diff --git a/crates/ironrdp-pdu/src/gcc/security_data.rs b/crates/ironrdp-pdu/src/gcc/security_data.rs
index 3d770048bc..be95b21758 100644
--- a/crates/ironrdp-pdu/src/gcc/security_data.rs
+++ b/crates/ironrdp-pdu/src/gcc/security_data.rs
@@ -59,8 +59,14 @@ impl<'de> Decode<'de> for ClientSecurityData {
     fn decode(src: &mut ReadCursor<'de>) -> DecodeResult<Self> {
         ensure_fixed_part_size!(in: src);
 
-        let encryption_methods = EncryptionMethod::from_bits(src.read_u32())
-            .ok_or_else(|| invalid_field_err!("encryptionMethods", "invalid encryption methods", in: src))?;
+        // [MS-RDPBCGR] 2.2.1.3.3: this field advertises the methods the
+        // client supports and the server "MUST select one of the methods
+        // specified by the client"; 3.3.5.3.3 never asks the server to
+        // validate the bit set, and the sibling extEncryptionMethods field
+        // below is already read unvalidated. Retain unknown bits (crate-wide
+        // policy since #1144) rather than failing the GCC exchange: the
+        // server simply never selects a method it does not know.
+        let encryption_methods = EncryptionMethod::from_bits_retain(src.read_u32());
         let ext_encryption_methods = src.read_u32();
 
         Ok(Self {
@@ -147,6 +153,12 @@ impl<'de> Decode<'de> for ServerSecurityData {
     fn decode(src: &mut ReadCursor<'de>) -> DecodeResult<Self> {
         ensure_fixed_part_size!(in: src);
 
+        // Deliberately strict, unlike ClientSecurityData above: this is the
+        // server's SELECTED method, not an advertisement. [MS-RDPBCGR]
+        // 2.2.1.3.3 obliges the server to select from the client's list, so a
+        // conforming server never sends an unknown bit here, and a client
+        // cannot implement a method it does not know. The value also drives
+        // the conditional serverRandom/serverCertificate parse below.
         let encryption_method = EncryptionMethod::from_bits(src.read_u32())
             .ok_or_else(|| invalid_field_err!("encryptionMethod", "invalid encryption method", in: src))?;
         let encryption_level = EncryptionLevel::from_u32(src.read_u32())
diff --git a/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs b/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs
index e34672339c..dcee3b17c4 100644
--- a/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs
+++ b/crates/ironrdp-testsuite-core/tests/pdu/gcc.rs
@@ -803,6 +803,24 @@ fn from_buffer_correctly_parses_client_security_data() {
     assert_eq!(*CLIENT_SECURITY_DATA, decode(buffer).unwrap());
 }
 
+#[test]
+fn client_security_data_with_undefined_method_bits_decodes_and_retains_them() {
+    // [MS-RDPBCGR] 2.2.1.3.3: encryptionMethods is an advertisement and the
+    // server selects only methods it knows; an unknown bit must not fail the
+    // GCC exchange. The bit is retained so re-encoding preserves the wire
+    // value. The server-selected method in ServerSecurityData stays strict.
+    const UNDEFINED_BIT: u32 = 0x0000_0004; // undefined in 2.2.1.3.3
+
+    let mut buffer = CLIENT_SECURITY_DATA_BUFFER.to_vec();
+    let methods = u32::from_le_bytes(buffer[0..4].try_into().unwrap()) | UNDEFINED_BIT;
+    buffer[0..4].copy_from_slice(&methods.to_le_bytes());
+
+    let data: ClientSecurityData = decode(buffer.as_slice()).expect("undefined method bits are not fatal");
+
+    assert_ne!(data.encryption_methods.bits() & UNDEFINED_BIT, 0);
+    assert_eq!(encode_vec(&data).unwrap(), buffer);
+}
+
 #[test]
 fn to_buffer_correctly_serializes_client_security_data() {
     let security_data = CLIENT_SECURITY_DATA.clone();