diff --git a/patch_v2/ncm_app/core/ncm_tag.c b/patch_v2/ncm_app/core/ncm_tag.c
index a439839..f79f56e 100644
--- a/patch_v2/ncm_app/core/ncm_tag.c
+++ b/patch_v2/ncm_app/core/ncm_tag.c
@@ -561,9 +561,10 @@ int ncm_tag_embed_file(const char *path, const char *ext, const ncm_tag_meta_t *
 {
     char tmp[NCM_TAG_PATH_MAX];
     struct stat st;
-    unsigned char magic[4];
+    unsigned char magic[12];
     unsigned char *buf;
     int in, out, rc, is_flac, n;
+    ssize_t mgot;
 
     if (!path || !path[0] || !m) return NCM_ERR_ARG;
     if (!m->title[0] && !m->artist[0] && !m->album[0]) return NCM_OK;   /* 无元数据：不动文件 */
@@ -579,12 +580,15 @@ int ncm_tag_embed_file(const char *path, const char *ext, const ncm_tag_meta_t *
     if (in < 0) return NCM_ERR_IO;
     if (fstat(in, &st) != 0 || st.st_size <= 0) { close(in); return NCM_ERR_IO; }
 
-    if (read_full(in, magic, sizeof magic) != (ssize_t)sizeof magic) { close(in); return NCM_ERR_IO; }
+    mgot = read_full(in, magic, sizeof magic);
+    if (mgot < 4) { close(in); return NCM_ERR_IO; }
     if (lseek(in, 0, SEEK_SET) < 0) { close(in); return NCM_ERR_IO; }
     is_flac = (memcmp(magic, "fLaC", 4) == 0);
     /* 声明 flac 却无 fLaC 魔数 = 残体/损坏：拒绝写（调用方 commit 裸流降级）。
-     * 声明 mp3 而内容实为 FLAC：按容器写 VORBIS_COMMENT（真机降级产物同理）。 */
+     * 声明 mp3 而内容实为 FLAC：按容器写 VORBIS_COMMENT（真机降级产物同理）。
+     * MP4/M4A（第 4..7 字节 "ftyp"）：ID3v2 头会破坏 MP4 容器 → 拒绝（裸流降级）。 */
     if (!is_flac && ext && strcasecmp(ext, "flac") == 0) { close(in); return NCM_ERR_ARG; }
+    if (!is_flac && mgot >= 8 && memcmp(magic + 4, "ftyp", 4) == 0) { close(in); return NCM_ERR_ARG; }
 
     buf = (unsigned char *)malloc(NCM_TAG_CHUNK);
     if (!buf) { close(in); return NCM_ERR_OOM; }
diff --git a/patch_v2/ncm_app/test/test_ncm_tag.c b/patch_v2/ncm_app/test/test_ncm_tag.c
index 81df582..a7f1403 100644
--- a/patch_v2/ncm_app/test/test_ncm_tag.c
+++ b/patch_v2/ncm_app/test/test_ncm_tag.c
@@ -347,6 +347,13 @@ static void test_edges(void)
         snprintf(tmp, sizeof tmp, "%s.tag.tmp", path);
         CHECK(stat(tmp, &st) != 0, "no .tag.tmp left behind");
     }
+    /* MP4/M4A（"....ftyp"）不是 mp3：ID3v2 前缀会破坏 MP4 容器 → 必须拒绝（降级裸流） */
+    snprintf(path, sizeof path, TMPDIR_PREFIX "/fake.m4a.mp3");
+    file_put(path, "\x00\x00\x00\x20""ftypisom\x00\x00\x00\x00", 16);
+    CHECK(ncm_tag_embed_file(path, "mp3", &m) != 0, "MP4 container rejected (no ID3 prefix)");
+    before = file_slurp(path, &blen);
+    CHECK(blen == 16 && memcmp(before + 4, "ftyp", 4) == 0, "MP4 file unchanged");
+    free(before);
     /* 不存在的文件 */
     CHECK(ncm_tag_embed_file(TMPDIR_PREFIX "/nope.mp3", "mp3", &m) != 0, "missing file -> error");
     CHECK(ncm_tag_read_file(TMPDIR_PREFIX "/nope.mp3", &m) == NCM_ERR_NOENT,