diff --git a/src-tauri/src/commands/secure_paste/reveal.rs b/src-tauri/src/commands/secure_paste/reveal.rs
index 9f33098..48d1eee 100644
--- a/src-tauri/src/commands/secure_paste/reveal.rs
+++ b/src-tauri/src/commands/secure_paste/reveal.rs
@@ -13,6 +13,18 @@ pub struct PasteRevealPayload {
     pub error: Option<String>,
 }
 
+/// Paste ids are 22-char base64url ShortGuids issued by the API. The id is interpolated into
+/// the DELETE path, so anything else (e.g. `../`, `?`) must be rejected before it gets there.
+fn is_paste_id(s: &str) -> bool {
+    s.len() == 22 && is_base64url(s)
+}
+
+fn is_base64url(s: &str) -> bool {
+    !s.is_empty()
+        && s.bytes()
+            .all(|b| b.is_ascii_alphanumeric() || b == b'-' || b == b'_')
+}
+
 /// Parse `cmdv://paste?id={pasteId}&key={encryptionKey}`.
 pub fn parse_paste_deep_link(url: &str) -> Option<(String, String)> {
     let url = url.trim();
@@ -29,7 +41,8 @@ pub fn parse_paste_deep_link(url: &str) -> Option<(String, String)> {
             _ => {}
         }
     }
-    Some((paste_id?, key?))
+    let (paste_id, key) = (paste_id?, key?);
+    (is_paste_id(&paste_id) && is_base64url(&key)).then_some((paste_id, key))
 }
 
 fn percent_decode(input: &str) -> String {
@@ -104,10 +117,29 @@ mod tests {
 
     #[test]
     fn parse_valid_paste_deep_link() {
-        let url = "cmdv://paste?id=abc123&key=YWJj";
+        let url = "cmdv://paste?id=AbCdEfGhIjKlMnOpQrSt_-&key=S2V5S2V5S2V5S2V5S2V5S2";
         let (id, key) = parse_paste_deep_link(url).unwrap();
-        assert_eq!(id, "abc123");
-        assert_eq!(key, "YWJj");
+        assert_eq!(id, "AbCdEfGhIjKlMnOpQrSt_-");
+        assert_eq!(key, "S2V5S2V5S2V5S2V5S2V5S2");
+    }
+
+    #[test]
+    fn parse_rejects_id_that_is_not_a_short_guid() {
+        for id in [
+            "..%2Fauth%2Fsessions%2F0f8fad5b-d9cb-469f-a165-70867728950e",
+            "abc%3Fx%3D1",
+            "abc123",
+            "AbCdEfGhIjKlMnOpQrSt_-x",
+        ] {
+            let url = format!("cmdv://paste?id={id}&key=S2V5S2V5S2V5S2V5S2V5S2");
+            assert!(parse_paste_deep_link(&url).is_none(), "accepted id {id}");
+        }
+    }
+
+    #[test]
+    fn parse_rejects_key_outside_base64url() {
+        let url = "cmdv://paste?id=AbCdEfGhIjKlMnOpQrSt_-&key=a%2Fb%2Bc";
+        assert!(parse_paste_deep_link(url).is_none());
     }
 
     #[test]