fix(negotiation): exclude SHA-1 MACs from Preferred::DEFAULT (#690)

Author: derek-codebridge

russh/src/negotiation.rs

@@ -131,6 +131,14 @@ const CIPHER_ORDER: &[cipher::Name] = &[
     cipher::AES_128_CTR,
 ];
 
+// SHA-1 MAC variants excluded; see HMAC_ORDER for the full list including legacy algorithms.
+const SAFE_HMAC_ORDER: &[mac::Name] = &[
+    mac::HMAC_SHA512_ETM,
+    mac::HMAC_SHA256_ETM,
+    mac::HMAC_SHA512,
+    mac::HMAC_SHA256,
+];
+
 const HMAC_ORDER: &[mac::Name] = &[
     mac::HMAC_SHA512_ETM,
     mac::HMAC_SHA256_ETM,
@@ -171,15 +179,15 @@ impl Preferred {
             Algorithm::Rsa { hash: None },
         ]),
         cipher: Cow::Borrowed(CIPHER_ORDER),
-        mac: Cow::Borrowed(HMAC_ORDER),
+        mac: Cow::Borrowed(SAFE_HMAC_ORDER),
         compression: Cow::Borrowed(COMPRESSION_ORDER),
     };
 
     pub const COMPRESSED: Preferred = Preferred {
         kex: Cow::Borrowed(SAFE_KEX_ORDER),
         key: Preferred::DEFAULT.key,
         cipher: Cow::Borrowed(CIPHER_ORDER),
-        mac: Cow::Borrowed(HMAC_ORDER),
+        mac: Cow::Borrowed(SAFE_HMAC_ORDER),
         compression: Cow::Borrowed(COMPRESSION_ORDER),
     };
 }