diff --git a/Libraries/LibWeb/SecureContexts/AbstractOperations.cpp b/Libraries/LibWeb/SecureContexts/AbstractOperations.cpp
index 7ae8f18aa8..844cd5a8e3 100644
--- a/Libraries/LibWeb/SecureContexts/AbstractOperations.cpp
+++ b/Libraries/LibWeb/SecureContexts/AbstractOperations.cpp
@@ -33,7 +33,7 @@ Trustworthiness is_origin_potentially_trustworthy(URL::Origin const& origin)
 
     // 4. If origin’s host matches one of the CIDR notations 127.0.0.0/8 or ::1/128 [RFC4632], return "Potentially Trustworthy".
     if (origin.host().has<IPv4Address>()) {
-        if ((origin.host().get<IPv4Address>().to_u32() & 0xff000000) != 0)
+        if ((origin.host().get<IPv4Address>().to_u32() & 0xff000000) == 0x7f000000)
             return Trustworthiness::PotentiallyTrustworthy;
     } else if (origin.host().has<IPv6Address>()) {
         auto ipv6_address = origin.host().get<IPv6Address>();
diff --git a/Tests/LibWeb/CMakeLists.txt b/Tests/LibWeb/CMakeLists.txt
index 7d4c8eafc2..91f14ed9c9 100644
--- a/Tests/LibWeb/CMakeLists.txt
+++ b/Tests/LibWeb/CMakeLists.txt
@@ -14,6 +14,7 @@ set(TEST_SOURCES
     TestMimeSniff.cpp
     TestNumbers.cpp
     TestRefCountedTreeNode.cpp
+    TestSecureContexts.cpp
     TestSourceHighlighter.cpp
     TestStrings.cpp
 )
@@ -27,6 +28,7 @@ ladybird_utility(css-tokenizer SOURCES css-tokenizer.cpp LIBS LibFileSystem LibM
 target_link_libraries(TestContentBlocker PRIVATE LibURL)
 target_link_libraries(TestControlMessageQueue PRIVATE LibSync)
 target_link_libraries(TestFetchURL PRIVATE LibURL)
+target_link_libraries(TestSecureContexts PRIVATE LibURL)
 target_link_libraries(TestSourceHighlighter PRIVATE LibURL LibWebView)
 
 if (NOT WIN32)
diff --git a/Tests/LibWeb/TestSecureContexts.cpp b/Tests/LibWeb/TestSecureContexts.cpp
new file mode 100644
index 0000000000..a0adfd34d8
--- /dev/null
+++ b/Tests/LibWeb/TestSecureContexts.cpp
@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2026, Luke Wilde <luke@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+#include <LibURL/Parser.h>
+#include <LibURL/URL.h>
+#include <LibWeb/SecureContexts/AbstractOperations.h>
+
+static bool is_potentially_trustworthy(StringView url_string)
+{
+    auto url = URL::Parser::basic_parse(url_string);
+    VERIFY(url.has_value());
+    return Web::SecureContexts::is_url_potentially_trustworthy(*url) == Web::SecureContexts::Trustworthiness::PotentiallyTrustworthy;
+}
+
+TEST_CASE(ipv4_loopback_is_potentially_trustworthy)
+{
+    EXPECT(is_potentially_trustworthy("http://127.0.0.1/"sv));
+    // The whole 127.0.0.0/8 block is loopback, not just 127.0.0.1.
+    EXPECT(is_potentially_trustworthy("http://127.0.0.2/"sv));
+    EXPECT(is_potentially_trustworthy("http://127.0.0.0/"sv));
+    EXPECT(is_potentially_trustworthy("http://127.255.255.255/"sv));
+}
+
+TEST_CASE(non_loopback_ipv4_is_not_potentially_trustworthy)
+{
+    EXPECT(!is_potentially_trustworthy("http://8.8.8.8/"sv));
+    EXPECT(!is_potentially_trustworthy("http://1.1.1.1/"sv));
+    EXPECT(!is_potentially_trustworthy("http://192.168.1.1/"sv));
+    // A non-zero final octet of 127 must not be mistaken for the leading octet.
+    EXPECT(!is_potentially_trustworthy("http://8.8.8.127/"sv));
+    // Just outside 127.0.0.0/8 on either side.
+    EXPECT(!is_potentially_trustworthy("http://126.0.0.1/"sv));
+    EXPECT(!is_potentially_trustworthy("http://128.0.0.1/"sv));
+}
+
+TEST_CASE(ipv6_loopback_is_potentially_trustworthy)
+{
+    EXPECT(is_potentially_trustworthy("http://[::1]/"sv));
+    EXPECT(!is_potentially_trustworthy("http://[::2]/"sv));
+}
+
+TEST_CASE(localhost_is_potentially_trustworthy)
+{
+    EXPECT(is_potentially_trustworthy("http://localhost/"sv));
+    EXPECT(is_potentially_trustworthy("http://foo.localhost/"sv));
+    EXPECT(!is_potentially_trustworthy("http://example.com/"sv));
+}
+
+TEST_CASE(https_and_wss_are_potentially_trustworthy)
+{
+    EXPECT(is_potentially_trustworthy("https://example.com/"sv));
+    EXPECT(is_potentially_trustworthy("wss://example.com/"sv));
+    EXPECT(!is_potentially_trustworthy("http://example.com/"sv));
+    EXPECT(!is_potentially_trustworthy("ws://example.com/"sv));
+}