install-scripts/install-safe-chain.ps1
+++ b/install-scripts/install-safe-chain.ps1
@@ -150,6 +150,19 @@ function Remove-VoltaInstallation {
 
 # Main installation
 function Install-SafeChain {
+    # Validate SAFE_CHAIN_DIR before using it to write files
+    if ($env:SAFE_CHAIN_DIR) {
+        if (-not [System.IO.Path]::IsPathRooted($env:SAFE_CHAIN_DIR)) {
+            Write-Error-Custom "SAFE_CHAIN_DIR must be an absolute path, got: $($env:SAFE_CHAIN_DIR)"
+        }
+        if ($env:SAFE_CHAIN_DIR -match '\.\.') {
+            Write-Error-Custom "SAFE_CHAIN_DIR must not contain path traversal (..)"
+        }
+        if ($env:SAFE_CHAIN_DIR -match '^[A-Za-z]:[/\\]?$' -or $env:SAFE_CHAIN_DIR -eq '/') {
+            Write-Error-Custom "SAFE_CHAIN_DIR cannot be a root or drive-root directory"
+        }
+    }
+
     # Show deprecation warning if SAFE_CHAIN_VERSION is set
     if (-not [string]::IsNullOrWhiteSpace($env:SAFE_CHAIN_VERSION)) {
         Write-Warn "SAFE_CHAIN_VERSION environment variable is deprecated."
diff --git a/install-scripts/install-safe-chain.sh b/install-scripts/install-safe-chain.sh
index f65b1d7..57b06d3 100755
--- a/install-scripts/install-safe-chain.sh
+++ b/install-scripts/install-safe-chain.sh
@@ -247,6 +247,20 @@ parse_arguments() {
 
 # Main installation
 main() {
+    # Validate SAFE_CHAIN_DIR before using it to write files
+    if [ -n "${SAFE_CHAIN_DIR}" ]; then
+        case "${SAFE_CHAIN_DIR}" in
+            /*) ;; # absolute path — OK
+            *) error "SAFE_CHAIN_DIR must be an absolute path, got: ${SAFE_CHAIN_DIR}" ;;
+        esac
+        case "${SAFE_CHAIN_DIR}" in
+            *../*|*/..*|..) error "SAFE_CHAIN_DIR must not contain path traversal (..)" ;;
+        esac
+        if [ "${SAFE_CHAIN_DIR}" = "/" ]; then
+            error "SAFE_CHAIN_DIR cannot be the root directory"
+        fi
+    fi
+
     # Initialize argument flags
     USE_CI_SETUP=false
 
diff --git a/install-scripts/uninstall-safe-chain.ps1 b/install-scripts/uninstall-safe-chain.ps1
index 32a27a5..a4f1fc1 100644
--- a/install-scripts/uninstall-safe-chain.ps1
+++ b/install-scripts/uninstall-safe-chain.ps1
@@ -75,6 +75,19 @@ function Remove-VoltaInstallation {
 
 # Main uninstallation
 function Uninstall-SafeChain {
+    # Validate SAFE_CHAIN_DIR before using it to delete files
+    if ($env:SAFE_CHAIN_DIR) {
+        if (-not [System.IO.Path]::IsPathRooted($env:SAFE_CHAIN_DIR)) {
+            Write-Error-Custom "SAFE_CHAIN_DIR must be an absolute path, got: $($env:SAFE_CHAIN_DIR)"
+        }
+        if ($env:SAFE_CHAIN_DIR -match '\.\.') {
+            Write-Error-Custom "SAFE_CHAIN_DIR must not contain path traversal (..)"
+        }
+        if ($env:SAFE_CHAIN_DIR -match '^[A-Za-z]:[/\\]?$' -or $env:SAFE_CHAIN_DIR -eq '/') {
+            Write-Error-Custom "SAFE_CHAIN_DIR cannot be a root or drive-root directory"
+        }
+    }
+
     Write-Info "Uninstalling safe-chain..."
 
     # Run teardown if safe-chain is available
diff --git a/install-scripts/uninstall-safe-chain.sh b/install-scripts/uninstall-safe-chain.sh
index fcb5153..5440730 100755
--- a/install-scripts/uninstall-safe-chain.sh
+++ b/install-scripts/uninstall-safe-chain.sh
@@ -139,6 +139,20 @@ remove_nvm_installation() {
 
 # Main uninstallation
 main() {
+    # Validate SAFE_CHAIN_DIR before using it to delete files
+    if [ -n "${SAFE_CHAIN_DIR}" ]; then
+        case "${SAFE_CHAIN_DIR}" in
+            /*) ;; # absolute path — OK
+            *) error "SAFE_CHAIN_DIR must be an absolute path, got: ${SAFE_CHAIN_DIR}" ;;
+        esac
+        case "${SAFE_CHAIN_DIR}" in
+            *../*|*/..*|..) error "SAFE_CHAIN_DIR must not contain path traversal (..)" ;;
+        esac
+        if [ "${SAFE_CHAIN_DIR}" = "/" ]; then
+            error "SAFE_CHAIN_DIR cannot be the root directory"
+        fi
+    fi
+
     SAFE_CHAIN_LOCATION="$DOT_SAFE_CHAIN/bin/safe-chain"
 
     if [ -x "$SAFE_CHAIN_LOCATION" ]; then
diff --git a/packages/safe-chain/src/shell-integration/startup-scripts/init-fish.fish b/packages/safe-chain/src/shell-integration/startup-scripts/init-fish.fish
index a705634..11d1d55 100644
--- a/packages/safe-chain/src/shell-integration/startup-scripts/init-fish.fish
+++ b/packages/safe-chain/src/shell-integration/startup-scripts/init-fish.fish
@@ -1,4 +1,8 @@
-set -l safe_chain_base (if set -q SAFE_CHAIN_DIR; echo $SAFE_CHAIN_DIR; else; echo $HOME/.safe-chain; end)
+# Guard against PATH separator injection: reject SAFE_CHAIN_DIR values containing ':'
+set -l safe_chain_base $HOME/.safe-chain
+if set -q SAFE_CHAIN_DIR; and not string match -q '*:*' -- $SAFE_CHAIN_DIR
+    set safe_chain_base $SAFE_CHAIN_DIR
+end
 set -gx PATH $PATH $safe_chain_base/bin
 
 function npx
diff --git a/packages/safe-chain/src/shell-integration/startup-scripts/init-posix.sh b/packages/safe-chain/src/shell-integration/startup-scripts/init-posix.sh
index b567902..45c6fd9 100644
--- a/packages/safe-chain/src/shell-integration/startup-scripts/init-posix.sh
+++ b/packages/safe-chain/src/shell-integration/startup-scripts/init-posix.sh
@@ -1,4 +1,10 @@
-export PATH="$PATH:${SAFE_CHAIN_DIR:-$HOME/.safe-chain}/bin"
+# Guard against PATH separator injection: reject SAFE_CHAIN_DIR values containing ':'
+case "${SAFE_CHAIN_DIR}" in
+    *:*) _sc_base="${HOME}/.safe-chain" ;;
+    *)   _sc_base="${SAFE_CHAIN_DIR:-${HOME}/.safe-chain}" ;;
+esac
+export PATH="$PATH:${_sc_base}/bin"
+unset _sc_base
 
 function npx() {
   wrapSafeChainCommand "npx" "$@"
diff --git a/packages/safe-chain/src/shell-integration/startup-scripts/init-pwsh.ps1 b/packages/safe-chain/src/shell-integration/startup-scripts/init-pwsh.ps1
index bcdd1c6..f814917 100644
--- a/packages/safe-chain/src/shell-integration/startup-scripts/init-pwsh.ps1
+++ b/packages/safe-chain/src/shell-integration/startup-scripts/init-pwsh.ps1
@@ -2,7 +2,8 @@
 # $IsWindows is only available in PowerShell Core 6.0+. If it doesn't exist, assume Windows PowerShell
 $isWindowsPlatform = if (Test-Path variable:IsWindows) { $IsWindows } else { $true }
 $pathSeparator = if ($isWindowsPlatform) { ';' } else { ':' }
-$safeChainBase = if ($env:SAFE_CHAIN_DIR) { $env:SAFE_CHAIN_DIR } else { Join-Path $HOME '.safe-chain' }
+# Guard against PATH separator injection: reject SAFE_CHAIN_DIR values containing the path separator
+$safeChainBase = if ($env:SAFE_CHAIN_DIR -and -not $env:SAFE_CHAIN_DIR.Contains($pathSeparator)) { $env:SAFE_CHAIN_DIR } else { Join-Path $HOME '.safe-chain' }
 $safeChainBin = Join-Path $safeChainBase 'bin'
 $env:PATH = "$env:PATH$pathSeparator$safeChainBin"