diff --git a/src/xmodem.cpp b/src/xmodem.cpp
index ce7ad8020d0..58339a369dc 100644
--- a/src/xmodem.cpp
+++ b/src/xmodem.cpp
@@ -62,10 +62,13 @@ bool XModemAdapter::isValidFilename(const char *name)
 {
     if (!name || name[0] == '\0')
         return false;
+    const bool driveLetter = (name[0] >= 'A' && name[0] <= 'Z') || (name[0] >= 'a' && name[0] <= 'z');
+    if (driveLetter && name[1] == ':')
+        return false;
     // Reject any ".." path component. Absolute paths and subdirectories are fine; they stay within
     // the filesystem root, so only traversal out of it needs blocking.
     for (const char *seg = name; *seg;) {
-        const char *slash = strchr(seg, '/');
+        const char *slash = strpbrk(seg, "/\\");
         const size_t len = slash ? (size_t)(slash - seg) : strlen(seg);
         if (len == 2 && seg[0] == '.' && seg[1] == '.')
             return false;
diff --git a/test/test_xmodem/test_main.cpp b/test/test_xmodem/test_main.cpp
index 816c78e9c01..c6a20fdf051 100644
--- a/test/test_xmodem/test_main.cpp
+++ b/test/test_xmodem/test_main.cpp
@@ -21,6 +21,22 @@ void test_xmodem_rejects_dotdot_traversal(void)
     TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("/.."));
 }
 
+void test_xmodem_rejects_backslash_traversal(void)
+{
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("..\\secret"));
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("..\\..\\Windows\\System32\\drivers\\etc\\hosts"));
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("dir\\..\\..\\x"));
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("dir/..\\x"));
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("dir\\.."));
+}
+
+void test_xmodem_rejects_drive_qualified(void)
+{
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("C:\\Windows\\System32\\x"));
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("C:/Windows/System32/x"));
+    TEST_ASSERT_FALSE(XModemAdapter::isValidFilename("c:relative.txt"));
+}
+
 void test_xmodem_rejects_empty(void)
 {
     TEST_ASSERT_FALSE(XModemAdapter::isValidFilename(""));
@@ -36,6 +52,9 @@ void test_xmodem_allows_legit_paths(void)
     // ".." only inside a name (not a whole component) is a valid filename, not traversal.
     TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("my..file"));
     TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("..."));
+    // A colon that cannot form a drive qualifier is a legal filename on the POSIX daemon.
+    TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("1:30pm.txt"));
+    TEST_ASSERT_TRUE(XModemAdapter::isValidFilename("dir/1:30pm.txt"));
 }
 
 #endif // FSCom
@@ -46,6 +65,8 @@ void setup()
     UNITY_BEGIN();
 #ifdef FSCom
     RUN_TEST(test_xmodem_rejects_dotdot_traversal);
+    RUN_TEST(test_xmodem_rejects_backslash_traversal);
+    RUN_TEST(test_xmodem_rejects_drive_qualified);
     RUN_TEST(test_xmodem_rejects_empty);
     RUN_TEST(test_xmodem_allows_legit_paths);
 #endif