diff --git a/crates/formats/src/der.rs b/crates/formats/src/der.rs
index 587900ad..ed7cc50e 100644
--- a/crates/formats/src/der.rs
+++ b/crates/formats/src/der.rs
@@ -88,10 +88,19 @@ pub fn children(data: &[u8], parent: &Tlv) -> Vec<Tlv> {
 
 /// Dotted form of an OBJECT IDENTIFIER's content.
 pub fn oid(content: &[u8]) -> String {
+    if content.is_empty() || content.len() > 64 {
+        return "invalid".into();
+    }
     let mut parts: Vec<String> = Vec::new();
     let mut v: u64 = 0;
     let mut first = true;
-    for &b in content.iter().take(64) {
+    let mut arc_start = true;
+    for &b in content {
+        // DER subidentifiers use the shortest base-128 encoding.
+        if arc_start && b == 0x80 {
+            return "invalid".into();
+        }
+        arc_start = b & 0x80 == 0;
         v = match v.checked_mul(128) {
             Some(x) => x | u64::from(b & 0x7f),
             None => return "invalid".into(),
@@ -114,6 +123,9 @@ pub fn oid(content: &[u8]) -> String {
             v = 0;
         }
     }
+    if !arc_start {
+        return "invalid".into();
+    }
     parts.join(".")
 }
 
diff --git a/crates/formats/tests/der_oid_validation.rs b/crates/formats/tests/der_oid_validation.rs
new file mode 100644
index 00000000..cecdcb68
--- /dev/null
+++ b/crates/formats/tests/der_oid_validation.rs
@@ -0,0 +1,28 @@
+use craft_anything_formats::der::{oid, oid_name};
+
+#[test]
+fn unfinished_oid_does_not_alias_a_known_algorithm() {
+    let mut algorithm = vec![0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 1, 1, 0x0b];
+    assert_eq!(oid_name(&oid(&algorithm)), Some("sha256WithRSAEncryption"));
+    algorithm.push(0x80);
+    assert_eq!(oid(&algorithm), "invalid");
+    assert!(oid_name(&oid(&algorithm)).is_none());
+}
+
+#[test]
+fn malformed_and_oversized_oids_are_not_returned_as_prefixes() {
+    assert_eq!(oid(&[]), "invalid");
+    assert_eq!(oid(&[0x80, 0x2a]), "invalid");
+    assert_eq!(oid(&[0x2a, 0x80, 0]), "invalid");
+    assert_eq!(oid(&[0; 65]), "invalid");
+    assert_eq!(oid(&[0xff; 11]), "invalid");
+}
+
+#[test]
+fn complete_oid_arcs_and_the_length_boundary_remain_supported() {
+    assert_eq!(oid(&[0x88, 0x37, 3]), "2.999.3");
+    let mut boundary = vec![0x2a];
+    boundary.resize(64, 0);
+    assert!(oid(&boundary).starts_with("1.2.0.0."));
+    assert_eq!(oid(&boundary).split('.').count(), 65);
+}