diff --git a/flake.nix b/flake.nix
index 895a081f2..094df9776 100644
--- a/flake.nix
+++ b/flake.nix
@@ -263,18 +263,46 @@
           flatMapAttrs
             (
               {
-                "" = nixpkgsFor.${system}.native;
+                # Run all tests with UBSAN enabled. Running both with ubsan and
+                # without doesn't seem to have much immediate benefit for doubling
+                # the GHA CI workaround.
+                #
+                # TODO: Work toward enabling "address,undefined" if it seems feasible.
+                # This would maybe require dropping Boost coroutines and ignoring intentional
+                # memory leaks with detect_leaks=0.
+                "" = rec {
+                  nixpkgs = nixpkgsFor.${system}.native;
+                  nixComponents = nixpkgs.nixComponents.overrideScope (
+                    nixCompFinal: nixCompPrev: {
+                      mesonComponentOverrides = _finalAttrs: prevAttrs: {
+                        mesonFlags =
+                          (prevAttrs.mesonFlags or [ ])
+                          # TODO: Macos builds instrumented with ubsan take very long
+                          # to run functional tests.
+                          ++ lib.optionals (!nixpkgs.stdenv.hostPlatform.isDarwin) [
+                            (lib.mesonOption "b_sanitize" "undefined")
+                          ];
+                      };
+                    }
+                  );
+                };
               }
               // lib.optionalAttrs (!nixpkgsFor.${system}.native.stdenv.hostPlatform.isDarwin) {
                 # TODO: enable static builds for darwin, blocked on:
                 #       https://github.com/NixOS/nixpkgs/issues/320448
                 # TODO: disabled to speed up GHA CI.
-                #"static-" = nixpkgsFor.${system}.native.pkgsStatic;
+                # "static-" = {
+                #   nixpkgs = nixpkgsFor.${system}.native.pkgsStatic;
+                # };
               }
             )
             (
-              nixpkgsPrefix: nixpkgs:
-              flatMapAttrs nixpkgs.nixComponents (
+              nixpkgsPrefix:
+              {
+                nixpkgs,
+                nixComponents ? nixpkgs.nixComponents,
+              }:
+              flatMapAttrs nixComponents (
                 pkgName: pkg:
                 flatMapAttrs pkg.tests or { } (
                   testName: test: {
@@ -283,7 +311,7 @@
                 )
               )
               // lib.optionalAttrs (nixpkgs.stdenv.hostPlatform == nixpkgs.stdenv.buildPlatform) {
-                "${nixpkgsPrefix}nix-functional-tests" = nixpkgs.nixComponents.nix-functional-tests;
+                "${nixpkgsPrefix}nix-functional-tests" = nixComponents.nix-functional-tests;
               }
             )
         // devFlake.checks.${system} or { }
diff --git a/src/libutil/strings.cc b/src/libutil/strings.cc
index 402b7ae98..785d81797 100644
--- a/src/libutil/strings.cc
+++ b/src/libutil/strings.cc
@@ -17,8 +17,10 @@ struct view_stringbuf : public std::stringbuf
     }
 };
 
-std::string_view toView(const std::ostringstream & os)
+__attribute__((no_sanitize("undefined"))) std::string_view toView(const std::ostringstream & os)
 {
+    /* Downcasting like this is very much undefined behavior, so we disable
+       UBSAN for this function. */
     auto buf = static_cast<view_stringbuf *>(os.rdbuf());
     return buf->toView();
 }