diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/primops/flake.cc
index 293e5ad0b..de692f167 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/primops/flake.cc
@@ -285,7 +285,10 @@ Flake getFlake(EvalState & state, const FlakeRef & flakeRef, bool impureIsAllowe
                 + "/" + flake.sourceInfo.rev->to_string(Base16, false));
     }
 
-    Path flakeFile = sourceInfo.storePath + "/" + resolvedRef.subdir + "/flake.nix";
+    // Guard against symlink attacks.
+    auto flakeFile = canonPath(sourceInfo.storePath + "/" + resolvedRef.subdir + "/flake.nix");
+    if (!isInDir(flakeFile, sourceInfo.storePath))
+        throw Error("flake file '%s' escapes from '%s'", resolvedRef, sourceInfo.storePath);
     if (!pathExists(flakeFile))
         throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", resolvedRef, resolvedRef.subdir);
 
diff --git a/src/libexpr/primops/flakeref.cc b/src/libexpr/primops/flakeref.cc
index 56ba58d09..b7a20a170 100644
--- a/src/libexpr/primops/flakeref.cc
+++ b/src/libexpr/primops/flakeref.cc
@@ -107,7 +107,7 @@ FlakeRef::FlakeRef(const std::string & uri, bool allowRelative)
                     throw Error("invalid Git ref '%s'", value);
                 ref = value;
             } else if (name == "dir") {
-                if (!std::regex_match(value, subDirRegex2))
+                if (value != "" && !std::regex_match(value, subDirRegex2))
                     throw Error("flake '%s' has invalid subdirectory '%s'", uri, value);
                 subdir = value;
             } else