diff --git a/changelog.d/unreleased/3220.security.md b/changelog.d/unreleased/3220.security.md
new file mode 100644
index 0000000000..46f4564c0d
--- /dev/null
+++ b/changelog.d/unreleased/3220.security.md
@@ -0,0 +1,20 @@
+---
+category: security
+issues:
+  - 3220
+affected:
+  - src/CodeIndex/Cli/DbPathResolver.cs
+  - src/CodeIndex/Cli/DbCommandRunner.cs
+  - src/CodeIndex/Cli/DiffCommandRunner.cs
+  - src/CodeIndex/Database/DbContext.cs
+  - tests/CodeIndex.Tests/DbPathResolverTests.cs
+  - tests/CodeIndex.Tests/DbCommandRunnerTests.cs
+---
+
+## English
+
+- **SQLite file URI database opens now avoid connection-string injection (#3220)** — user-supplied `file:` URI database paths are built with `SqliteConnectionStringBuilder`, so `;Mode=...` payloads stay inside the data source value instead of becoming connection options.
+
+## 日本語
+
+- **SQLite file URI の DB open で connection-string injection を防止しました (#3220)** — ユーザー指定の `file:` URI DB パスは `SqliteConnectionStringBuilder` で組み立て、`;Mode=...` payload が接続オプションではなく data source 値に留まるようにしました。
diff --git a/src/CodeIndex/Cli/DbCommandRunner.cs b/src/CodeIndex/Cli/DbCommandRunner.cs
index 53ab56ec63..63f7dea9c1 100644
--- a/src/CodeIndex/Cli/DbCommandRunner.cs
+++ b/src/CodeIndex/Cli/DbCommandRunner.cs
@@ -422,13 +422,7 @@ private static DbIntegrityCheckReadResult RunIntegrityCheckPragma(string dbPath)
         if (IntegrityCheckRowsForTesting != null)
             return BoundIntegrityRows(IntegrityCheckRowsForTesting());
 
-        var connectionString = dbPath.StartsWith("file:", StringComparison.OrdinalIgnoreCase)
-            ? $"Data Source={dbPath}"
-            : new SqliteConnectionStringBuilder
-            {
-                DataSource = dbPath,
-                Mode = SqliteOpenMode.ReadOnly,
-            }.ConnectionString;
+        var connectionString = DbPathResolver.BuildSqliteConnectionString(dbPath, SqliteOpenMode.ReadOnly);
 
         using var connection = new SqliteConnection(connectionString);
         connection.Open();
@@ -571,13 +565,9 @@ FROM reference_lines rl
 
     private static SqliteConnection OpenConnection(string dbPath, bool writable)
     {
-        var connectionString = dbPath.StartsWith("file:", StringComparison.OrdinalIgnoreCase)
-            ? $"Data Source={dbPath}"
-            : new SqliteConnectionStringBuilder
-            {
-                DataSource = dbPath,
-                Mode = writable ? SqliteOpenMode.ReadWrite : SqliteOpenMode.ReadOnly,
-            }.ConnectionString;
+        var connectionString = DbPathResolver.BuildSqliteConnectionString(
+            dbPath,
+            writable ? SqliteOpenMode.ReadWrite : SqliteOpenMode.ReadOnly);
         var connection = new SqliteConnection(connectionString);
         connection.Open();
         return connection;
diff --git a/src/CodeIndex/Cli/DbPathResolver.cs b/src/CodeIndex/Cli/DbPathResolver.cs
index 1124e54045..b836182602 100644
--- a/src/CodeIndex/Cli/DbPathResolver.cs
+++ b/src/CodeIndex/Cli/DbPathResolver.cs
@@ -291,6 +291,17 @@ public static bool TryResolveWritableMutationDbPath(string dbPath, out string wr
         return true;
     }
 
+    internal static string BuildSqliteConnectionString(string dbPath, SqliteOpenMode? mode = null)
+    {
+        var builder = new SqliteConnectionStringBuilder
+        {
+            DataSource = dbPath,
+        };
+        if (mode.HasValue)
+            builder.Mode = mode.Value;
+        return builder.ConnectionString;
+    }
+
     private static string? TryReadIndexedProjectRoot(string dbPath)
         => TryReadMetaString(dbPath, CodeIndex.Database.DbContext.IndexedProjectRootMetaKey);
 
@@ -412,15 +423,7 @@ private static bool SiblingRootMatchesIndexedContents(string dbPath, string full
 
     private static SqliteConnection OpenMetadataConnection(string dbPath)
     {
-        if (dbPath.StartsWith("file:", StringComparison.OrdinalIgnoreCase) && UriRequestsReadOnly(dbPath))
-            return new SqliteConnection($"Data Source={dbPath}");
-
-        var builder = new SqliteConnectionStringBuilder
-        {
-            DataSource = dbPath,
-            Mode = SqliteOpenMode.ReadOnly,
-        };
-        return new SqliteConnection(builder.ConnectionString);
+        return new SqliteConnection(BuildSqliteConnectionString(dbPath, SqliteOpenMode.ReadOnly));
     }
 
     public static bool UriRequestsReadOnly(string uriText)
diff --git a/src/CodeIndex/Cli/DiffCommandRunner.cs b/src/CodeIndex/Cli/DiffCommandRunner.cs
index bb5f979637..2511388b99 100644
--- a/src/CodeIndex/Cli/DiffCommandRunner.cs
+++ b/src/CodeIndex/Cli/DiffCommandRunner.cs
@@ -626,13 +626,7 @@ private static SqliteConnection OpenReadOnlyConnection(string dbPath)
         if (!isUri && !File.Exists(LongPath.EnsureWindowsPrefix(dbPath)))
             throw new IOException($"database not found: {dbPath}");
 
-        var connectionString = isUri
-            ? $"Data Source={dbPath}"
-            : new SqliteConnectionStringBuilder
-            {
-                DataSource = dbPath,
-                Mode = SqliteOpenMode.ReadOnly,
-            }.ConnectionString;
+        var connectionString = DbPathResolver.BuildSqliteConnectionString(dbPath, SqliteOpenMode.ReadOnly);
 
         var connection = new SqliteConnection(connectionString);
         connection.Open();
diff --git a/src/CodeIndex/Database/DbContext.cs b/src/CodeIndex/Database/DbContext.cs
index 8b5eaab032..b60bd09ffd 100644
--- a/src/CodeIndex/Database/DbContext.cs
+++ b/src/CodeIndex/Database/DbContext.cs
@@ -252,7 +252,7 @@ public DbContext(string dbPath)
             {
                 try
                 {
-                    _connection = new SqliteConnection($"Data Source={dbPath}");
+                    _connection = new SqliteConnection(DbPathResolver.BuildSqliteConnectionString(dbPath, SqliteOpenMode.ReadOnly));
                     _connection.Open();
                     Execute("PRAGMA busy_timeout=5000");
                     ApplyConnectionPerformancePragmas();
diff --git a/tests/CodeIndex.Tests/DbCommandRunnerTests.cs b/tests/CodeIndex.Tests/DbCommandRunnerTests.cs
index f09f9fc848..02b4b65bbb 100644
--- a/tests/CodeIndex.Tests/DbCommandRunnerTests.cs
+++ b/tests/CodeIndex.Tests/DbCommandRunnerTests.cs
@@ -121,6 +121,27 @@ public void Run_MissingDb_ReturnsNotFoundWithHint()
         Assert.Contains("cdidx index <projectPath>", stderr);
     }
 
+    [Fact]
+    public void Run_IntegrityCheck_FileUriSemicolonPayloadDoesNotCreateDatabase_Issue3220()
+    {
+        var missingDb = Path.Combine(Path.GetTempPath(), $"cdidx_db_uri_injection_{Guid.NewGuid():N}.db");
+        var uri = new Uri(missingDb).AbsoluteUri + ";Mode=ReadWriteCreate";
+        try
+        {
+            var (exitCode, _, stderr) = RunAndCaptureStreams(["--integrity-check", "--db", uri]);
+
+            Assert.Equal(CommandExitCodes.DatabaseError, exitCode);
+            Assert.Contains("failed to run integrity check", stderr);
+            Assert.False(File.Exists(missingDb));
+        }
+        finally
+        {
+            SqliteConnection.ClearAllPools();
+            if (File.Exists(missingDb))
+                File.Delete(missingDb);
+        }
+    }
+
     [Fact]
     public void Run_MissingDb_JsonShapeIncludesHint()
     {
diff --git a/tests/CodeIndex.Tests/DbPathResolverTests.cs b/tests/CodeIndex.Tests/DbPathResolverTests.cs
index b81156b4ae..7655d4f15b 100644
--- a/tests/CodeIndex.Tests/DbPathResolverTests.cs
+++ b/tests/CodeIndex.Tests/DbPathResolverTests.cs
@@ -36,6 +36,19 @@ public void ResolveForIndex_PrefersExplicitPath()
         Assert.Equal(explicitPath, dbPath);
     }
 
+    [Fact]
+    public void BuildSqliteConnectionString_FileUriKeepsSemicolonPayloadInDataSource_Issue3220()
+    {
+        const string uri = "file:///tmp/codeindex.db?immutable=1;Mode=ReadWriteCreate;Cache=Shared";
+
+        var connectionString = DbPathResolver.BuildSqliteConnectionString(uri, SqliteOpenMode.ReadOnly);
+        var parsed = new SqliteConnectionStringBuilder(connectionString);
+
+        Assert.Equal(uri, parsed.DataSource);
+        Assert.Equal(SqliteOpenMode.ReadOnly, parsed.Mode);
+        Assert.NotEqual(SqliteOpenMode.ReadWriteCreate, parsed.Mode);
+    }
+
     [Fact]
     public void ResolveForIndex_PrefersExplicitDataDirWhenDbPathMissing()
     {