github · michaelnebel · Sep 2, 2026 · Sep 2, 2026 · Sep 3, 2026 · Sep 4, 2026
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Immutable;
 using System.Collections.Generic;
 using System.IO;
 using System.Security.Cryptography.X509Certificates;
@@ -14,13 +15,39 @@ public class DependabotProxy : IDependabotProxy
  /// <summary>
  /// Represents configurations for package registries.
  /// </summary>
- /// <param name="Type">The type of package registry.</param>
- /// <param name="URL">The URL of the package registry.</param>
- public record class RegistryConfig(string Type, string URL);
+ public class RegistryConfig
+ {
+ /// <summary>
+ /// The type of the package registry.
+ /// </summary>
+ public string Type { get; init; } = "";
+
+ /// <summary>
+ /// The URL of the package registry.
+ /// </summary>
+ public string URL { get; init; } = "";
+
+ /// <summary>
+ /// A boolean indicating whether this registry replaces the base registry.
+ /// </summary>
+ [JsonProperty("replaces-base")]
+ public bool ReplacesBase { get; init; } = false;
+ };
 
  public string Address { get; }
 
- public HashSet<string> RegistryURLs { get; } = [];
+ /// <summary>
+ /// A dictionary mapping registry URLs to a boolean indicating whether they replace the base registry.
+ /// </summary>
+ private readonly Dictionary<string, bool> registryMapping = [];
+
+ private ImmutableHashSet<string>? registryURLs;
+ public ImmutableHashSet<string> RegistryURLs =>
+ registryURLs ??= registryMapping.Keys.ToImmutableHashSet();
+
+ private ImmutableHashSet<string>? registryBaseURLs;
+ public ImmutableHashSet<string> RegistryBaseURLs =>
+ registryBaseURLs ??= registryMapping.Where(kvp => kvp.Value).Select(kvp => kvp.Key).ToImmutableHashSet();
 
  public string? CertificatePath { get; private set; }
 
@@ -65,7 +92,7 @@ private DependabotProxy(IDependabotProxyConfiguration config, ILogger logger, Te
  }
 
  logger.LogInfo($"Found private registry at '{registry.URL}'");
-RegistryURLs.Add(registry.URL);
+registryMapping.AddOrUpdateToLatest(registry.URL, registry.ReplacesBase);
  }
  }
  }

@@ -56,7 +56,6 @@ internal static class EnvironmentVariableNames
 
  /// <summary>
  /// Specifies the NuGet feeds to use for fallback NuGet dependency fetching. The value is a space-separated list of feed URLs.
- /// The default value is `https://api.nuget.org/v3/index.json`.
  /// </summary>
  public const string FallbackNugetFeeds = "CODEQL_EXTRACTOR_CSHARP_BUILDLESS_NUGET_FEEDS_FALLBACK";
 

@@ -10,13 +10,17 @@ namespace Semmle.Extraction.CSharp.DependencyFetching
 {
  internal sealed partial class FeedManager : IDisposable
  {
- internal const string PublicNugetOrgFeed = "https://api.nuget.org/v3/index.json";
+ private const string PublicNugetOrg = "nuget.org";
+ private const string PublicDotNugetOrg = $".{PublicNugetOrg}";
+ internal const string PublicApiNugetOrgFeed = $"https://api{PublicDotNugetOrg}/v3/index.json";
 
  private readonly ILogger logger;
  private readonly IDotNet dotnet;
  private readonly IFileProvider fileProvider;
  private readonly DependencyDirectory emptyPackageDirectory;
  private readonly ImmutableHashSet<string> privateRegistryFeeds;
+ private readonly bool hasPrivateRegistryBaseFeeds;
+ private readonly ImmutableHashSet<string> privateRegistryBaseFeeds;
  private readonly IFeedManagerIO feedManagerIo;
 
  /// <summary>
@@ -72,14 +76,33 @@ internal sealed partial class FeedManager : IDisposable
  /// </summary>
  public ImmutableHashSet<string> ReachableFallbackFeeds => lazyReachableFallbackFeeds.Value;
 
+ private readonly Lazy<ImmutableHashSet<string>> lazyReachableDefaultFeeds;
+
+ /// <summary>
+ /// Gets the list of default NuGet feeds that are configured in the environment.
+ /// This is either the public NuGet feed or a set of feeds specified by the environment.
+ /// </summary>
+ public ImmutableHashSet<string> DefaultFeeds { get; init; }
+
+ /// <summary>
+ /// Gets the list of reachable default NuGet feeds.
+ /// </summary>
+ public ImmutableHashSet<string> ReachableDefaultFeeds => lazyReachableDefaultFeeds.Value;
+
  public FeedManager(ILogger logger, IDotNet dotnet, IDependabotProxy? dependabotProxy, IFileProvider fileProvider, IFeedManagerIO feedManagerIo)
  {
  this.logger = logger;
  this.dotnet = dotnet;
  this.fileProvider = fileProvider;
  this.feedManagerIo = feedManagerIo;
- privateRegistryFeeds = dependabotProxy?.RegistryURLs.ToImmutableHashSet() ?? [];
+ privateRegistryFeeds = dependabotProxy?.RegistryURLs ?? [];
  HasPrivateRegistryFeeds = privateRegistryFeeds.Count > 0;
+ privateRegistryBaseFeeds = dependabotProxy?.RegistryBaseURLs ?? [];
+ hasPrivateRegistryBaseFeeds = privateRegistryBaseFeeds.Count > 0;
+
+ DefaultFeeds = hasPrivateRegistryBaseFeeds
+ ? privateRegistryBaseFeeds
+ : [PublicApiNugetOrgFeed];
  emptyPackageDirectory = new DependencyDirectory("empty", "empty package", logger);
 
  lazyExplicitFeeds = new Lazy<ImmutableHashSet<string>>(GetExplicitFeeds);
@@ -96,13 +119,28 @@ public FeedManager(ILogger logger, IDotNet dotnet, IDependabotProxy? dependabotP
  var reachableFallbackFeeds = GetReachableFallbackNugetFeeds();
  return reachableFallbackFeeds.ToImmutableHashSet();
  });
+ lazyReachableDefaultFeeds = new Lazy<ImmutableHashSet<string>>(() => CheckSpecifiedFeeds(DefaultFeeds));
  }
 
  public FeedManager(ILogger logger, IDotNet dotnet, IDependabotProxy? dependabotProxy, IFileProvider fileProvider)
  : this(logger, dotnet, dependabotProxy, fileProvider, new FeedManagerIO(logger, dependabotProxy))
  {
  }
 
+ private bool IsNugetOrgFeed(string url)
+ {
+ try
+ {
+ var uri = new Uri(url);
+ return uri.Host.EndsWith(PublicDotNugetOrg, StringComparison.InvariantCultureIgnoreCase) ||
+ string.Equals(uri.Host, PublicNugetOrg, StringComparison.InvariantCultureIgnoreCase);
+ }
+ catch (UriFormatException)
+ {
+ return false;
+ }
+ }
+
  private IEnumerable<string> GetFeeds(Func<IList<string>> getNugetFeeds)
  {
  var results = getNugetFeeds();
@@ -124,10 +162,18 @@ private IEnumerable<string> GetFeeds(Func<IList<string>> getNugetFeeds)
  continue;
  }
 
- if (!string.IsNullOrWhiteSpace(url))
+ if (hasPrivateRegistryBaseFeeds && IsNugetOrgFeed(url))
  {
- yield return url;
+ // Use private registry base feeds.
+ foreach (var feed in privateRegistryBaseFeeds)
+ {
+ logger.LogDebug($"Using private registry base feed '{feed}'.");
+ yield return feed;
+ }
+ continue;
  }
+
+ yield return url;
  }
  }
 
@@ -266,22 +312,6 @@ private ImmutableHashSet<string> CheckSpecifiedFeeds(ImmutableHashSet<string> fe
  return reachable.Union(feeds.Where(feed => excludedFeeds.Contains(feed))).ToImmutableHashSet();
  }
 
- /// <summary>
- /// Return true if the default NuGet feed is reachable, false otherwise.
- /// If the reachability check is disabled, this method will always return true.
- /// </summary>
- /// <returns>True if the default NuGet feed is reachable, false otherwise.</returns>
- public bool IsDefaultFeedReachable()
- {
- if (CheckNugetFeedResponsiveness)
- {
- var (initialTimeout, tryCount) = GetFeedRequestSettings(isFallback: false);
- return feedManagerIo.IsFeedReachable(PublicNugetOrgFeed, initialTimeout, tryCount);
- }
-
- return true;
- }
-
  /// <summary>
  /// Tests which of the feeds given by <paramref name="feedsToCheck"/> are reachable.
  /// </summary>
@@ -315,8 +345,8 @@ private List<string> GetReachableFallbackNugetFeeds()
  var fallbackFeeds = EnvironmentVariables.GetURLs(EnvironmentVariableNames.FallbackNugetFeeds).ToHashSet();
  if (fallbackFeeds.Count == 0)
  {
- fallbackFeeds.Add(PublicNugetOrgFeed);
- logger.LogInfo($"No fallback NuGet feeds specified. Adding default feed: {PublicNugetOrgFeed}");
+ fallbackFeeds.UnionWith(DefaultFeeds);
+ logger.LogInfo($"No fallback NuGet feeds specified. Adding default feeds: {string.Join(", ", DefaultFeeds.OrderBy(f => f))}");
 
  var shouldAddNugetConfigFeeds = EnvironmentVariables.GetBooleanOptOut(EnvironmentVariableNames.AddNugetConfigFeedsToFallback);
  logger.LogInfo($"Adding feeds from nuget.config to fallback restore: {shouldAddNugetConfigFeeds}");

@@ -1,5 +1,5 @@
 using System;
-using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Security.Cryptography.X509Certificates;
 
 namespace Semmle.Extraction.CSharp.DependencyFetching
@@ -14,7 +14,12 @@ public interface IDependabotProxy : IDisposable
  /// <summary>
  /// The URLs of package registries that are configured for the proxy.
  /// </summary>
- HashSet<string> RegistryURLs { get; }
+ ImmutableHashSet<string> RegistryURLs { get; }
+
+ /// <summary>
+ /// The URLs of package registries that replace the base registry.
+ /// </summary>
+ ImmutableHashSet<string> RegistryBaseURLs { get; }
 
  /// <summary>
  /// The path to the temporary file where the certificate is stored.

@@ -460,7 +460,7 @@ private bool TryRestorePackageManually(string package, List<string> nugetSources
  return true;
  }
 
- if (!feedManager.CheckNugetFeedResponsiveness && res.HasNugetPackageSourceError && nugetSources.Count > 0)
+ if (!feedManager.CheckNugetFeedResponsiveness && !feedManager.HasPrivateRegistryFeeds && res.HasNugetPackageSourceError && nugetSources.Count > 0)
  {
  logger.LogDebug($"Trying to restore '{package}' without explicitly providing NuGet sources.");
  // Restore could not be completed because the listed source is unavailable. Try without an explicit restore source argument.

@@ -67,10 +67,6 @@ private class NugetExeWrapper : IPackagesConfigRestore
 
  private bool IsWindows => SystemBuildActions.Instance.IsWindows();
 
- private bool? isDefaultFeedReachable;
- private bool IsDefaultFeedReachable =>
- isDefaultFeedReachable ??= feedManager.IsDefaultFeedReachable();
-
  /// <summary>
  /// Create the package manager for a specified source tree.
  /// </summary>
@@ -169,15 +165,18 @@ private bool TryRestoreNugetPackage(string packagesConfig)
 
  List<string> sourcesArgument = [];
  var feedsToUse = feedManager.FeedsToUse(packagesConfig).ToList();
- var useDefaultFeed = feedsToUse.Count == 0 && IsDefaultFeedReachable;
+ var defaultFeeds = feedManager.CheckNugetFeedResponsiveness
+ ? feedManager.ReachableDefaultFeeds
+ : feedManager.DefaultFeeds;
+ var useDefaultFeeds = feedsToUse.Count == 0 && defaultFeeds.Count > 0;
 
  // Explicitly construct the sources to be used for the restore command when checking feed
- // responsiveness, using private registries, or falling back to nuget.org.
- if (feedManager.CheckNugetFeedResponsiveness || feedManager.HasPrivateRegistryFeeds || useDefaultFeed)
+ // responsiveness, using private registries, or falling back to default feeds.
+ if (feedManager.CheckNugetFeedResponsiveness || feedManager.HasPrivateRegistryFeeds || useDefaultFeeds)
  {
- if (useDefaultFeed)
+ if (useDefaultFeeds)
  {
- feedsToUse.Add(FeedManager.PublicNugetOrgFeed);
+ feedsToUse.AddRange(defaultFeeds);
  }
  var restoreFeeds = feedManager.RestoreFeeds(feedsToUse);
  sourcesArgument = restoreFeeds.SelectMany<string, string>(feed => ["-Source", feed]).ToList();

@@ -135,7 +135,8 @@ public void TestDependabotRegistryUrls1()
 
  // Verify
  Assert.NotNull(proxy);
- Assert.Equal([], proxy.RegistryURLs);
+ Assert.Empty(proxy.RegistryURLs);
+ Assert.Empty(proxy.RegistryBaseURLs);
  }
 
  [Fact]
@@ -158,6 +159,7 @@ public void TestDependabotRegistryUrls2()
  Assert.Equal([
  "https://nuget.pkg.github.com/org/index.json"
  ], proxy.RegistryURLs);
+ Assert.Empty(proxy.RegistryBaseURLs);
  }
 
  [Fact]
@@ -180,6 +182,33 @@ public void TestDependabotRegistryUrls3()
  Assert.Equal([
  "https://example.com/org/index.json"
  ], proxy.RegistryURLs);
+ Assert.Empty(proxy.RegistryBaseURLs);
+ }
+
+ [Fact]
+ public void TestDependabotReplacesBase1()
+ {
+ // Setup
+ var config = new DependabotConfigurationStub
+ {
+ Port = "8080",
+ Host = "localhost",
+ RegistryURLs = "[ { \"type\": \"nuget_feed\", \"url\": \"https://example.com/org/index.json\", \"replaces-base\": true }, { \"type\": \"nuget_feed\", \"url\": \"https://example2.com/org/index.json\", \"replaces-base\": false } ]"
+ };
+
+ // Execute
+ using var tempWorkingDirectory = MakeTemporaryDirectory();
+ using var proxy = DependabotProxy.Make(config, new LoggerStub(), new DiagnosticsWriterStub(), tempWorkingDirectory);
+
+ // Verify
+ Assert.NotNull(proxy);
+ Assert.Equal([
+ "https://example.com/org/index.json",
+ "https://example2.com/org/index.json"
+ ], proxy.RegistryURLs);
+ Assert.Equal([
+ "https://example.com/org/index.json",
+ ], proxy.RegistryBaseURLs);
  }
  }
 }