diff --git a/src/engine/hot_loop/ccp.rs b/src/engine/hot_loop/ccp.rs
index d0a8100a..3cb165a6 100644
--- a/src/engine/hot_loop/ccp.rs
+++ b/src/engine/hot_loop/ccp.rs
@@ -224,7 +224,7 @@ impl CcpState {
                 for frame in frames {
                     match frame {
                         Frame::FixComp(raw) => {
-                            let (unsigned, _) = conn.unsign(&raw);
+                            let Some(unsigned) = conn.unsign(&raw) else { continue };
                             match fixcomp::fixcomp_decompress(&unsigned) {
                                 Ok(inner) => {
                                     if log::log_enabled!(log::Level::Trace) {
@@ -243,14 +243,14 @@ impl CcpState {
                             }
                         }
                         Frame::Fix(raw) => {
-                            let (unsigned, _) = conn.unsign(&raw);
+                            let Some(unsigned) = conn.unsign(&raw) else { continue };
                             if log::log_enabled!(log::Level::Trace) {
                                 log::trace!("WIRE< ccp/fix {}", fix::fmt_pipe(&unsigned));
                             }
                             msgs.push(unsigned);
                         }
                         Frame::Binary(raw) => {
-                            let (unsigned, _) = conn.unsign(&raw);
+                            let Some(unsigned) = conn.unsign(&raw) else { continue };
                             if log::log_enabled!(log::Level::Trace) {
                                 log::trace!("WIRE< ccp/bin {}", fix::fmt_pipe(&unsigned));
                             }
diff --git a/src/engine/hot_loop/farm.rs b/src/engine/hot_loop/farm.rs
index 8b9b2075..f92ef46a 100644
--- a/src/engine/hot_loop/farm.rs
+++ b/src/engine/hot_loop/farm.rs
@@ -85,7 +85,7 @@ impl FarmState {
             for frame in &frames {
                 match frame {
                     Frame::FixComp(raw) => {
-                        let (unsigned, _valid) = conn.unsign(raw);
+                        let Some(unsigned) = conn.unsign(raw) else { continue };
                         match fixcomp::fixcomp_decompress(&unsigned) {
                             Ok(inner) => {
                                 if log::log_enabled!(log::Level::Trace) {
@@ -104,14 +104,14 @@ impl FarmState {
                         }
                     }
                     Frame::Binary(raw) => {
-                        let (unsigned, _valid) = conn.unsign(raw);
+                        let Some(unsigned) = conn.unsign(raw) else { continue };
                         if log::log_enabled!(log::Level::Trace) {
                             log::trace!("WIRE< farm/bin {}", fix::fmt_pipe(&unsigned));
                         }
                         self.farm_msg_buf.push(unsigned);
                     }
                     Frame::Fix(raw) => {
-                        let (unsigned, _valid) = conn.unsign(raw);
+                        let Some(unsigned) = conn.unsign(raw) else { continue };
                         if log::log_enabled!(log::Level::Trace) {
                             log::trace!("WIRE< farm/fix {}", fix::fmt_pipe(&unsigned));
                         }
diff --git a/src/engine/hot_loop/hmds.rs b/src/engine/hot_loop/hmds.rs
index e5a24e39..f30e3cbd 100644
--- a/src/engine/hot_loop/hmds.rs
+++ b/src/engine/hot_loop/hmds.rs
@@ -115,7 +115,7 @@ impl HmdsState {
                 for frame in &frames {
                     match frame {
                         Frame::FixComp(raw) => {
-                            let (unsigned, _valid) = conn.unsign(raw);
+                            let Some(unsigned) = conn.unsign(raw) else { continue };
                             match fixcomp::fixcomp_decompress(&unsigned) {
                                 Ok(inner) => {
                                     if log::log_enabled!(log::Level::Trace) {
@@ -134,14 +134,14 @@ impl HmdsState {
                             }
                         }
                         Frame::Binary(raw) => {
-                            let (unsigned, _valid) = conn.unsign(raw);
+                            let Some(unsigned) = conn.unsign(raw) else { continue };
                             if log::log_enabled!(log::Level::Trace) {
                                 log::trace!("WIRE< hmds/bin {}", crate::protocol::fix::fmt_pipe(&unsigned));
                             }
                             msgs.push(unsigned);
                         }
                         Frame::Fix(raw) => {
-                            let (unsigned, _valid) = conn.unsign(raw);
+                            let Some(unsigned) = conn.unsign(raw) else { continue };
                             if log::log_enabled!(log::Level::Trace) {
                                 log::trace!("WIRE< hmds/fix {}", crate::protocol::fix::fmt_pipe(&unsigned));
                             }
diff --git a/src/gateway.rs b/src/gateway.rs
index c238ee2d..ac2cb62d 100644
--- a/src/gateway.rs
+++ b/src/gateway.rs
@@ -279,10 +279,18 @@ pub fn farm_logon_exchange(
 
         // FIX.4.1 message
         if msg.starts_with(b"8=FIX.4.1\x01") {
-            let has_sig = msg.windows(5).any(|w| w == b"8349=");
+            let has_sig = msg.windows(6).any(|w| w == b"\x018349=");
             // Check for HMAC signature → unsign
             let parsed_msg = if has_sig {
-                let (unsigned, new_iv, _valid) = fix::fix_unsign(&msg, read_mac_key, &read_iv);
+                let (unsigned, new_iv, valid) = fix::fix_unsign(&msg, read_mac_key, &read_iv);
+                if !valid {
+                    // Same rule as `Connection::unsign`: a frame that does not
+                    // verify is neither parsed nor allowed to move the chain,
+                    // since the IV it would move to is derived from a body the
+                    // MAC just declined to vouch for.
+                    log::warn!("auth frame failed signature verification — dropped");
+                    continue;
+                }
                 read_iv = new_iv;
                 unsigned
             } else {
@@ -565,7 +573,7 @@ pub fn connect_farm(
     for frame in &frames {
         match frame {
             crate::protocol::connection::Frame::FixComp(raw) => {
-                let (unsigned, _valid) = conn.unsign(raw);
+                let Some(unsigned) = conn.unsign(raw) else { continue };
                 let inner = fixcomp::fixcomp_decompress(&unsigned).unwrap_or_else(|e| {
                     log::warn!("{}: dropping malformed FIXCOMP frame: {}", farm_id, e);
                     Vec::new()
@@ -586,7 +594,7 @@ pub fn connect_farm(
                 }
             }
             crate::protocol::connection::Frame::Fix(raw) => {
-                let (unsigned, _valid) = conn.unsign(raw);
+                let Some(unsigned) = conn.unsign(raw) else { continue };
                 let parsed = fix_parse(&unsigned);
                 let mt = parsed.get(&35).map(|s| s.as_str()).unwrap_or("");
                 log::debug!("{} routing FIX 35={}", farm_id, mt);
@@ -601,7 +609,7 @@ pub fn connect_farm(
                 }
             }
             crate::protocol::connection::Frame::Binary(raw) => {
-                let (_unsigned, _valid) = conn.unsign(raw);
+                let Some(_unsigned) = conn.unsign(raw) else { continue };
                 log::info!("{} routing 8=O: {} bytes", farm_id, raw.len());
             }
             crate::protocol::connection::Frame::Control(raw) => {
diff --git a/src/protocol/connection.rs b/src/protocol/connection.rs
index 7b65de4e..2f833cfa 100644
--- a/src/protocol/connection.rs
+++ b/src/protocol/connection.rs
@@ -271,19 +271,60 @@ impl Connection {
         frames
     }
 
-    /// Unsign a received frame using the read IV. Chains the IV.
-    /// Returns the undistorted message bytes and whether the signature was valid.
-    pub fn unsign(&mut self, msg: &[u8]) -> (Vec<u8>, bool) {
+    /// Unsign a received frame using the read IV, chaining the IV.
+    ///
+    /// `None` means the frame did not verify and must not be parsed. The result
+    /// used to be a `(bytes, bool)` pair and every one of the twelve callers
+    /// discarded the flag, so a tampered frame — an order ack, a fill, an
+    /// account push — was applied exactly like an authentic one. Returning no
+    /// message is the same information in a form a caller cannot ignore.
+    ///
+    /// A failed frame also leaves the IV alone, because the IV it would advance
+    /// to is derived from the body the signature just failed to vouch for.
+    /// Advancing would let one injected frame steer the receiver's chain and
+    /// drop every genuine frame after it.
+    ///
+    /// This costs the case where a genuine frame is damaged in exactly its
+    /// signature: its body is intact, so the derived IV would have been the
+    /// sender's true next one, and the connection is left unable to verify what
+    /// follows until it reconnects. That case cannot be told apart from an
+    /// injection here, and the reasoning for preferring this side is set out
+    /// where the decision is made, below.
+    pub fn unsign(&mut self, msg: &[u8]) -> Option<Vec<u8>> {
         if self.read_key.is_empty() {
-            return (msg.to_vec(), true); // no signing configured
+            return Some(msg.to_vec()); // no signing configured
         }
-        // Only unsign if 8349= HMAC tag is present (matching Python _unsign_conn)
-        if !msg.windows(5).any(|w| w == b"8349=") {
-            return (msg.to_vec(), true);
+        // A frame carrying no 8349 tag is still accepted, as the reference
+        // client does. Whether the gateway ever sends one on a keyed
+        // connection is not established here, and refusing them on that
+        // assumption would drop real traffic; the warning makes the case
+        // visible so the question can be settled from logs rather than guessed.
+        if !msg.windows(6).any(|w| w == b"\x018349=") {
+            log::warn!("inbound frame carries no 8349 signature on a signed connection");
+            return Some(msg.to_vec());
         }
         let (undistorted, new_iv, valid) = fix::fix_unsign(msg, &self.read_key, &self.read_iv);
+        if !valid {
+            // The chain is not advanced. `new_iv` is derived from the received
+            // body, and a failed MAC is exactly the statement that this body
+            // cannot be vouched for — so advancing would let one injected frame
+            // steer the receiver's chain state and drop every genuine frame
+            // after it.
+            //
+            // The cost is a genuine frame whose signature alone was damaged in
+            // transit: its body is intact, so `new_iv` would have been the
+            // sender's true next one, and holding it back leaves the connection
+            // stuck. That case is indistinguishable from an injection at this
+            // point, and a channel where a MAC failure has occurred is not one
+            // whose state can be inferred either way. Failing closed on
+            // unauthenticated input is the side to err on; the connection
+            // wanting a teardown rather than a guess is the real answer, and is
+            // a larger change than this.
+            log::warn!("inbound frame failed signature verification — dropped");
+            return None;
+        }
         self.read_iv = new_iv;
-        (undistorted, valid)
+        Some(undistorted)
     }
 
     /// Build a FIX message, sign it, and send it. Increments seq and chains sign IV.
@@ -664,4 +705,120 @@ mod tests {
         // windows(0) panics, so empty needle panics
         find_subsequence(b"hello", b"");
     }
+
+    /// Build a connection with signing configured, over a loopback pair.
+    fn signed_conn(key: &[u8], iv: &[u8]) -> Connection {
+        let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+        let stream = std::net::TcpStream::connect(listener.local_addr().unwrap()).unwrap();
+        let (_peer, _) = listener.accept().unwrap();
+        let mut conn = Connection::new_raw(stream).unwrap();
+        conn.read_key = key.to_vec();
+        conn.read_iv = iv.to_vec();
+        conn
+    }
+
+    /// A frame that does not verify must not reach a parser. The result used to
+    /// be a pair whose validity flag every caller discarded, so a tampered
+    /// order ack or fill was applied like an authentic one.
+    #[test]
+    fn a_frame_that_fails_verification_is_not_returned() {
+        let key = b"0123456789abcdef";
+        let iv = vec![0u8; 16];
+        let (frame, _) = crate::protocol::fix::fix_sign(&fix_build(&[(35, "8")], 1), key, &iv);
+
+        let mut conn = signed_conn(key, &iv);
+        assert!(conn.unsign(&frame).is_some(), "the genuine frame verifies");
+
+        // Flip a byte in the body, leaving the signature tag in place.
+        let mut tampered = frame.clone();
+        let at = tampered.len() / 2;
+        tampered[at] ^= 0x01;
+        let mut conn = signed_conn(key, &iv);
+        assert!(conn.unsign(&tampered).is_none(), "a tampered frame is dropped");
+    }
+
+    /// A frame that fails is dropped without moving the chain, so a genuine
+    /// frame after it still verifies. The batch is the case that matters —
+    /// good, bad, good — because an injected frame between two authentic ones
+    /// is what advancing on unauthenticated input would let poison the rest.
+    #[test]
+    fn an_injected_frame_does_not_poison_the_authentic_ones_around_it() {
+        let key = b"0123456789abcdef";
+        let iv0 = vec![0u8; 16];
+
+        // Two frames as a sender produces them: the second signed from the IV
+        // the first chained to.
+        let (first, iv1) =
+            crate::protocol::fix::fix_sign(&fix_build(&[(35, "8")], 1), key, &iv0);
+        let (second, _) =
+            crate::protocol::fix::fix_sign(&fix_build(&[(35, "8")], 2), key, &iv1);
+
+        // Something else entirely, arriving between them.
+        let (mut injected, _) =
+            crate::protocol::fix::fix_sign(&fix_build(&[(35, "8"), (58, "x")], 9), b"wrongkey00000000", &iv1);
+        let at = find_subsequence(&injected, b"\x018349=").expect("signed") + 6;
+        injected[at] = if injected[at] == b'0' { b'1' } else { b'0' };
+
+        let mut conn = signed_conn(key, &iv0);
+        assert!(conn.unsign(&first).is_some(), "the first authentic frame");
+        assert_eq!(conn.read_iv, iv1, "and it advanced the chain");
+
+        assert!(conn.unsign(&injected).is_none(), "the injected frame is dropped");
+        assert_eq!(conn.read_iv, iv1, "without moving the chain");
+
+        assert!(
+            conn.unsign(&second).is_some(),
+            "so the authentic frame after it still verifies",
+        );
+    }
+
+    /// Two shapes that must keep working, because refusing either would drop
+    /// real traffic rather than protect anything: an unsigned connection, and a
+    /// frame carrying no signature tag on a signed one.
+    #[test]
+    fn unsigned_connections_and_untagged_frames_still_pass() {
+        let plain = fix_build(&[(35, "0")], 1);
+
+        let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+        let stream = std::net::TcpStream::connect(listener.local_addr().unwrap()).unwrap();
+        let (_peer, _) = listener.accept().unwrap();
+        let mut conn = Connection::new_raw(stream).unwrap();
+        assert_eq!(conn.unsign(&plain), Some(plain.clone()), "no key configured");
+
+        let mut conn = signed_conn(b"0123456789abcdef", &vec![0u8; 16]);
+        assert_eq!(conn.unsign(&plain), Some(plain), "no 8349 tag on the frame");
+    }
+
+    /// The same rule at the pre-check. An *unsigned* frame quoting the tag in a
+    /// value must not be routed into verification at all: it carries no
+    /// signature field, so it would be judged invalid and dropped. The signed
+    /// case below cannot catch this — its pre-check passes under either needle.
+    #[test]
+    fn an_unsigned_frame_quoting_the_signature_tag_is_not_verified() {
+        let quoting = fix_build(&[(35, "8"), (58, "rejected: 8349= missing")], 1);
+
+        let mut conn = signed_conn(b"0123456789abcdef", &vec![0u8; 16]);
+        assert_eq!(
+            conn.unsign(&quoting),
+            Some(quoting.clone()),
+            "the tag text in a value is not a signature field",
+        );
+    }
+
+    /// A signature is a field, not a substring. A legitimate signed frame whose
+    /// *value* happens to contain the same text — a reject reason quoting it,
+    /// say — was reported invalid, and enforcing that verdict would drop it.
+    #[test]
+    fn a_signed_frame_quoting_the_signature_tag_in_a_value_still_verifies() {
+        let key = b"0123456789abcdef";
+        let iv = vec![0u8; 16];
+        let quoting = fix_build(&[(35, "8"), (58, "rejected: 8349= missing")], 1);
+        let (frame, _) = crate::protocol::fix::fix_sign(&quoting, key, &iv);
+
+        let mut conn = signed_conn(key, &iv);
+        assert!(
+            conn.unsign(&frame).is_some(),
+            "the tag text inside a value is not the signature field",
+        );
+    }
 }
diff --git a/src/protocol/fix.rs b/src/protocol/fix.rs
index daef728f..3d85d7f3 100644
--- a/src/protocol/fix.rs
+++ b/src/protocol/fix.rs
@@ -349,13 +349,17 @@ pub fn fix_unsign(msg: &[u8], mac_key: &[u8], iv: &[u8]) -> (Vec<u8>, Vec<u8>, b
         None => return (msg_bytes, iv.to_vec(), false),
     };
 
-    // Find 8349= tag
-    let sig_needle = b"8349=";
+    // Find the 8349 signature field. Matched with its leading delimiter: a
+    // bare `8349=` search also matches the same text inside a field *value* —
+    // a reject reason quoting it, say — and then the body boundary lands in the
+    // middle of the message and a legitimate frame reports invalid (ibx#275).
+    let sig_needle = b"\x018349=";
     let t8349 = match msg_bytes
         .windows(sig_needle.len())
         .position(|w| w == sig_needle)
     {
-        Some(p) => p,
+        // Keep the delimiter in the body, where the un-delimited match left it.
+        Some(p) => p + 1,
         None => return (msg_bytes, iv.to_vec(), false),
     };
 
@@ -368,7 +372,7 @@ pub fn fix_unsign(msg: &[u8], mac_key: &[u8], iv: &[u8]) -> (Vec<u8>, Vec<u8>, b
     let expected = xor_fold_bytes(&hmac_res);
 
     // Extract actual signature and compare as bytes (no String alloc)
-    let sig_start = t8349 + sig_needle.len();
+    let sig_start = t8349 + (sig_needle.len() - 1);
     let sig_end = msg_bytes[sig_start..]
         .iter()
         .position(|&b| b == SOH)
diff --git a/tests/depth_wire_test.rs b/tests/depth_wire_test.rs
index 97de3060..81a875ca 100644
--- a/tests/depth_wire_test.rs
+++ b/tests/depth_wire_test.rs
@@ -63,10 +63,10 @@ fn raw_farm_subscribe_test() {
             };
             // Decompress if FIXCOMP
             let msgs = if raw.starts_with(b"8=FIXCOMP") {
-                let (unsigned, _) = farm.unsign(raw);
+                let Some(unsigned) = farm.unsign(raw) else { continue };
                 fixcomp::fixcomp_decompress(&unsigned).unwrap_or_default()
             } else {
-                let (unsigned, _) = farm.unsign(raw);
+                let Some(unsigned) = farm.unsign(raw) else { continue };
                 vec![unsigned]
             };
             for msg in &msgs {
diff --git a/tests/ib_paper_compat/common.rs b/tests/ib_paper_compat/common.rs
index 0aee62ec..7a099d1c 100644
--- a/tests/ib_paper_compat/common.rs
+++ b/tests/ib_paper_compat/common.rs
@@ -103,7 +103,7 @@ pub(super) fn ccp_keepalive(ccp: &mut Connection) {
                 // Control-state frames are not consumed downstream (ibx#185).
                 Frame::Control(_) => continue,
             };
-            let (unsigned, _) = ccp.unsign(raw);
+            let Some(unsigned) = ccp.unsign(raw) else { continue };
             let msg = if matches!(frame, Frame::FixComp(_)) {
                 fixcomp::fixcomp_decompress(&unsigned)
                     .ok()
diff --git a/tests/ib_paper_compat/contracts.rs b/tests/ib_paper_compat/contracts.rs
index d8e0970f..0d91160e 100644
--- a/tests/ib_paper_compat/contracts.rs
+++ b/tests/ib_paper_compat/contracts.rs
@@ -136,7 +136,7 @@ pub(super) fn phase_trading_hours(conns: &mut Conns) {
         }
         for frame in conns.ccp.extract_frames() {
             let messages = match frame {
-                Frame::FixComp(raw) => { let (u, _) = conns.ccp.unsign(&raw); fixcomp::fixcomp_decompress(&u).unwrap_or_default() }
+                Frame::FixComp(raw) => { let Some(u) = conns.ccp.unsign(&raw) else { continue }; fixcomp::fixcomp_decompress(&u).unwrap_or_default() }
                 Frame::Fix(raw) => vec![raw],
                 _ => continue,
             };
diff --git a/tests/ib_paper_compat/main.rs b/tests/ib_paper_compat/main.rs
index 91c8e679..da38eb59 100644
--- a/tests/ib_paper_compat/main.rs
+++ b/tests/ib_paper_compat/main.rs
@@ -100,7 +100,8 @@ fn compat_suite() {
                             Frame::Fix(r) => (r, "FIX"),
                             Frame::Control(r) => (r, "Control"),
                         };
-                        let (unsigned, valid) = conn.unsign(raw);
+                        let Some(unsigned) = conn.unsign(raw) else { continue };
+                        let valid = true;
                         if label == "FIXCOMP" {
                             let inner = fixcomp::fixcomp_decompress(&unsigned).unwrap_or_default();
                             for m in &inner {
diff --git a/tests/ib_paper_compat/market_data.rs b/tests/ib_paper_compat/market_data.rs
index b46400f7..7a8ca864 100644
--- a/tests/ib_paper_compat/market_data.rs
+++ b/tests/ib_paper_compat/market_data.rs
@@ -537,7 +537,7 @@ pub(super) fn phase_forex_market_data(conns: Conns) -> Conns {
         for frame in ccp.extract_frames() {
             let messages = match frame {
                 Frame::FixComp(raw) => {
-                    let (unsigned, _) = ccp.unsign(&raw);
+                    let Some(unsigned) = ccp.unsign(&raw) else { continue };
                     fixcomp::fixcomp_decompress(&unsigned).unwrap_or_default()
                 }
                 Frame::Fix(raw) => vec![raw],
diff --git a/tests/ib_paper_compat/multi_asset.rs b/tests/ib_paper_compat/multi_asset.rs
index bb00fb69..fd19d234 100644
--- a/tests/ib_paper_compat/multi_asset.rs
+++ b/tests/ib_paper_compat/multi_asset.rs
@@ -36,7 +36,7 @@ pub(super) fn phase_forex_order(conns: Conns) -> Conns {
         for frame in ccp.extract_frames() {
             let messages = match frame {
                 Frame::FixComp(raw) => {
-                    let (unsigned, _) = ccp.unsign(&raw);
+                    let Some(unsigned) = ccp.unsign(&raw) else { continue };
                     fixcomp::fixcomp_decompress(&unsigned).unwrap_or_default()
                 }
                 Frame::Fix(raw) => vec![raw],
@@ -152,7 +152,7 @@ pub(super) fn phase_futures_order(conns: Conns) -> Conns {
         for frame in ccp.extract_frames() {
             let messages = match frame {
                 Frame::FixComp(raw) => {
-                    let (unsigned, _) = ccp.unsign(&raw);
+                    let Some(unsigned) = ccp.unsign(&raw) else { continue };
                     fixcomp::fixcomp_decompress(&unsigned).unwrap_or_default()
                 }
                 Frame::Fix(raw) => vec![raw],
@@ -275,7 +275,7 @@ pub(super) fn phase_options_order(conns: Conns) -> Conns {
         for frame in ccp.extract_frames() {
             let messages = match frame {
                 Frame::FixComp(raw) => {
-                    let (unsigned, _) = ccp.unsign(&raw);
+                    let Some(unsigned) = ccp.unsign(&raw) else { continue };
                     fixcomp::fixcomp_decompress(&unsigned).unwrap_or_default()
                 }
                 Frame::Fix(raw) => vec![raw],