diff --git a/crates/bashkit/src/interpreter/mod.rs b/crates/bashkit/src/interpreter/mod.rs
index 26d5ee7fe..e8a9eba23 100644
--- a/crates/bashkit/src/interpreter/mod.rs
+++ b/crates/bashkit/src/interpreter/mod.rs
@@ -5012,7 +5012,16 @@ impl Interpreter {
     }
 
     fn ensure_persistent_fd_capacity(&self, fd: i32) -> Result<()> {
-        if fd <= 2 || self.exec_fd_table.contains_key(&fd) || self.coproc_buffers.contains_key(&fd)
+        if fd < 0 {
+            return Err(crate::error::Error::Execution(format!(
+                "invalid file descriptor: {}",
+                fd
+            )));
+        }
+
+        if (0..=2).contains(&fd)
+            || self.exec_fd_table.contains_key(&fd)
+            || self.coproc_buffers.contains_key(&fd)
         {
             return Ok(());
         }
diff --git a/crates/bashkit/tests/integration/threat_model_tests.rs b/crates/bashkit/tests/integration/threat_model_tests.rs
index 7bf0b4be9..08cd10125 100644
--- a/crates/bashkit/tests/integration/threat_model_tests.rs
+++ b/crates/bashkit/tests/integration/threat_model_tests.rs
@@ -74,6 +74,41 @@ mod resource_exhaustion {
         assert_eq!(result.stdout.trim(), "ok");
     }
 
+    /// TM-DOS-063: Negative fd vars must be rejected and must not bypass fd limits.
+    #[tokio::test]
+    async fn fd_limit_rejects_negative_fd_var_output() {
+        let limits = ExecutionLimits::new().max_file_descriptors(1);
+        let mut bash = Bash::builder().limits(limits).build();
+
+        let result = bash.exec("v=-1; exec {v}>/tmp/neg-out").await;
+
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(
+            err.contains("invalid file descriptor"),
+            "Expected invalid file descriptor error, got: {}",
+            err
+        );
+    }
+
+    /// TM-DOS-063: Negative fd vars for input must be rejected.
+    #[tokio::test]
+    async fn fd_limit_rejects_negative_fd_var_input() {
+        let limits = ExecutionLimits::new().max_file_descriptors(1);
+        let mut bash = Bash::builder().limits(limits).build();
+        bash.exec("echo hi >/tmp/neg-in").await.unwrap();
+
+        let result = bash.exec("v=-1; exec {v}</tmp/neg-in").await;
+
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(
+            err.contains("invalid file descriptor"),
+            "Expected invalid file descriptor error, got: {}",
+            err
+        );
+    }
+
     /// Subsequent exec() calls recover after a prior call hits the command limit.
     /// Each exec() is a separate script invocation and gets its own budget.
     #[tokio::test]