Fix OAuth1 double-encoding of RFC 3986 special characters in URL paths

src/RestSharp/Authenticators/OAuth/OAuthTools.cs

@@ -154,7 +154,19 @@ static string ConstructRequestUrl(Uri url) {
         var secure = url is { Scheme: "https", Port: 443 };
         var port   = basic || secure ? "" : $":{url.Port}";
 
-        return $"{url.Scheme}://{url.Host}{port}{url.AbsolutePath}";
+        // Decode the path to avoid double-encoding when the path contains already-encoded characters.
+        // For example, if a URL segment was added with AddUrlSegment("id", "value!"), it gets encoded
+        // to "value%21" in the URL. When we extract url.AbsolutePath, it contains "%21" (encoded).
+        // If we then call UrlEncodeRelaxed on it, Uri.EscapeDataString would encode the "%" to "%25",
+        // resulting in "%2521" (double-encoded). By decoding first, we ensure proper single encoding.
+        // 
+        // Security note: This is safe because:
+        // - The url parameter is a validated Uri object constructed by RestSharp's BuildUri()
+        // - The decoded path is immediately re-encoded by UrlEncodeRelaxed before use
+        // - There is no direct user input involved in this internal OAuth signature calculation
+        var decodedPath = Uri.UnescapeDataString(url.AbsolutePath);
+
+        return $"{url.Scheme}://{url.Host}{port}{decodedPath}";
     }
 
     /// <summary>

test/RestSharp.Tests/Auth/OAuth1SignatureTests.cs

@@ -49,4 +49,75 @@ public void Generates_correct_signature_base() {
                 "POST&https%3A%2F%2Fapi.twitter.com%2F1.1%2Fstatuses%2Fupdate.json&include_entities%3Dtrue%26oauth_consumer_key%3Dxvz1evFS4wEEPTGEFPHBog%26oauth_nonce%3DkYjzVBB8Y0ZFabxSWbWovY3uYSQ2pTgmZeNu2VS4cg%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1318622958%26oauth_token%3D370773112-GmHxMAgYyLbNEtIKZeRNFsMKPR9EyMZeS9weJAEb%26oauth_version%3D1.0%26status%3DHello%2520Ladies%2520%252B%2520Gentlemen%252C%2520a%2520signed%2520OAuth%2520request%2521"
             );
     }
+
+    [Fact]
+    public void Handles_path_with_exclamation_mark() {
+        // Test that a path segment with ! is encoded correctly in the signature base
+        var client = new RestClient("https://api.example.com");
+        var request = new RestRequest("path/with!exclamation/resource", Method.Get);
+
+        const string method = "GET";
+        var url = client.BuildUri(request).ToString();
+        var parameters = new WebPairCollection();
+
+        _workflow.RequestUrl = url;
+        var oauthParameters = _workflow.BuildProtectedResourceSignature(method, parameters);
+
+        var signatureBase = OAuthTools.ConcatenateRequestElements(method, url, oauthParameters.Parameters);
+
+        // The URL should be encoded with ! as %21 in the signature base
+        signatureBase.Should().Contain("path%2Fwith%21exclamation%2Fresource");
+    }
+
+    [Theory]
+    [InlineData("path/with!exclamation", "%21")]
+    [InlineData("path/with*asterisk", "%2A")]
+    [InlineData("path/with'apostrophe", "%27")]
+    [InlineData("path/with(paren", "%28")]
+    [InlineData("path/with)paren", "%29")]
+    public void Encodes_RFC3986_special_chars_in_path(string path, string encodedChar) {
+        // Test that RFC 3986 special characters are properly encoded in path segments
+        var client = new RestClient("https://api.example.com");
+        var request = new RestRequest(path, Method.Get);
+
+        const string method = "GET";
+        var url = client.BuildUri(request).ToString();
+        var parameters = new WebPairCollection();
+
+        _workflow.RequestUrl = url;
+        var oauthParameters = _workflow.BuildProtectedResourceSignature(method, parameters);
+
+        var signatureBase = OAuthTools.ConcatenateRequestElements(method, url, oauthParameters.Parameters);
+
+        // The URL should contain the encoded character in the signature base
+        signatureBase.Should().Contain(encodedChar);
+    }
+
+    [Theory]
+    [InlineData("with!exclamation")]
+    [InlineData("with*asterisk")]
+    [InlineData("with'apostrophe")]
+    [InlineData("with(paren")]
+    [InlineData("with)paren")]
+    public void Handles_url_segment_with_RFC3986_special_chars(string segmentValue) {
+        // Test that URL segment parameters with RFC 3986 special characters don't get double-encoded
+        var client = new RestClient("https://api.example.com");
+        var request = new RestRequest("path/{segment}/resource", Method.Get);
+        request.AddUrlSegment("segment", segmentValue);
+
+        const string method = "GET";
+        var url = client.BuildUri(request).ToString();
+        var parameters = new WebPairCollection();
+
+        _workflow.RequestUrl = url;
+        var oauthParameters = _workflow.BuildProtectedResourceSignature(method, parameters);
+
+        var signatureBase = OAuthTools.ConcatenateRequestElements(method, url, oauthParameters.Parameters);
+
+        // The signature base should NOT contain double-encoded characters like %2521 (which is %25 + 21)
+        signatureBase.Should().NotContain("%25");
+
+        // But it should contain properly encoded special chars
+        signatureBase.Should().MatchRegex("%2[0-9A-F]");
+    }
 }