diff --git a/lib/state/block.rs b/lib/state/block.rs
index 549f2bf..09d5708 100644
--- a/lib/state/block.rs
+++ b/lib/state/block.rs
@@ -84,6 +84,21 @@ pub fn validate(
         };
         return Err(err);
     }
+    // The authorization and address checks below pair authorizations with
+    // spent UTXOs positionally via `zip`, which silently ignores any trailing
+    // inputs when there are fewer authorizations than inputs requiring one.
+    // Require an exact count so that every input requiring authorization is
+    // covered by a provided authorization.
+    let n_authorizations_required: usize = filled_txs
+        .iter()
+        .map(|t| t.spent_utxos_requiring_auth().len())
+        .sum();
+    if body.authorizations.len() != n_authorizations_required {
+        return Err(Error::WrongNumberOfAuthorizations {
+            expected: n_authorizations_required,
+            received: body.authorizations.len(),
+        });
+    }
     let spent_utxos = filled_txs
         .iter()
         .flat_map(|t| t.spent_utxos_requiring_auth().into_iter());
@@ -168,6 +183,21 @@ pub fn prevalidate(
         };
         return Err(err);
     }
+    // The authorization and address checks below pair authorizations with
+    // spent UTXOs positionally via `zip`, which silently ignores any trailing
+    // inputs when there are fewer authorizations than inputs requiring one.
+    // Require an exact count so that every input requiring authorization is
+    // covered by a provided authorization.
+    let n_authorizations_required: usize = filled_transactions
+        .iter()
+        .map(|t| t.spent_utxos_requiring_auth().len())
+        .sum();
+    if body.authorizations.len() != n_authorizations_required {
+        return Err(Error::WrongNumberOfAuthorizations {
+            expected: n_authorizations_required,
+            received: body.authorizations.len(),
+        });
+    }
     let spent_utxos = filled_transactions
         .iter()
         .flat_map(|t| t.spent_utxos_requiring_auth().into_iter());
diff --git a/lib/state/error.rs b/lib/state/error.rs
index 2c7a8e9..1d16103 100644
--- a/lib/state/error.rs
+++ b/lib/state/error.rs
@@ -265,6 +265,10 @@ pub enum Error {
     UtxoDoubleSpent,
     #[error(transparent)]
     WithdrawalBundle(#[from] WithdrawalBundleError),
+    #[error(
+        "wrong number of authorizations: expected {expected}, but received {received}"
+    )]
+    WrongNumberOfAuthorizations { expected: usize, received: usize },
     #[error("wrong public key for address")]
     WrongPubKeyForAddress,
 }
diff --git a/lib/state/mod.rs b/lib/state/mod.rs
index fe2fb85..c50d1ab 100644
--- a/lib/state/mod.rs
+++ b/lib/state/mod.rs
@@ -484,6 +484,17 @@ impl State {
     ) -> Result<bitcoin::Amount, Error> {
         let filled_transaction =
             self.fill_transaction(rotxn, &transaction.transaction)?;
+        // Pairing authorizations with spent UTXOs via `zip` silently ignores
+        // trailing inputs when there are too few authorizations. Require an
+        // exact count so that every input requiring authorization is covered.
+        let n_authorizations_required =
+            filled_transaction.spent_utxos_requiring_auth().len();
+        if transaction.authorizations.len() != n_authorizations_required {
+            return Err(Error::WrongNumberOfAuthorizations {
+                expected: n_authorizations_required,
+                received: transaction.authorizations.len(),
+            });
+        }
         for (authorization, spent_utxo) in transaction
             .authorizations
             .iter()
@@ -650,3 +661,111 @@ impl Watchable<()> for State {
         tokio_stream::wrappers::WatchStream::new(self.tip.watch().clone())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use ed25519_dalek::SigningKey;
+
+    use super::{Error, State};
+    use crate::{
+        authorization,
+        types::{
+            Address, AuthorizedTransaction, BitcoinOutputContent, FilledOutput,
+            FilledOutputContent, OutPoint, OutPointKey, Output, OutputContent,
+            Transaction, VerifyingKey,
+        },
+    };
+
+    /// Open a fresh state in a unique temporary directory.
+    fn new_state() -> (sneed::Env, State) {
+        let path = std::env::temp_dir()
+            .join(format!("plain_bitnames_auth_count_{}", std::process::id()));
+        let _remove_result = std::fs::remove_dir_all(&path);
+        std::fs::create_dir_all(&path).unwrap();
+        let env = {
+            let mut opts = heed::EnvOpenOptions::new();
+            opts.map_size(10 * 1024 * 1024).max_dbs(State::NUM_DBS);
+            unsafe { sneed::Env::open(&opts, &path) }.unwrap()
+        };
+        let state = State::new(&env).unwrap();
+        (env, state)
+    }
+
+    /// Fund `address` with a single bitcoin UTXO of `value` sats, returning its
+    /// outpoint.
+    fn fund(
+        env: &sneed::Env,
+        state: &State,
+        address: Address,
+        value: u64,
+    ) -> OutPoint {
+        let outpoint = OutPoint::Regular {
+            txid: Default::default(),
+            vout: 0,
+        };
+        let output = FilledOutput::new(
+            address,
+            FilledOutputContent::Bitcoin(BitcoinOutputContent(
+                bitcoin::Amount::from_sat(value),
+            )),
+        );
+        let mut rwtxn = env.write_txn().unwrap();
+        state
+            .utxos
+            .put(&mut rwtxn, &OutPointKey::from(&outpoint), &output)
+            .unwrap();
+        rwtxn.commit().unwrap();
+        outpoint
+    }
+
+    /// A transaction that spends an input without supplying an authorization
+    /// for it must be rejected. Otherwise the `zip` of authorizations and
+    /// spent UTXOs silently skips the unauthorized input, allowing any UTXO to
+    /// be spent without a signature.
+    #[test]
+    fn validate_transaction_rejects_missing_authorization() {
+        let (env, state) = new_state();
+        let signing_key = SigningKey::from_bytes(&[1u8; 32]);
+        let verifying_key: VerifyingKey = signing_key.verifying_key().into();
+        let address = authorization::get_address(&verifying_key);
+        let outpoint = fund(&env, &state, address, 1000);
+
+        let transaction = Transaction::new(
+            vec![outpoint],
+            vec![Output::new(
+                address,
+                OutputContent::Bitcoin(BitcoinOutputContent(
+                    bitcoin::Amount::from_sat(900),
+                )),
+            )],
+        );
+
+        // The attack: spend the input while providing no authorization for it.
+        let unauthorized = AuthorizedTransaction {
+            transaction: transaction.clone(),
+            authorizations: Vec::new(),
+        };
+        let rotxn = env.read_txn().unwrap();
+        let err = state
+            .validate_transaction(&rotxn, &unauthorized)
+            .expect_err("tx with no authorizations must be rejected");
+        assert!(
+            matches!(
+                err,
+                Error::WrongNumberOfAuthorizations {
+                    expected: 1,
+                    received: 0
+                }
+            ),
+            "unexpected error: {err:?}"
+        );
+
+        // The same transaction with a valid authorization is accepted.
+        let authorized =
+            authorization::authorize(&[(address, &signing_key)], transaction)
+                .unwrap();
+        state
+            .validate_transaction(&rotxn, &authorized)
+            .expect("correctly authorized tx should validate");
+    }
+}