diff --git a/mypyc/lib-rt/librt_internal.c b/mypyc/lib-rt/librt_internal.c
index 6cae63cfadcb..eaf451eff22b 100644
--- a/mypyc/lib-rt/librt_internal.c
+++ b/mypyc/lib-rt/librt_internal.c
@@ -319,6 +319,11 @@ read_str_internal(PyObject *data) {
     CPyTagged tagged_size = _read_short_int(data, first);
     if (tagged_size == CPY_INT_TAG)
         return NULL;
+    if ((Py_ssize_t)tagged_size < 0) {
+        // Fail fast for invalid/tampered data.
+        PyErr_SetString(PyExc_ValueError, "invalid str size");
+        return NULL;
+    }
     Py_ssize_t size = tagged_size >> 1;
     // Read string content.
     char *buf = ((BufferObject *)data)->buf;
@@ -437,6 +442,11 @@ read_bytes_internal(PyObject *data) {
     CPyTagged tagged_size = _read_short_int(data, first);
     if (tagged_size == CPY_INT_TAG)
         return NULL;
+    if ((Py_ssize_t)tagged_size < 0) {
+        // Fail fast for invalid/tampered data.
+        PyErr_SetString(PyExc_ValueError, "invalid bytes size");
+        return NULL;
+    }
     Py_ssize_t size = tagged_size >> 1;
     // Read bytes content.
     char *buf = ((BufferObject *)data)->buf;
@@ -601,6 +611,10 @@ read_int_internal(PyObject *data) {
     Py_ssize_t size_and_sign = _read_short_int(data, first);
     if (size_and_sign == CPY_INT_TAG)
         return CPY_INT_TAG;
+    if ((Py_ssize_t)size_and_sign < 0) {
+        PyErr_SetString(PyExc_ValueError, "invalid int data");
+        return CPY_INT_TAG;
+    }
     bool sign = (size_and_sign >> 1) & 1;
     Py_ssize_t size = size_and_sign >> 2;
 
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index b02d10446800..0805da184e1a 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -5359,3 +5359,71 @@ def test_deletable_attr() -> None:
     assert i.del_counter == 1
 
 test_deletable_attr()
+
+[case testBufferCorruptedData_librt_internal]
+from librt.internal import (
+    Buffer, read_bool, read_str, read_float, read_int, read_tag, read_bytes
+)
+from random import randbytes
+
+def check(data: bytes) -> None:
+    b = Buffer(data)
+    try:
+        while True:
+            read_bool(b)
+    except ValueError:
+        pass
+    b = Buffer(data)
+    read_tag(b)  # Always succeeds
+    try:
+        while True:
+            read_int(b)
+    except ValueError:
+        pass
+    b = Buffer(data)
+    try:
+        while True:
+            read_str(b)
+    except ValueError:
+        pass
+    b = Buffer(data)
+    try:
+        while True:
+            read_bytes(b)
+    except ValueError:
+        pass
+    b = Buffer(data)
+    try:
+        while True:
+            read_float(b)
+    except ValueError:
+        pass
+
+import time
+
+def test_read_corrupted_data() -> None:
+    # Test various deterministic byte sequences (1 to 4 bytes).
+    t0 = time.time()
+    for a in range(256):
+        check(bytes([a]))
+    for a in range(256):
+        for b in range(256):
+            check(bytes([a, b]))
+    for a in range(32):
+        for b in range(48):
+            for c in range(48):
+                check(bytes([a, b, c]))
+    for a in range(32):
+        for b in (0, 5, 17, 34):
+            for c in (0, 5, 17, 34):
+                for d in (0, 5, 17, 34):
+                    check(bytes([a, b, c, d]))
+    # Also test some random data.
+    for i in range(20000):
+        data = randbytes(16)
+        try:
+            check(data)
+        except BaseException as e:
+            print("RANDOMIZED TEST FAILURE -- please open an issue with the following context:")
+            print(">>>", e, data)
+            raise