diff --git a/crates/engine/src/templates.rs b/crates/engine/src/templates.rs
index a8009f47..500308b7 100644
--- a/crates/engine/src/templates.rs
+++ b/crates/engine/src/templates.rs
@@ -486,10 +486,10 @@ pub fn encode_thumb(w: u32, h: u32, rgba: &[u8]) -> String {
 /// Decode a thumbnail to RGB bytes (THUMB_W×THUMB_H×3).
 pub fn decode_thumb(text: &str) -> Option<Vec<u8>> {
     let t = text.trim();
-    if t.len() != THUMB_W * THUMB_H * 6 {
+    if t.len() != THUMB_W * THUMB_H * 6 || !t.is_ascii() {
         return None;
     }
-    (0..t.len()).step_by(2).map(|i| u8::from_str_radix(&t[i..i + 2], 16).ok()).collect()
+    (0..t.len()).step_by(2).map(|i| u8::from_str_radix(t.get(i..i + 2)?, 16).ok()).collect()
 }
 
 /// Render a project's main comp at its poster time into a thumbnail.
@@ -943,6 +943,12 @@ mod tests {
     use super::*;
     use crate::config::{DirConfig, MemoryConfig};
 
+    #[test]
+    fn non_ascii_thumbnail_is_rejected_without_panicking() {
+        let malformed = format!("aé{}", "0".repeat(THUMB_W * THUMB_H * 6 - 3));
+        assert!(decode_thumb(&malformed).is_none());
+    }
+
     #[test]
     fn every_builtin_builds_with_controls_and_renders() {
         let s = Session::default();
diff --git a/crates/ui-egui/src/panels/home.rs b/crates/ui-egui/src/panels/home.rs
index 269f2f44..fcfd0170 100644
--- a/crates/ui-egui/src/panels/home.rs
+++ b/crates/ui-egui/src/panels/home.rs
@@ -166,12 +166,13 @@ pub fn encode_thumb(img: &egui::ColorImage) -> String {
     out
 }
 
+/// Decode a stored hex thumbnail, rejecting malformed or non-ASCII text.
 pub fn decode_thumb(text: &str) -> Option<egui::ColorImage> {
     let t = text.trim();
-    if t.len() != THUMB_W * THUMB_H * 6 {
+    if t.len() != THUMB_W * THUMB_H * 6 || !t.is_ascii() {
         return None;
     }
-    let b: Vec<u8> = (0..t.len()).step_by(2).map(|i| u8::from_str_radix(&t[i..i + 2], 16)).collect::<Result<_, _>>().ok()?;
+    let b: Vec<u8> = (0..t.len()).step_by(2).map(|i| u8::from_str_radix(t.get(i..i + 2)?, 16).ok()).collect::<Option<_>>()?;
     let px = b.as_chunks::<3>().0.iter().map(|c| Color32::from_rgb(c[0], c[1], c[2])).collect();
     Some(egui::ColorImage::new([THUMB_W, THUMB_H], px))
 }
@@ -496,6 +497,12 @@ fn run_actions(app: &mut EffectcraftApp, ctx: &egui::Context, actions: Vec<(&str
 mod tests {
     use super::*;
 
+    #[test]
+    fn non_ascii_thumbnail_is_rejected_without_panicking() {
+        let malformed = format!("aé{}", "0".repeat(THUMB_W * THUMB_H * 6 - 3));
+        assert!(decode_thumb(&malformed).is_none());
+    }
+
     #[test]
     fn recent_list_comes_from_settings() {
         let mut prefs = effectcraft_engine::prefs::Prefs::default();