njaremko · maxammann · Apr 1, 2026 · Apr 2, 2026 · Apr 8, 2026
diff --git a/src/crypto/mod.rs b/src/crypto/mod.rs
@@ -65,11 +65,14 @@ impl From<Vec<u8>> for CertificateDer {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ReduceMode {
     /// Returns xmlsec's pre-digest content for exactly one verified reference across the document.
+    ///
+    /// This is the strictest mode. It only works if there is one Signature element
+    /// in the XML.
     PreDigest,
     /// Legacy mode that preserves the verified content and every element ancestor up to the
     /// document root.
     ///
-    /// This is kept for compatibility with older callers. It is not the default because unsigned
+    /// This is kept for compatibility with older callers. It is not recommended because unsigned
     /// ancestors can survive reduction in this mode.
     ValidateAndMark,
     /// Returns a rooted XML document containing only xmlsec-verified content.

diff --git a/src/crypto/xmlsec/mod.rs b/src/crypto/xmlsec/mod.rs
@@ -208,10 +208,20 @@ impl super::CryptoProvider for XmlSec {
         }
 
         if reduce_mode == ReduceMode::PreDigest {
-            if predigest_results.len() != 1 {
-                return Err(CryptoError::InvalidSignature);
+            if predigest_results.len() == 1 {
+                return Ok(predigest_results.remove(0));
+            } else {
+                for predigest in predigest_results {
+                    return match crate::service_provider::root_element_local_name(&predigest)
+                        .as_deref()
+                    {
+                        // We want to trust the full response as that includes the relevant data.
+                        // Everything is signed so even if we found a signed Assertion in here we could trust it, for now lets keep it minimal.
+                        Some("Response") => Ok(predigest),
+                        _ => Err(CryptoError::InvalidSignature),
+                    };
+                }
             }
-            return Ok(predigest_results.remove(0));
         }
 
         if selections.is_empty() {

diff --git a/src/crypto/xmlsec/wrapper/xmldsig.rs b/src/crypto/xmlsec/wrapper/xmldsig.rs
@@ -68,8 +68,7 @@ impl XmlSecSignatureContext {
                 let uri = if ref_ctx.uri.is_null() {
                     None
                 } else {
-                    let uri_cstr =
-                        std::ffi::CStr::from_ptr(ref_ctx.uri as *const std::ffi::c_char);
+                    let uri_cstr = std::ffi::CStr::from_ptr(ref_ctx.uri as *const std::ffi::c_char);
                     Some(
                         uri_cstr
                             .to_str()
@@ -85,8 +84,7 @@ impl XmlSecSignatureContext {
 
                 let data_ptr = bindings::xmlSecBufferGetData(predigest_buf);
                 let data_size = bindings::xmlSecBufferGetSize(predigest_buf);
-                let predigest_xml =
-                    predigest_xml_from_raw_buffer(data_ptr, data_size as usize)?;
+                let predigest_xml = predigest_xml_from_raw_buffer(data_ptr, data_size as usize)?;
 
                 result.push(VerifiedReference { uri, predigest_xml });
             }

diff --git a/src/idp/tests.rs b/src/idp/tests.rs
@@ -371,18 +371,18 @@ fn test_malicious_ancestors_not_included() {
         "/test_vectors/idp_2_metadata_public.pem"
     )));
 
-    for (reduce_mode, result) in [
-        (ReduceMode::PreDigest, true),
-        (ReduceMode::ValidateAndMark, true),
-        (ReduceMode::ValidateAndMarkNoAncestors, true),
+    for (reduce_mode, should_contain_attacker_url) in [
+        (ReduceMode::PreDigest, false),
+        (ReduceMode::ValidateAndMark, false),
+        (ReduceMode::ValidateAndMarkNoAncestors, false),
     ] {
         let reduced = XmlSec::reduce_xml_to_signed(signed_xml, &[cert.clone()], reduce_mode)
             .expect("reduce_xml_to_signed should succeed");
 
         assert_eq!(
-            !reduced.contains("https://attacker.evil.com"),
-            result,
-            "ancestor should not be contained in {reduce_mode:?}"
+            reduced.contains("https://attacker.evil.com"),
+            should_contain_attacker_url,
+            "Attacker URL containment mismatch for {reduce_mode:?}: expected {should_contain_attacker_url}"
         );
     }
 }
@@ -398,18 +398,18 @@ fn test_object_reference_removed() {
         "/test_vectors/idp_2_metadata_public.pem"
     )));
 
-    for (reduce_mode, result) in [
-        (ReduceMode::PreDigest, true),
-        (ReduceMode::ValidateAndMark, true),
-        (ReduceMode::ValidateAndMarkNoAncestors, true),
+    for (reduce_mode, should_contain_object) in [
+        (ReduceMode::PreDigest, false),
+        (ReduceMode::ValidateAndMark, false),
+        (ReduceMode::ValidateAndMarkNoAncestors, false),
     ] {
         let reduced = XmlSec::reduce_xml_to_signed(signed_xml, &[cert.clone()], reduce_mode)
             .expect("reduce_xml_to_signed should succeed");
 
         assert_eq!(
-            !reduced.contains("ds:Object"),
-            result,
-            "object should not be present in {reduce_mode:?}"
+            reduced.contains("ds:Object"),
+            should_contain_object,
+            "Object containment mismatch for {reduce_mode:?}: expected {should_contain_object}"
         );
     }
 }
@@ -448,18 +448,18 @@ fn test_xpath_transforms_validated() {
         "/test_vectors/idp_2_metadata_public.pem"
     )));
 
-    for (reduce_mode, result) in [
-        (ReduceMode::PreDigest, true),
-        (ReduceMode::ValidateAndMark, true),
-        (ReduceMode::ValidateAndMarkNoAncestors, true),
+    for (reduce_mode, should_contain_malicious) in [
+        (ReduceMode::PreDigest, false),
+        (ReduceMode::ValidateAndMark, false),
+        (ReduceMode::ValidateAndMarkNoAncestors, false),
     ] {
         let reduced = XmlSec::reduce_xml_to_signed(signed_xml, &[cert.clone()], reduce_mode)
             .expect("reduce_xml_to_signed should succeed");
 
         assert_eq!(
-            !reduced.contains("malicious"),
-            result,
-            "malicious content should not be present in {reduce_mode:?}"
+            reduced.contains("malicious"),
+            should_contain_malicious,
+            "Malicious content containment mismatch for {reduce_mode:?}: expected {should_contain_malicious}"
         );
     }
 }
diff --git a/src/service_provider/mod.rs b/src/service_provider/mod.rs
@@ -364,10 +364,24 @@ impl ServiceProvider {
         &self,
         encoded_resp: &str,
         possible_request_ids: Option<&[&str]>,
+    ) -> Result<Assertion, Box<dyn std::error::Error>> {
+        self.parse_base64_response_with_mode(
+            encoded_resp,
+            possible_request_ids,
+            ReduceMode::default(),
+        )
+    }
+
+    pub fn parse_base64_response_with_mode(
+        &self,
+        encoded_resp: &str,
+        possible_request_ids: Option<&[&str]>,
+        reduce_mode: ReduceMode,
     ) -> Result<Assertion, Box<dyn std::error::Error>> {
         let bytes = general_purpose::STANDARD.decode(encoded_resp)?;
         let decoded = std::str::from_utf8(&bytes)?;
-        let assertion = self.parse_xml_response(decoded, possible_request_ids)?;
+        let assertion =
+            self.parse_xml_response_with_mode(decoded, possible_request_ids, reduce_mode)?;
         Ok(assertion)
     }
 
@@ -376,11 +390,7 @@ impl ServiceProvider {
         response_xml: &str,
         possible_request_ids: Option<&[&str]>,
     ) -> Result<Assertion, Error> {
-        self.parse_xml_response_with_mode(
-            response_xml,
-            possible_request_ids,
-            ReduceMode::ValidateAndMarkNoAncestors,
-        )
+        self.parse_xml_response_with_mode(response_xml, possible_request_ids, ReduceMode::default())
     }
 
     pub fn parse_xml_response_with_mode(
@@ -701,7 +711,7 @@ impl ServiceProvider {
     }
 }
 
-fn root_element_local_name(xml: &str) -> Option<String> {
+pub(crate) fn root_element_local_name(xml: &str) -> Option<String> {
     let mut reader = Reader::from_str(xml);
 
     loop {

diff --git a/src/service_provider/tests.rs b/src/service_provider/tests.rs
@@ -412,6 +412,29 @@ mod encrypted_assertion_tests {
         );
     }
 
+    // TODO: this should work, but it does not with ValidateAndMark
+    #[test]
+    fn test_validate_and_mark_only_assertion_signed() {
+        let sp = create_predigest_assertion_sp("http://sp.example.com/demo1/index.php?acs");
+        let response_xml = include_str!(concat!(
+        env!("CARGO_MANIFEST_DIR"),
+        "/test_vectors/response_signed_assertion.xml"
+        ));
+
+        let assertion = sp
+            .parse_xml_response_with_mode(
+                &response_xml,
+                Some(&["ONELOGIN_4fee3b046395c4e751011e97f8900b5273d56685"]),
+                ReduceMode::ValidateAndMark
+            )
+            .unwrap();
+
+        assert_eq!(
+            assertion.issuer.value.as_deref(),
+            Some("http://idp.example.com/metadata.php")
+        );
+    }
+
     #[test]
     fn test_response_validation_requires_assertion_recipient_binding() {
         let sp = create_predigest_assertion_sp("http://sp.example.com/demo1/index.php?acs");
@@ -583,4 +606,33 @@ mod encrypted_assertion_tests {
             Error::AssertionSubjectConfirmationExpiredBefore { .. }
         ));
     }
+
+    #[test]
+    fn test_parse_xml_response_with_empty_saml_response() {
+        let mut sp = create_predigest_assertion_sp_with_metadata("https://api.dev.zoo.dev/auth/saml/00000000-00000000-00000000-00000000/login", r#"<?xml version="1.0"?>
+<md:EntityDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata" entityID="https://some.idp.test/blah/">
+  <md:IDPSSODescriptor WantAuthnRequestsSigned="false" protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol">
+    <md:KeyDescriptor use="signing">
+      <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
+        <ds:X509Data>
+          <ds:X509Certificate>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</ds:X509Certificate>
+        </ds:X509Data>
+      </ds:KeyInfo>
+    </md:KeyDescriptor>
+    <md:SingleLogoutService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect" Location="https://some.idp.test/blah/"/>
+    <md:NameIDFormat>urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress</md:NameIDFormat>
+    <md:SingleSignOnService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect" Location="https://some.idp.test/blah/"/>
+  </md:IDPSSODescriptor>
+</md:EntityDescriptor>
+"#.parse().unwrap());
+        sp.allow_idp_initiated = true;
+        let response_xml = include_str!(concat!(
+            env!("CARGO_MANIFEST_DIR"),
+            "/test_vectors/multi_saml_response_signed_2.xml"
+        ));
+
+        let assertion = sp
+            .parse_xml_response_with_mode(response_xml, None, ReduceMode::PreDigest)
+            .expect("signed assertion should parse in pre-digest mode");
+    }
 }
diff --git a/test_vectors/cert.pem b/test_vectors/cert.pem
@@ -0,0 +1,13 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/test_vectors/multi_saml_response.sh b/test_vectors/multi_saml_response.sh
@@ -0,0 +1,6 @@
+#!/bin/env bash
+
+openssl req -new -x509 -key test_vectors/private.der -inform DER -out test_vectors/cert.pem -days 365 -subj "/CN=Test"
+openssl pkey -in test_vectors/private.der -inform DER -out test_vectors/private.pem
+xmlsec1 --sign --privkey-pem:signing test_vectors/private.pem,test_vectors/cert.pem --id-attr:ID urn:oasis:names:tc:SAML:2.0:assertion:Assertion --id-attr:ID urn:oasis:names:tc:SAML:2.0:protocol:Response --node-xpath "//*[local-name()='Assertion' and namespace-uri()='urn:oasis:names:tc:SAML:2.0:assertion']/*[local-name()='Signature' and namespace-uri()='http://www.w3.org/2000/09/xmldsig#']" --output test_vectors/multi_saml_response_signed.xml --verbose test_vectors/multi_saml_response_template.xml
+xmlsec1 --sign --privkey-pem:signing test_vectors/private.pem,test_vectors/cert.pem --id-attr:ID urn:oasis:names:tc:SAML:2.0:assertion:Assertion --id-attr:ID urn:oasis:names:tc:SAML:2.0:protocol:Response --node-xpath "/*[local-name()='Response' and namespace-uri()='urn:oasis:names:tc:SAML:2.0:protocol']/*[local-name()='Signature' and namespace-uri()='http://www.w3.org/2000/09/xmldsig#']" --output test_vectors/multi_saml_response_signed_2.xml --verbose test_vectors/multi_saml_response_signed.xml
diff --git a/test_vectors/multi_saml_response_signed.xml b/test_vectors/multi_saml_response_signed.xml
@@ -0,0 +1,92 @@
+<?xml version="1.0"?>
+<samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" Version="2.0" ID="_f2f02ca12d7292f8a4e03b7b99d71a45f8896c2677" IssueInstant="2022-05-04T15:36:12.631Z" Destination="https://api.dev.zoo.dev/auth/saml/00000000-00000000-00000000-00000000/login">
+  <saml:Issuer>https://some.idp.test/blah/</saml:Issuer>
+  <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
+    <ds:SignedInfo>
+      <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
+      <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
+      <ds:Reference URI="#_f2f02ca12d7292f8a4e03b7b99d71a45f8896c2677">
+        <ds:Transforms>
+          <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
+          <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
+        </ds:Transforms>
+        <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
+        <ds:DigestValue/>
+      </ds:Reference>
+    </ds:SignedInfo>
+    <ds:SignatureValue/>
+    <ds:KeyInfo>
+      <ds:KeyName>signing</ds:KeyName>
+      <ds:X509Data>
+        <ds:X509Certificate/>
+      </ds:X509Data>
+    </ds:KeyInfo>
+  </ds:Signature>
+  <samlp:Status>
+    <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
+  </samlp:Status>
+  <saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" Version="2.0" ID="_a8d644a6c83fc56337b0330d8b0ff684978a5fe56c" IssueInstant="2022-05-04T15:36:12.556Z">
+    <saml:Issuer>https://some.idp.test/blah/</saml:Issuer>
+    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
+      <ds:SignedInfo>
+        <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
+        <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"/>
+        <ds:Reference URI="#_a8d644a6c83fc56337b0330d8b0ff684978a5fe56c">
+          <ds:Transforms>
+            <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
+            <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
+          </ds:Transforms>
+          <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>
+          <ds:DigestValue>ptR4sQtcSeB51h3ggeXGv6dlvp3V3NEcvF33GhgdTlE=</ds:DigestValue>
+        </ds:Reference>
+      </ds:SignedInfo>
+      <ds:SignatureValue>SoTLlmi1vhoGBgwjlwFY4+CLkczw81PFDXOeK1vSlrC0eOhe89wc2Ve7fFCGe5N/
+L5luYt+QcWyoi7M3E48EuuxyYnJtznG6nLjezTUe2Rd1HvQwtDptmjyes3SD1o1/
+FM9UpYbnPAQgvdmfr/oM5R2xIxwhw7ZBDyPhk5HpwL8=</ds:SignatureValue>
+      <ds:KeyInfo>
+        <ds:KeyName>signing</ds:KeyName>
+        <ds:X509Data>
+          <ds:X509Certificate>MIIB+jCCAWOgAwIBAgIUdcXUPTE+mOSWxRCJh8ldmDMPzREwDQYJKoZIhvcNAQEL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+</ds:X509Certificate>
+        </ds:X509Data>
+      </ds:KeyInfo>
+    </ds:Signature>
+    <saml:Subject>
+      <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">some@customer.com</saml:NameID>
+      <saml:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
+        <saml:SubjectConfirmationData NotOnOrAfter="3923-09-01T02:16:12.556Z" Recipient="https://api.dev.zoo.dev/auth/saml/00000000-00000000-00000000-00000000/login"/>
+      </saml:SubjectConfirmation>
+    </saml:Subject>
+    <saml:Conditions NotBefore="2022-05-04T15:36:12.556Z" NotOnOrAfter="3923-09-01T02:16:12.556Z"/>
+    <saml:AuthnStatement AuthnInstant="2022-05-04T15:36:12.556Z" SessionNotOnOrAfter="3923-09-01T02:16:12.556Z" SessionIndex="_samling_4356273_859641652">
+      <saml:AuthnContext>
+        <saml:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:unspecified</saml:AuthnContextClassRef>
+      </saml:AuthnContext>
+    </saml:AuthnStatement>
+    <saml:AttributeStatement>
+      <saml:Attribute Name="email" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified">
+        <saml:AttributeValue>thing@thing.com</saml:AttributeValue>
+      </saml:Attribute>
+    </saml:AttributeStatement>
+    <saml:AttributeStatement>
+      <saml:Attribute Name="first_name" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified">
+        <saml:AttributeValue>Test</saml:AttributeValue>
+      </saml:Attribute>
+    </saml:AttributeStatement>
+    <saml:AttributeStatement>
+      <saml:Attribute Name="last_name" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified">
+        <saml:AttributeValue>User</saml:AttributeValue>
+      </saml:Attribute>
+    </saml:AttributeStatement>
+  </saml:Assertion>
+</samlp:Response>