diff --git a/src/libmongoc/src/mongoc/mongoc-gridfs-bucket.c b/src/libmongoc/src/mongoc/mongoc-gridfs-bucket.c
index d61f7d4384..1f8f95cfe0 100644
--- a/src/libmongoc/src/mongoc/mongoc-gridfs-bucket.c
+++ b/src/libmongoc/src/mongoc/mongoc-gridfs-bucket.c
@@ -182,6 +182,7 @@ mongoc_gridfs_bucket_open_upload_stream_with_id(mongoc_gridfs_bucket_t *bucket,
    file->bucket = bucket;
    file->chunk_size = gridfs_opts.chunkSizeBytes;
    file->metadata = bson_copy(&gridfs_opts.metadata);
+   BSON_ASSERT(gridfs_opts.chunkSizeBytes > 0); // Validated in _mongoc_gridfs_bucket_opts_parse.
    file->buffer = bson_malloc((size_t)gridfs_opts.chunkSizeBytes);
    file->in_buffer = 0;
 
@@ -347,6 +348,26 @@ mongoc_gridfs_bucket_open_download_stream(mongoc_gridfs_bucket_t *bucket,
 
    bson_destroy(&file_doc);
 
+   if (file->chunk_size <= 0) {
+      _mongoc_set_error(error,
+                        MONGOC_ERROR_GRIDFS,
+                        MONGOC_ERROR_GRIDFS_CORRUPT,
+                        "File document contains invalid chunk size: %" PRId32,
+                        file->chunk_size);
+      _mongoc_gridfs_bucket_file_destroy(file);
+      return NULL;
+   }
+
+   if (file->length < 0) {
+      _mongoc_set_error(error,
+                        MONGOC_ERROR_GRIDFS,
+                        MONGOC_ERROR_GRIDFS_CORRUPT,
+                        "File document contains invalid length: %" PRId64,
+                        file->length);
+      _mongoc_gridfs_bucket_file_destroy(file);
+      return NULL;
+   }
+
    file->file_id = (bson_value_t *)bson_malloc0(sizeof *(file->file_id));
    bson_value_copy(file_id, file->file_id);
    file->bucket = bucket;
diff --git a/src/libmongoc/tests/test-mongoc-gridfs-bucket.c b/src/libmongoc/tests/test-mongoc-gridfs-bucket.c
index c12f8d7b82..3cd48cdc79 100644
--- a/src/libmongoc/tests/test-mongoc-gridfs-bucket.c
+++ b/src/libmongoc/tests/test-mongoc-gridfs-bucket.c
@@ -1048,6 +1048,180 @@ test_gridfs_bucket_opts(void)
    mongoc_client_destroy(client);
 }
 
+// Regression test for CDRIVER-6125
+static void
+test_bad_sizes(void)
+{
+   mongoc_client_t *client = test_framework_new_default_client();
+   bson_error_t error;
+
+   mongoc_database_t *db = mongoc_client_get_database(client, "test_bad_sizes");
+   mongoc_database_drop(db, NULL);
+
+   // Test negative chunkSize:
+   {
+      bson_t *doc = tmp_bson(BSON_STR({
+         "_id" : 0,
+         "filename" : "foo.txt",
+         "length" : 1000,
+         "chunkSize" : -1, // Negative!
+         "uploadDate" : {"$date" : 1234567890000}
+      }));
+
+      bson_iter_t id_iter;
+      ASSERT(bson_iter_init_find(&id_iter, doc, "_id"));
+      const bson_value_t *id_value = bson_iter_value(&id_iter);
+
+      // Insert manually:
+      {
+         mongoc_collection_t *files = mongoc_database_get_collection(db, "fs.files");
+         mongoc_collection_insert_one(files, doc, NULL, NULL, &error);
+         mongoc_collection_destroy(files);
+      }
+
+      // Try to read:
+      {
+         mongoc_gridfs_bucket_t *bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);
+         ASSERT_OR_PRINT(bucket, error);
+         mongoc_stream_t *stream = mongoc_gridfs_bucket_open_download_stream(bucket, id_value, &error);
+         ASSERT(!stream);
+         ASSERT_ERROR_CONTAINS(error, MONGOC_ERROR_GRIDFS, MONGOC_ERROR_GRIDFS_CORRUPT, "invalid chunk size");
+         mongoc_gridfs_bucket_destroy(bucket);
+      }
+
+      // Try to write:
+      {
+         mongoc_gridfs_bucket_t *bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);
+         ASSERT_OR_PRINT(bucket, error);
+         bson_t *opts = tmp_bson(BSON_STR({"chunkSizeBytes" : 0}));
+         mongoc_stream_t *stream = mongoc_gridfs_bucket_open_upload_stream(bucket, "foo.txt", opts, NULL, &error);
+         ASSERT(!stream);
+         ASSERT_ERROR_CONTAINS(
+            error, MONGOC_ERROR_COMMAND, MONGOC_ERROR_COMMAND_INVALID_ARG, "should be greater than 0");
+         mongoc_gridfs_bucket_destroy(bucket);
+      }
+   }
+
+   mongoc_database_drop(db, NULL);
+
+   // Test zero chunkSize:
+   {
+      bson_t *doc = tmp_bson(BSON_STR({
+         "_id" : 0,
+         "filename" : "foo.txt",
+         "length" : 1000,
+         "chunkSize" : 0, // Zero!
+         "uploadDate" : {"$date" : 1234567890000}
+      }));
+
+      bson_iter_t id_iter;
+      ASSERT(bson_iter_init_find(&id_iter, doc, "_id"));
+      const bson_value_t *id_value = bson_iter_value(&id_iter);
+
+      // Insert manually:
+      {
+         mongoc_collection_t *files = mongoc_database_get_collection(db, "fs.files");
+         mongoc_collection_insert_one(files, doc, NULL, NULL, &error);
+         mongoc_collection_destroy(files);
+      }
+
+      // Try to read:
+      {
+         mongoc_gridfs_bucket_t *bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);
+         ASSERT_OR_PRINT(bucket, error);
+         mongoc_stream_t *stream = mongoc_gridfs_bucket_open_download_stream(bucket, id_value, &error);
+         ASSERT(!stream);
+         ASSERT_ERROR_CONTAINS(error, MONGOC_ERROR_GRIDFS, MONGOC_ERROR_GRIDFS_CORRUPT, "invalid chunk size");
+         mongoc_gridfs_bucket_destroy(bucket);
+      }
+
+      // Try to write:
+      {
+         mongoc_gridfs_bucket_t *bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);
+         ASSERT_OR_PRINT(bucket, error);
+         bson_t *opts = tmp_bson(BSON_STR({"chunkSizeBytes" : -1}));
+         mongoc_stream_t *stream = mongoc_gridfs_bucket_open_upload_stream(bucket, "foo.txt", opts, NULL, &error);
+         ASSERT(!stream);
+         ASSERT_ERROR_CONTAINS(
+            error, MONGOC_ERROR_COMMAND, MONGOC_ERROR_COMMAND_INVALID_ARG, "should be greater than 0");
+         mongoc_gridfs_bucket_destroy(bucket);
+      }
+   }
+
+   mongoc_database_drop(db, NULL);
+
+   // Test negative length:
+   {
+      bson_t *doc = tmp_bson(BSON_STR({
+         "_id" : 0,
+         "filename" : "foo.txt",
+         "length" : -1, // Negative!
+         "chunkSize" : 10,
+         "uploadDate" : {"$date" : 1234567890000}
+      }));
+
+      bson_iter_t id_iter;
+      ASSERT(bson_iter_init_find(&id_iter, doc, "_id"));
+      const bson_value_t *id_value = bson_iter_value(&id_iter);
+
+      // Insert manually:
+      {
+         mongoc_collection_t *files = mongoc_database_get_collection(db, "fs.files");
+         mongoc_collection_insert_one(files, doc, NULL, NULL, &error);
+         mongoc_collection_destroy(files);
+      }
+
+      // Try to read:
+      {
+         mongoc_gridfs_bucket_t *bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);
+         mongoc_stream_t *stream = mongoc_gridfs_bucket_open_download_stream(bucket, id_value, &error);
+         ASSERT(!stream);
+         ASSERT_ERROR_CONTAINS(error, MONGOC_ERROR_GRIDFS, MONGOC_ERROR_GRIDFS_CORRUPT, "invalid length");
+         mongoc_gridfs_bucket_destroy(bucket);
+      }
+   }
+
+   mongoc_database_drop(db, NULL);
+
+   // Test a zero length (OK):
+   {
+      bson_t *doc = tmp_bson(BSON_STR({
+         "_id" : 0,
+         "filename" : "foo.txt",
+         "length" : 0, // Zero!
+         "chunkSize" : 10,
+         "uploadDate" : {"$date" : 1234567890000}
+      }));
+
+      bson_iter_t id_iter;
+      ASSERT(bson_iter_init_find(&id_iter, doc, "_id"));
+      const bson_value_t *id_value = bson_iter_value(&id_iter);
+
+      // Insert manually:
+      {
+         mongoc_collection_t *files = mongoc_database_get_collection(db, "fs.files");
+         mongoc_collection_insert_one(files, doc, NULL, NULL, &error);
+         mongoc_collection_destroy(files);
+      }
+
+      // Try to read:
+      {
+         mongoc_gridfs_bucket_t *bucket = mongoc_gridfs_bucket_new(db, NULL, NULL, &error);
+         mongoc_stream_t *stream = mongoc_gridfs_bucket_open_download_stream(bucket, id_value, &error);
+         ASSERT(stream);
+         // OK. Gets back an empty read.
+         uint8_t buf[64];
+         ssize_t r = mongoc_stream_read(stream, buf, sizeof buf, 0, 0);
+         ASSERT_CMPINT(r, ==, 0);
+         mongoc_stream_destroy(stream);
+         mongoc_gridfs_bucket_destroy(bucket);
+      }
+   }
+
+   mongoc_database_destroy(db);
+   mongoc_client_destroy(client);
+}
+
 void
 test_gridfs_bucket_install(TestSuite *suite)
 {
@@ -1070,4 +1244,5 @@ test_gridfs_bucket_install(TestSuite *suite)
                      test_framework_skip_if_no_sessions,
                      test_framework_skip_if_no_crypto);
    TestSuite_AddLive(suite, "/gridfs/options", test_gridfs_bucket_opts);
+   TestSuite_AddLive(suite, "/gridfs/bad_sizes", test_bad_sizes);
 }