diff --git a/changelog.d/unreleased/2866.security.md b/changelog.d/unreleased/2866.security.md
new file mode 100644
index 0000000000..9129c6e2f5
--- /dev/null
+++ b/changelog.d/unreleased/2866.security.md
@@ -0,0 +1,16 @@
+---
+category: security
+issues:
+  - 2866
+affected:
+  - src/CodeIndex/Cli/DbPathResolver.cs
+  - tests/CodeIndex.Tests/DbPathResolverTests.cs
+---
+
+## English
+
+- **DbPathResolver now ignores indexed-file samples that escape candidate roots (#2866)** — project-root probing for explicit `.cdidx/codeindex.db` paths now normalizes candidate sample paths and skips `../` or absolute-like entries before reading files.
+
+## 日本語
+
+- **DbPathResolver が candidate root 外へ逃げる indexed-file sample を無視するようになりました (#2866)** — 明示指定された `.cdidx/codeindex.db` の project-root 推定で、サンプルpathを正規化し、`../` や絶対path風の entries はファイル読取前にスキップします。
diff --git a/src/CodeIndex/Cli/DbPathResolver.cs b/src/CodeIndex/Cli/DbPathResolver.cs
index f730b3921d..357b70f30c 100644
--- a/src/CodeIndex/Cli/DbPathResolver.cs
+++ b/src/CodeIndex/Cli/DbPathResolver.cs
@@ -508,8 +508,9 @@ private static SampleMatchResult CountMatchingSamples(string candidateRoot, IRea
         {
             try
             {
-                var absolutePath = Path.Combine(candidateRoot, sample.RelativePath.Replace('/', Path.DirectorySeparatorChar));
-                var ioPath = LongPath.EnsureWindowsPrefix(absolutePath);
+                if (!TryResolveIndexedFileSampleIoPath(candidateRoot, sample.RelativePath, out var ioPath))
+                    continue;
+
                 if (!File.Exists(ioPath))
                     continue;
 
@@ -532,6 +533,37 @@ private static SampleMatchResult CountMatchingSamples(string candidateRoot, IRea
         return new SampleMatchResult(checksumMatches, pathExistsMatches);
     }
 
+    internal static bool TryResolveIndexedFileSampleIoPath(string candidateRoot, string sampleRelativePath, out string ioPath)
+    {
+        ioPath = string.Empty;
+        if (string.IsNullOrWhiteSpace(sampleRelativePath) || IsRootedOrAbsoluteLikeSamplePath(sampleRelativePath))
+            return false;
+
+        try
+        {
+            var normalizedRoot = Path.GetFullPath(candidateRoot);
+            var relativePath = NormalizeSampleRelativePath(sampleRelativePath);
+            var absolutePath = Path.GetFullPath(Path.Combine(normalizedRoot, relativePath));
+            if (!IsUnderDirectory(normalizedRoot, absolutePath))
+                return false;
+
+            ioPath = LongPath.EnsureWindowsPrefix(absolutePath);
+            return true;
+        }
+        catch (Exception ex) when (ex is ArgumentException or IOException or NotSupportedException or PathTooLongException)
+        {
+            return false;
+        }
+    }
+
+    private static bool IsRootedOrAbsoluteLikeSamplePath(string samplePath)
+        => Path.IsPathRooted(samplePath);
+
+    private static string NormalizeSampleRelativePath(string sampleRelativePath)
+        => Path.DirectorySeparatorChar == '\\'
+            ? sampleRelativePath.Replace('/', Path.DirectorySeparatorChar)
+            : sampleRelativePath;
+
     private readonly record struct SampleMatchResult(int ChecksumMatches, int PathExistsMatches);
     private sealed record IndexedFileSample(string RelativePath, string Checksum);
 }
diff --git a/tests/CodeIndex.Tests/DbPathResolverTests.cs b/tests/CodeIndex.Tests/DbPathResolverTests.cs
index 47cff4fbb8..3905fff8ce 100644
--- a/tests/CodeIndex.Tests/DbPathResolverTests.cs
+++ b/tests/CodeIndex.Tests/DbPathResolverTests.cs
@@ -358,6 +358,124 @@ public void ResolveProjectRootForQuery_ExplicitProjectLocalDbDoesNotCaseFoldPers
         }
     }
 
+    [Fact]
+    public void ResolveProjectRootForQuery_ExplicitProjectLocalDbIgnoresEscapingSampleMatches()
+    {
+        var projectParent = TestProjectHelper.CreateTempProject("cdidx_db_path_resolver_escape_parent");
+        var staleParent = TestProjectHelper.CreateTempProject("cdidx_db_path_resolver_escape_stale_parent");
+        var projectRoot = Path.Combine(projectParent, "project");
+        var staleRoot = Path.Combine(staleParent, "stale");
+        var dbPath = Path.Combine(projectRoot, ".cdidx", "codeindex.db");
+        try
+        {
+            Directory.CreateDirectory(projectRoot);
+            Directory.CreateDirectory(staleRoot);
+            Directory.CreateDirectory(Path.GetDirectoryName(dbPath)!);
+            Directory.CreateDirectory(Path.Combine(projectParent, "outside"));
+
+            const string outsideContent = "class Outside {}\n";
+            File.WriteAllText(Path.Combine(projectParent, "outside", "outside.cs"), outsideContent);
+
+            using (var db = new DbContext(dbPath))
+            {
+                db.InitializeSchema();
+                var writer = new DbWriter(db.Connection);
+                writer.SetMeta(DbContext.IndexedProjectRootMetaKey, staleRoot);
+            }
+            TestProjectHelper.InsertIndexedFile(dbPath, "../outside/outside.cs", "csharp", outsideContent);
+
+            var resolved = DbPathResolver.ResolveProjectRootForQuery(dbPath, dbPathExplicit: true);
+
+            Assert.Equal(staleRoot, resolved);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectParent);
+            TestProjectHelper.DeleteDirectory(staleParent);
+        }
+    }
+
+    [Theory]
+    [InlineData("../outside.cs")]
+    [InlineData("src/../../outside.cs")]
+    public void TryResolveIndexedFileSampleIoPath_RejectsEscapingRelativeSamples(string samplePath)
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_db_path_resolver_escape_sample");
+        try
+        {
+            var resolved = DbPathResolver.TryResolveIndexedFileSampleIoPath(projectRoot, samplePath, out var ioPath);
+
+            Assert.False(resolved);
+            Assert.Equal(string.Empty, ioPath);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
+    [Theory]
+    [InlineData("/outside.cs")]
+    public void TryResolveIndexedFileSampleIoPath_RejectsRootedSamples(string samplePath)
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_db_path_resolver_rooted_sample");
+        try
+        {
+            var resolved = DbPathResolver.TryResolveIndexedFileSampleIoPath(projectRoot, samplePath, out var ioPath);
+
+            Assert.False(resolved);
+            Assert.Equal(string.Empty, ioPath);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
+    [Fact]
+    public void TryResolveIndexedFileSampleIoPath_OnWindowsRejectsDriveAndUncSamples()
+    {
+        if (!OperatingSystem.IsWindows())
+            return;
+
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_db_path_resolver_windows_absolute_sample");
+        try
+        {
+            foreach (var samplePath in new[] { "\\outside.cs", "C:/outside.cs", @"C:\outside.cs", @"\\server\share\outside.cs" })
+            {
+                var resolved = DbPathResolver.TryResolveIndexedFileSampleIoPath(projectRoot, samplePath, out var ioPath);
+
+                Assert.False(resolved);
+                Assert.Equal(string.Empty, ioPath);
+            }
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
+    [Fact]
+    public void TryResolveIndexedFileSampleIoPath_OnPosixPreservesBackslashInFilename()
+    {
+        if (OperatingSystem.IsWindows())
+            return;
+
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_db_path_resolver_posix_backslash_sample");
+        try
+        {
+            const string samplePath = "back\\slash.py";
+            var resolved = DbPathResolver.TryResolveIndexedFileSampleIoPath(projectRoot, samplePath, out var ioPath);
+
+            Assert.True(resolved);
+            Assert.Equal(Path.GetFullPath(Path.Combine(projectRoot, samplePath)), ioPath);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
     [Fact]
     public void ResolveProjectRootForQuery_ExplicitProjectLocalReadOnlyUriWithoutMetadataReturnsNull()
     {