diff --git a/.github/workflows/main.yaml b/.github/workflows/main.yaml
index f7225e2..38f0413 100644
--- a/.github/workflows/main.yaml
+++ b/.github/workflows/main.yaml
@@ -26,7 +26,7 @@ jobs:
       dry-run: ${{ steps.check-dry-run.outputs.enabled }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
         with:
           fetch-depth: 2
 
@@ -64,22 +64,25 @@ jobs:
       contents: read
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
+
+      - name: Lint pinned actions
+        run: bash tools/linter_actions_pinned.sh
 
       - name: Get list of changed C# files
         id: changed-files
-        uses: tj-actions/changed-files@v46
+        uses: tj-actions/changed-files@ed68ef82c095e0d48ec87eccea555d944a631a4c # v46
         with:
           files: |
             **.cs
 
       - name: Setup .NET
-        uses: actions/setup-dotnet@v5
+        uses: actions/setup-dotnet@c2fa09f4bde5ebb9d1777cf28262a3eb3db3ced7 # v5
         with:
           dotnet-version: ${{ env.DOTNET_VERSION }}
 
       - name: Cache NuGet packages
-        uses: actions/cache@v4
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
         with:
           path: ~/.nuget/packages
           key: ${{ runner.os }}-nuget-${{ hashFiles('**/*.*proj') }}
@@ -116,7 +119,7 @@ jobs:
           fi
 
       - name: Upload test results
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
         if: failure()
         with:
           name: test-results-unit
@@ -124,7 +127,7 @@ jobs:
           retention-days: 7
 
       - name: Upload coverage data
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
         if: always()
         with:
           name: coverage-unit
@@ -166,15 +169,15 @@ jobs:
       pull-requests: write
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
 
       - name: Setup .NET
-        uses: actions/setup-dotnet@v5
+        uses: actions/setup-dotnet@c2fa09f4bde5ebb9d1777cf28262a3eb3db3ced7 # v5
         with:
           dotnet-version: ${{ env.DOTNET_VERSION }}
 
       - name: Download all coverage artifacts
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4
         with:
           pattern: coverage-*
           path: ./all-coverage
@@ -192,21 +195,21 @@ jobs:
             -title:"Weaviate C# Client Coverage"
 
       - name: Upload HTML coverage report
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
         with:
           name: coverage-report-html
           path: ./coveragereport
           retention-days: 7
 
       - name: Add coverage to PR comment
-        uses: marocchino/sticky-pull-request-comment@v2
+        uses: marocchino/sticky-pull-request-comment@773744901bac0e8cbb5a0dc842800d45e9b2b405 # v2
         if: github.event_name == 'pull_request'
         with:
           header: coverage
           path: ./coveragereport/SummaryGithub.md
 
       - name: Download all test results
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4
         continue-on-error: true
         with:
           pattern: test-results-*
@@ -249,18 +252,18 @@ jobs:
           if [ -z "${{ secrets.NUGET_APIKEY }}" ]; then echo "Warning: NUGET_APIKEY is not set"; fi
 
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
         with:
           fetch-depth: 0
           fetch-tags: true
 
       - name: Setup .NET
-        uses: actions/setup-dotnet@v5
+        uses: actions/setup-dotnet@c2fa09f4bde5ebb9d1777cf28262a3eb3db3ced7 # v5
         with:
           dotnet-version: ${{ env.DOTNET_VERSION }}
 
       - name: Cache NuGet packages
-        uses: actions/cache@v4
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
         with:
           path: ~/.nuget/packages
           key: ${{ runner.os }}-nuget-${{ hashFiles('**/.*proj') }}
@@ -275,7 +278,7 @@ jobs:
         run: dotnet nuget push './out/*.nupkg' --skip-duplicate --api-key ${{ secrets.NUGET_APIKEY }} --source https://api.nuget.org/v3/index.json
 
       - name: GH Release
-        uses: softprops/action-gh-release@v1
+        uses: softprops/action-gh-release@de2c0eb89ae2a093876385947365aca7b0e5f844 # v1
         with:
           generate_release_notes: true
           draft: true
diff --git a/.github/workflows/pr-security-lint.yaml b/.github/workflows/pr-security-lint.yaml
new file mode 100644
index 0000000..3dc27e5
--- /dev/null
+++ b/.github/workflows/pr-security-lint.yaml
@@ -0,0 +1,27 @@
+name: PR Security Lint
+
+on:
+  pull_request_target:
+    types: [opened, synchronize, reopened]
+
+# No permissions at workflow level — grant only what's needed at job level
+permissions: {}
+
+jobs:
+  hidden-unicode-check:
+    name: Check for hidden Unicode characters
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
+    steps:
+      - name: Checkout base branch
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
+        with:
+          ref: ${{ github.event.pull_request.base.sha }}
+
+      - name: Check PR diff for hidden Unicode
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          gh pr diff ${{ github.event.pull_request.number }} | bash tools/linter_hidden_unicode.sh --stdin
diff --git a/.github/workflows/test-on-weaviate-version.yml b/.github/workflows/test-on-weaviate-version.yml
index 658c08e..9e5e92d 100644
--- a/.github/workflows/test-on-weaviate-version.yml
+++ b/.github/workflows/test-on-weaviate-version.yml
@@ -36,15 +36,15 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@93cb6efe18208431cddfb8368fd83d5badbf9bfd # v5
 
       - name: Setup .NET
-        uses: actions/setup-dotnet@v5
+        uses: actions/setup-dotnet@c2fa09f4bde5ebb9d1777cf28262a3eb3db3ced7 # v5
         with:
           dotnet-version: ${{ env.DOTNET_VERSION }}
 
       - name: Cache NuGet packages
-        uses: actions/cache@v4
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
         with:
           path: ~/.nuget/packages
           key: ${{ runner.os }}-nuget-${{ hashFiles('**/*.*proj') }}
@@ -55,7 +55,7 @@ jobs:
         run: dotnet restore
 
       - name: Login to Docker Hub
-        uses: docker/login-action@v3
+        uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3
         if: ${{ !github.event.pull_request.head.repo.fork && github.triggering_actor != 'dependabot[bot]' }}
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
@@ -86,7 +86,7 @@ jobs:
         run: /bin/bash ci/stop_weaviate.sh ${{ inputs.weaviate-version }}
 
       - name: Upload test results
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
         if: failure()
         with:
           name: test-results-integration-${{ inputs.weaviate-version }}
@@ -94,7 +94,7 @@ jobs:
           retention-days: 7
 
       - name: Upload coverage data
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
         if: always()
         with:
           name: coverage-integration-${{ inputs.weaviate-version }}
@@ -102,7 +102,7 @@ jobs:
           retention-days: 7
 
       - name: Test Report
-        uses: dorny/test-reporter@v1
+        uses: dorny/test-reporter@3eeb9fc888e82e8be2fb356bbeec2750231672bc # v1
         if: failure() && !inputs.dry-run
         with:
           name: Integration Tests - Weaviate ${{ inputs.weaviate-version }}
diff --git a/tools/linter_actions_pinned.sh b/tools/linter_actions_pinned.sh
new file mode 100755
index 0000000..ebe39b3
--- /dev/null
+++ b/tools/linter_actions_pinned.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+# Lint GitHub Actions workflow files to ensure all external actions are pinned to SHA hashes.
+# Usage: bash tools/linter_actions_pinned.sh
+
+set -euo pipefail
+
+ERRORS=0
+
+for workflow in .github/workflows/*.yaml .github/workflows/*.yml; do
+    [ -f "$workflow" ] || continue
+
+    while IFS= read -r line; do
+        lineno=$(echo "$line" | cut -d: -f1)
+        content=$(echo "$line" | cut -d: -f2-)
+
+        # Extract the action reference (everything after "uses:")
+        action_ref=$(echo "$content" | sed -n 's/.*uses:[[:space:]]*//p' | xargs)
+
+        # Skip local actions (starting with ./)
+        if [[ "$action_ref" == ./* ]]; then
+            continue
+        fi
+
+        # Extract the version part (after @, before space or # comment)
+        version=$(echo "$action_ref" | sed -n 's/.*@\([^ #]*\).*/\1/p')
+
+        if [ -z "$version" ]; then
+            echo "::error file=${workflow},line=${lineno}::Action missing version pin: ${action_ref}"
+            ERRORS=$((ERRORS + 1))
+            continue
+        fi
+
+        # Check that the version is a 40-character hex SHA
+        if ! echo "$version" | grep -qE '^[0-9a-f]{40}$'; then
+            echo "::error file=${workflow},line=${lineno}::Action not pinned to SHA: ${action_ref} (version: ${version})"
+            ERRORS=$((ERRORS + 1))
+        fi
+    done < <(grep -n 'uses:' "$workflow")
+done
+
+if [ "$ERRORS" -gt 0 ]; then
+    echo ""
+    echo "ERROR: Found ${ERRORS} action(s) not pinned to a SHA hash."
+    echo "Replace tag references (e.g., @v5) with the full commit SHA (e.g., @93cb6ef...)"
+    echo "Preserve the tag as a comment: uses: actions/checkout@<SHA> # v5"
+    exit 1
+fi
+
+echo "All GitHub Actions are pinned to SHA hashes."
diff --git a/tools/linter_hidden_unicode.sh b/tools/linter_hidden_unicode.sh
new file mode 100755
index 0000000..c63a6ed
--- /dev/null
+++ b/tools/linter_hidden_unicode.sh
@@ -0,0 +1,143 @@
+#!/usr/bin/env bash
+# Lint for hidden/invisible Unicode characters in diffs (trojan-source attack prevention).
+# Requires Perl (pre-installed on GitHub Actions Ubuntu runners).
+#
+# Usage:
+#   bash tools/linter_hidden_unicode.sh --stdin      # read diff from stdin (CI mode)
+#   bash tools/linter_hidden_unicode.sh <base-ref>   # diff against a base ref
+#   bash tools/linter_hidden_unicode.sh              # diff staged changes (git diff --cached)
+
+set -euo pipefail
+
+# Binary file extensions to skip
+BINARY_PATTERN='\.(png|jpg|jpeg|gif|ico|svg|woff2?|ttf|eot|otf|zip|tar|gz|bz2|xz|7z|rar|pdf|dll|exe|so|dylib|o|obj|class|jar|war|pyc|pyo|wasm|bin|dat|db|sqlite|nupkg|snupkg)$'
+
+get_diff() {
+    if [[ "${1:-}" == "--stdin" ]]; then
+        cat
+    elif [[ -n "${1:-}" ]]; then
+        # Validate ref argument to prevent command injection
+        if ! [[ "$1" =~ ^[a-zA-Z0-9._/-]+$ ]]; then
+            echo "ERROR: Invalid ref argument: $1" >&2
+            exit 1
+        fi
+        if ! git rev-parse --verify "$1" >/dev/null 2>&1; then
+            echo "ERROR: Git ref not found: $1" >&2
+            exit 2
+        fi
+        git diff "$1"
+    else
+        git diff --cached
+    fi
+}
+
+# Perl script that:
+# 1. Tracks current file from diff headers
+# 2. Skips binary files
+# 3. Scans only added lines (starting with +, excluding +++ headers)
+# 4. Detects ~30+ categories of invisible/suspicious Unicode characters
+PERL_SCRIPT='
+use utf8;
+use strict;
+use warnings;
+
+sub escape_property {
+    my ($s) = @_;
+    $s =~ s/%/%25/g;
+    $s =~ s/\r/%0D/g;
+    $s =~ s/\n/%0A/g;
+    $s =~ s/:/%3A/g;
+    $s =~ s/,/%2C/g;
+    return $s;
+}
+
+sub escape_message {
+    my ($s) = @_;
+    $s =~ s/%/%25/g;
+    $s =~ s/\r/%0D/g;
+    $s =~ s/\n/%0A/g;
+    return $s;
+}
+
+my $file = "";
+my $line_in_file = 0;
+my $errors = 0;
+my $in_binary = 0;
+my $binary_pattern = qr/'"$BINARY_PATTERN"'/i;
+
+while (<STDIN>) {
+    chomp;
+
+    # Track file from diff headers
+    if (/^\+\+\+ b\/(.+)$/) {
+        $file = $1;
+        $line_in_file = 0;
+        $in_binary = ($file =~ $binary_pattern) ? 1 : 0;
+        next;
+    }
+
+    # Skip binary file markers
+    if (/^Binary files/) {
+        $in_binary = 1;
+        next;
+    }
+
+    # Track hunk headers for line numbers
+    if (/^@@ -\d+(?:,\d+)? \+(\d+)/) {
+        $line_in_file = $1 - 1;
+        next;
+    }
+
+    # Count lines in the new file
+    if (/^\+/ || /^ /) {
+        $line_in_file++;
+    }
+
+    # Only scan added lines, skip binary files
+    next if $in_binary;
+    next unless /^\+/;
+    next if /^\+\+\+ (?:$|b\/|\/dev\/null)/;
+
+    # Remove the leading + for scanning
+    my $content = substr($_, 1);
+
+    # Check for suspicious invisible Unicode characters:
+    # - Bidi overrides and isolates (U+200E-200F, U+202A-202E, U+2066-2069)
+    # - Zero-width characters (U+200B-200D, U+2060)
+    # - Byte order mark mid-line (U+FEFF)
+    # - Soft hyphen (U+00AD)
+    # - Mongolian vowel separator (U+180E)
+    # - Combining grapheme joiner (U+034F)
+    # - Function application and invisible operators (U+2061-2064)
+    # - Hangul fillers (U+115F, U+1160, U+3164, U+FFA0)
+    # - Interlinear annotation (U+FFF9-FFFB)
+    # - Object replacement / replacement char (U+FFFC-FFFD) -- FFFD is sometimes legitimate
+    # - Unicode tag block (U+E0001, U+E0020-E007F)
+    # - Deprecated format chars (U+206A-206F)
+    if ($content =~ /([\x{00AD}\x{034F}\x{115F}\x{1160}\x{180E}\x{200B}-\x{200F}\x{202A}-\x{202E}\x{2060}-\x{2064}\x{2066}-\x{2069}\x{206A}-\x{206F}\x{3164}\x{FE00}-\x{FE0F}\x{FEFF}\x{FFA0}\x{FFF9}-\x{FFFB}\x{E0001}\x{E0020}-\x{E007F}])/) {
+        my $char = $1;
+        my $codepoint = sprintf("U+%04X", ord($char));
+        my $col = $-[1] + 1;
+
+        if ($ENV{GITHUB_ACTIONS}) {
+            my $efile = escape_property($file);
+            my $emsg = escape_message("Hidden Unicode character ${codepoint} found");
+            print "::error file=${efile},line=${line_in_file},col=${col}::${emsg}\n";
+        } else {
+            print "ERROR: $file:$line_in_file:$col - Hidden Unicode character $codepoint found\n";
+        }
+        $errors++;
+    }
+}
+
+if ($errors > 0) {
+    print "\nFound $errors hidden Unicode character(s) in added lines.\n";
+    print "These may indicate a trojan-source attack. See https://trojansource.codes/\n";
+    exit 1;
+} else {
+    print "No hidden Unicode characters detected.\n";
+    exit 0;
+}
+'
+
+get_diff "$@" | perl -CS -e "$PERL_SCRIPT"