diff --git a/sdk/include/reactos/imm32_undoc.h b/sdk/include/reactos/imm32_undoc.h
index 5897579654a..8b1d1a12b78 100644
--- a/sdk/include/reactos/imm32_undoc.h
+++ b/sdk/include/reactos/imm32_undoc.h
@@ -31,6 +31,18 @@ extern "C" {
 
 #define WM_IME_SYSTEM 0x00000287
 
+/* The additional dwIndex value for ImmGetCompositionStringA/W */
+#define GCS_PRIVATE 0x8000
+
+/* Used in ImmGetCompositionStringA/W */
+typedef struct tagCOMPSTR_PRIVATE
+{
+    DWORD dwUnknown0; /* FIXME: Rename */
+    DWORD dwUnknown1; /* FIXME: Rename */
+    DWORD dwLen;
+    DWORD dwOffset;
+} COMPSTR_PRIVATE, *PCOMPSTR_PRIVATE, *LPCOMPSTR_PRIVATE;
+
 /* wParam for WM_IME_SYSTEM */
 #define IMS_NOTIFYIMESHOW       0x05
 #define IMS_UPDATEIMEUI         0x06
@@ -330,6 +342,7 @@ HRESULT WINAPI CtfImmLastEnabledWndDestroy(_In_ BOOL bCreate);
 HRESULT WINAPI CtfImmTIMActivate(_In_ HKL hKL);
 BOOL WINAPI CtfImmIsTextFrameServiceDisabled(VOID);
 BOOL WINAPI CtfImmIsCiceroEnabled(VOID);
+BOOL WINAPI CtfImmIsGuidMapEnable(_In_ HIMC hIMC);
 
 LRESULT WINAPI
 CtfImmDispatchDefImeMessage(
diff --git a/win32ss/user/imm32/compstr.c b/win32ss/user/imm32/compstr.c
index 8358f42dc43..f2be03aa0fd 100644
--- a/win32ss/user/imm32/compstr.c
+++ b/win32ss/user/imm32/compstr.c
@@ -6,7 +6,7 @@
  *              Copyright 2002, 2003, 2007 CodeWeavers, Aric Stewart
  *              Copyright 2017 James Tabor <james.tabor@reactos.org>
  *              Copyright 2018 Amine Khaldi <amine.khaldi@reactos.org>
- *              Copyright 2020-2021 Katayama Hirofumi MZ <katayama.hirofumi.mz@gmail.com>
+ *              Copyright 2020-2026 Katayama Hirofumi MZ <katayama.hirofumi.mz@gmail.com>
  */
 
 #include "precomp.h"
@@ -214,6 +214,35 @@ Imm32CompClauseWideToAnsi(const DWORD *source, INT slen, LPCWSTR text,
 #define CS_DoAttr CS_DoStrA
 #define CS_DoClause CS_DoStrA
 
+static DWORD CS_DoPrivate(HIMC hIMC, const COMPOSITIONSTRING *pCS, PVOID pBuffer, DWORD dwBufLen)
+{
+    if (!CtfImmIsGuidMapEnable(hIMC) || pCS->dwPrivateSize < sizeof(COMPSTR_PRIVATE))
+        return IMM_ERROR_GENERAL;
+
+    /* Check boundary #1 (ReactOS only) */
+    DWORD dwPrivateOffset = pCS->dwPrivateOffset;
+    if (dwPrivateOffset >= pCS->dwSize || dwPrivateOffset + pCS->dwPrivateSize > pCS->dwSize)
+        return IMM_ERROR_GENERAL;
+
+    const BYTE *pbCS = (const BYTE *)pCS;
+    const COMPSTR_PRIVATE *pPrivate = (const COMPSTR_PRIVATE *)(pbCS + dwPrivateOffset);
+    DWORD dwLen = pPrivate->dwLen, dwOffset = pPrivate->dwOffset;
+
+    /* Check boundary #2 (ReactOS only) */
+    if (dwPrivateOffset + dwOffset > pCS->dwPrivateSize ||
+        dwPrivateOffset + dwOffset + dwLen > pCS->dwPrivateSize)
+    {
+        return IMM_ERROR_GENERAL;
+    }
+
+    if (!dwBufLen)
+        return dwLen;
+
+    DWORD ret = min(dwBufLen, dwLen);
+    CopyMemory(pBuffer, (const BYTE *)pPrivate + dwOffset, ret);
+    return ret;
+}
+
 static LONG
 Imm32GetCompStrA(HIMC hIMC, const COMPOSITIONSTRING *pCS, DWORD dwIndex,
                  LPVOID lpBuf, DWORD dwBufLen, BOOL bAnsiClient, UINT uCodePage)
@@ -270,6 +299,9 @@ Imm32GetCompStrA(HIMC hIMC, const COMPOSITIONSTRING *pCS, DWORD dwIndex,
                 CS_DoClause(pCS, ResultClause);
                 break;
 
+            case GCS_PRIVATE:
+                return CS_DoPrivate(hIMC, pCS, lpBuf, dwBufLen);
+
             default:
                 FIXME("0x%X\n", dwIndex);
                 return IMM_ERROR_GENERAL;
@@ -355,6 +387,9 @@ Imm32GetCompStrA(HIMC hIMC, const COMPOSITIONSTRING *pCS, DWORD dwIndex,
                                                      lpBuf, dwBufLen, uCodePage);
                 break;
 
+            case GCS_PRIVATE:
+                return CS_DoPrivate(hIMC, pCS, lpBuf, dwBufLen);
+
             default:
                 FIXME("0x%X\n", dwIndex);
                 return IMM_ERROR_GENERAL;
@@ -447,6 +482,9 @@ Imm32GetCompStrW(HIMC hIMC, const COMPOSITIONSTRING *pCS, DWORD dwIndex,
                                                      lpBuf, dwBufLen, uCodePage);
                 break;
 
+            case GCS_PRIVATE:
+                return CS_DoPrivate(hIMC, pCS, lpBuf, dwBufLen);
+
             default:
                 FIXME("0x%X\n", dwIndex);
                 return IMM_ERROR_GENERAL;
@@ -504,6 +542,9 @@ Imm32GetCompStrW(HIMC hIMC, const COMPOSITIONSTRING *pCS, DWORD dwIndex,
                 CS_DoClause(pCS, ResultClause);
                 break;
 
+            case GCS_PRIVATE:
+                return CS_DoPrivate(hIMC, pCS, lpBuf, dwBufLen);
+
             default:
                 FIXME("0x%X\n", dwIndex);
                 return IMM_ERROR_GENERAL;