diff --git a/virtweb_backend/Cargo.lock b/virtweb_backend/Cargo.lock
index 66d3308..6516a15 100644
--- a/virtweb_backend/Cargo.lock
+++ b/virtweb_backend/Cargo.lock
@@ -1982,6 +1982,16 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "libyml"
+version = "0.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3302702afa434ffa30847a83305f0a69d6abd74293b6554c18ec85c7ef30c980"
+dependencies = [
+ "anyhow",
+ "version_check",
+]
+
 [[package]]
 name = "light-openid"
 version = "1.0.4"
@@ -3093,6 +3103,21 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serde_yml"
+version = "0.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59e2dd588bf1597a252c3b920e0143eb99b0f76e4e082f4c92ce34fbc9e71ddd"
+dependencies = [
+ "indexmap",
+ "itoa",
+ "libyml",
+ "memchr",
+ "ryu",
+ "serde",
+ "version_check",
+]
+
 [[package]]
 name = "sha1"
 version = "0.10.6"
@@ -3754,6 +3779,7 @@ dependencies = [
  "rust-embed",
  "serde",
  "serde_json",
+ "serde_yml",
  "sysinfo",
  "tempfile",
  "thiserror 2.0.12",
diff --git a/virtweb_backend/Cargo.toml b/virtweb_backend/Cargo.toml
index 9c75bd0..ee19a16 100644
--- a/virtweb_backend/Cargo.toml
+++ b/virtweb_backend/Cargo.toml
@@ -22,6 +22,7 @@ actix-ws = "0.3.0"
 actix-http = "3.10.0"
 serde = { version = "1.0.219", features = ["derive"] }
 serde_json = "1.0.140"
+serde_yml = "0.0.12"
 quick-xml = { version = "0.37.5", features = ["serialize", "overlapped-lists"] }
 futures-util = "0.3.31"
 anyhow = "1.0.98"
diff --git a/virtweb_backend/src/libvirt_rest_structures/mod.rs b/virtweb_backend/src/libvirt_rest_structures/mod.rs
index 8984e81..cdbbcd1 100644
--- a/virtweb_backend/src/libvirt_rest_structures/mod.rs
+++ b/virtweb_backend/src/libvirt_rest_structures/mod.rs
@@ -13,4 +13,6 @@ enum LibVirtStructError {
     ParseFilteringChain(String),
     #[error("NetworkFilterExtractionError: {0}")]
     NetworkFilterExtraction(String),
+    #[error("CloudInitConfigurationError: {0}")]
+    CloudInitConfiguration(String),
 }
diff --git a/virtweb_backend/src/libvirt_rest_structures/vm.rs b/virtweb_backend/src/libvirt_rest_structures/vm.rs
index 68f10f1..8042aa7 100644
--- a/virtweb_backend/src/libvirt_rest_structures/vm.rs
+++ b/virtweb_backend/src/libvirt_rest_structures/vm.rs
@@ -3,7 +3,9 @@ use crate::constants;
 use crate::libvirt_lib_structures::XMLUuid;
 use crate::libvirt_lib_structures::domain::*;
 use crate::libvirt_rest_structures::LibVirtStructError;
-use crate::libvirt_rest_structures::LibVirtStructError::StructureExtraction;
+use crate::libvirt_rest_structures::LibVirtStructError::{
+    CloudInitConfiguration, StructureExtraction,
+};
 use crate::utils::cloud_init_utils::CloudInitConfig;
 use crate::utils::file_size_utils::FileSize;
 use crate::utils::files_utils;
@@ -142,6 +144,10 @@ impl VMInfo {
             return Err(StructureExtraction("Invalid number of vCPU specified!").into());
         }
 
+        if let Some(e) = self.cloud_init.check_error() {
+            return Err(CloudInitConfiguration(e).into());
+        }
+
         let mut iso_absolute_files = vec![];
 
         // Process cloud init image
diff --git a/virtweb_backend/src/utils/cloud_init_utils.rs b/virtweb_backend/src/utils/cloud_init_utils.rs
index d689b1a..8891035 100644
--- a/virtweb_backend/src/utils/cloud_init_utils.rs
+++ b/virtweb_backend/src/utils/cloud_init_utils.rs
@@ -35,6 +35,27 @@ pub struct CloudInitConfig {
 }
 
 impl CloudInitConfig {
+    /// Check cloud init configuration
+    pub fn check_error(&self) -> Option<String> {
+        if !self.user_data.is_empty() {
+            // Check YAML content
+            if let Err(e) = serde_yml::from_str::<serde_json::Value>(&self.user_data) {
+                return Some(format!(
+                    "user data is an invalid YAML file! Deserialization error: {e}"
+                ));
+            }
+
+            // Check first line
+            if !self.user_data.starts_with("#cloud-config\n") {
+                return Some(
+                    "user data file MUST start with '#cloud-config' as first line!".to_string(),
+                );
+            }
+        }
+
+        None
+    }
+
     /// Generate disk image for nocloud usage
     pub fn generate_nocloud_disk(&self) -> anyhow::Result<Vec<u8>> {
         let temp_path = tempfile::tempdir_in(&AppConfig::get().temp_dir)?;