diff --git a/Manifest.toml b/Manifest.toml
index ebed2653c7..ad121a1b07 100644
--- a/Manifest.toml
+++ b/Manifest.toml
@@ -1,5 +1,11 @@
 # This file is machine-generated - editing it directly is not advised
 
+[[Artifacts]]
+deps = ["Pkg"]
+git-tree-sha1 = "c30985d8821e0cd73870b17b0ed0ce6dc44cb744"
+uuid = "56f22d72-fd6d-98f1-02f0-08ddc0907c33"
+version = "1.3.0"
+
 [[AssetRegistry]]
 deps = ["Distributed", "JSON", "Pidfile", "SHA", "Test"]
 git-tree-sha1 = "b25e88db7944f98789130d7b503276bc34bc098e"
@@ -32,6 +38,11 @@ uuid = "ade2ca70-3891-5945-98fb-dc099432e06a"
 deps = ["Random", "Serialization", "Sockets"]
 uuid = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 
+[[ExprTools]]
+git-tree-sha1 = "10407a39b87f29d47ebaca8edbc75d7c302ff93e"
+uuid = "e2ba6199-217a-4e67-a87a-7c52f15ade04"
+version = "0.1.3"
+
 [[FileWatching]]
 uuid = "7b1f6079-737a-58dc-b8bc-7a2ca5c1b5ee"
 
@@ -74,6 +85,11 @@ git-tree-sha1 = "a3f24677c21f5bbe9d2a714f95dcd58337fb2856"
 uuid = "82899510-4779-5014-852e-03e436cf321d"
 version = "1.0.0"
 
+[[JLLWrappers]]
+git-tree-sha1 = "c70593677bbf2c3ccab4f7500d0f4dacfff7b75c"
+uuid = "692b3bcd-3c85-4b1f-b108-f13ce0eb3210"
+version = "1.1.3"
+
 [[JSON]]
 deps = ["Dates", "Mmap", "Parsers", "Unicode"]
 git-tree-sha1 = "b34d7cef7b337321e97d22242c3c2b91f476748e"
@@ -131,6 +147,12 @@ version = "2.16.6+1"
 [[Mmap]]
 uuid = "a63ad114-7e13-5084-954f-fe012c677804"
 
+[[Mocking]]
+deps = ["ExprTools"]
+git-tree-sha1 = "916b850daad0d46b8c71f65f719c49957e9513ed"
+uuid = "78c3b35d-d492-501b-9361-3d52fe80e533"
+version = "0.7.1"
+
 [[Mustache]]
 deps = ["Printf", "Tables"]
 git-tree-sha1 = "fcfc8266461f2905534aa00c0fc59b8751b1026e"
@@ -228,7 +250,7 @@ uuid = "c2297ded-f4af-51ae-bb23-16f91089e4e1"
 version = "1.2.1"
 
 [[ZeroMQ_jll]]
-deps = ["Libdl", "Pkg"]
-git-tree-sha1 = "733352667c60ce39dfd3017db9b798b288c87417"
+deps = ["Artifacts", "JLLWrappers", "Libdl", "Pkg"]
+git-tree-sha1 = "bba617292e040408cb72baa03c20f43583bf239f"
 uuid = "8f1865be-045e-5c20-9c9f-bfbfb0764568"
-version = "4.3.2+4"
+version = "4.3.2+5"
diff --git a/Project.toml b/Project.toml
index ff3a6b60bb..15c2a30707 100644
--- a/Project.toml
+++ b/Project.toml
@@ -16,6 +16,7 @@ JSON = "682c06a0-de6a-54ab-a142-c8b1cf79cde6"
 LibGit2 = "76f85450-5226-5b5a-8eaa-529ad045b433"
 Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"
 MbedTLS = "739be429-bea8-5141-9913-cc70e7f3736d"
+Mocking = "78c3b35d-d492-501b-9361-3d52fe80e533"
 Mustache = "ffc61752-8dc7-55ee-8c37-f3e9cdd09e70"
 Mux = "a975b10e-0019-58db-a62f-e48ff68538c9"
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
diff --git a/src/webui/gitutils.jl b/src/webui/gitutils.jl
index 885686ae75..1c8bb4e1e3 100644
--- a/src/webui/gitutils.jl
+++ b/src/webui/gitutils.jl
@@ -1,4 +1,5 @@
 using ..Registrator: decodeb64
+using Mocking
 
 # # Run some GitForge function, warning on error but still returning the value.
 macro gf(ex::Expr)
@@ -25,13 +26,23 @@ getrepo(::GitLabAPI, owner::AbstractString, name::AbstractString) =
 getrepo(::GitHubAPI, owner::AbstractString, name::AbstractString) =
     @gf get_repo(PROVIDERS["github"].client, owner, name)
 
+
+abstract type AuthResult end
+struct AuthSuccess <: AuthResult end
+struct AuthFailure <: AuthResult
+    reason::AbstractString
+end
+
+is_success(res::AuthSuccess) = true
+is_success(res::AuthFailure) = false
+
 # Check for a user's authorization to release a package.
 # The criteria is simply whether the user is a collaborator for user-owned repos,
 # or whether they're an organization member or collaborator for organization-owned repos.
-isauthorized(u, repo) = false
+isauthorized(u, repo) = AuthFailure("Unkown user type or repo type")
 function isauthorized(u::User{GitHub.User}, repo::GitHub.Repo)
     if !get(CONFIG, "allow_private", false)
-        repo.private && return false
+        repo.private && return AuthFailure("Repo $(repo.name) is private")
     end
 
     if repo.private
@@ -40,20 +51,29 @@ function isauthorized(u::User{GitHub.User}, repo::GitHub.Repo)
         forge = u.forge
     end
 
-    hasauth = if repo.organization === nothing
-        @gf is_collaborator(forge, repo.owner.login, repo.name, u.user.login)
+    if repo.organization === nothing
+        hasauth = @gf @mock is_collaborator(forge, repo.owner.login, repo.name, u.user.login)
+        if something(hasauth, false)
+            return AuthSuccess()
+        else
+            return AuthFailure("User $(u.user.login) is not a collaborator on repo $(repo.name)")
+        end
     else
         # First check for organization membership, and fall back to collaborator status.
-        ismember = @gf is_member(forge, repo.organization.login, u.user.login)
-        something(ismember, false) ||
-            @gf is_collaborator(forge, repo.organization.login, repo.name, u.user.login)
+        ismember = @gf @mock is_member(forge, repo.organization.login, u.user.login)
+        hasauth = something(ismember, false) ||
+            @gf @mock is_collaborator(forge, repo.organization.login, repo.name, u.user.login)
+        if something(hasauth, false)
+            return AuthSuccess()
+        else
+            return AuthFailure("User $(u.user.login) is not a member of the org $(repo.organization.login) and not a collaborator on repo $(repo.name)")
+        end
     end
-    return something(hasauth, false)
 end
 
 function isauthorized(u::User{GitLab.User}, repo::GitLab.Project)
     if !get(CONFIG, "allow_private", false)
-        repo.visibility == "private" && return false
+        repo.visibility == "private" && return AuthFailure("Project $(repo.name) is private")
     end
 
     if repo.visibility == "private"
@@ -62,23 +82,31 @@ function isauthorized(u::User{GitLab.User}, repo::GitLab.Project)
         forge = u.forge
     end
 
-    hasauth = if repo.namespace.kind == "user"
-        @gf is_collaborator(forge, repo.owner.username, repo.name, u.user.id)
+    if repo.namespace.kind == "user"
+        hasauth = @gf @mock is_collaborator(forge, repo.owner.username, repo.name, u.user.id)
+        if something(hasauth, false)
+            return AuthSuccess()
+        else
+            return AuthFailure("User $(u.user.name) is not a member of project $(repo.name)") # GitLab terminology "member" (not "collaborator")
+        end
     else
         # Same as above: group membership then collaborator check.
         nspath = split(repo.namespace.full_path, "/")
-        ismember = @gf is_collaborator(u.forge, repo.namespace.full_path, repo.name, u.user.id)
+        ismember = @gf @mock is_collaborator(u.forge, repo.namespace.full_path, repo.name, u.user.id)
         if !something(ismember, false)
             accns = ""
             for ns in nspath
                 accns = joinpath(accns, ns)
-                ismember = @gf is_member(forge, accns, u.user.id)
+                ismember = @gf @mock is_member(forge, accns, u.user.id)
                 something(ismember, false) && break
             end
         end
-        ismember
+        if ismember
+            return AuthSuccess()
+        else
+            return AuthFailure("Project $(repo.name) belongs to the group $(repo.namespace.full_path), and user $(u.user.name) is not a member of that group or its parent group(s)")
+        end
     end
-    return something(hasauth, false)
 end
 
 # Get the raw (Julia)Project.toml text from a repository.
diff --git a/src/webui/routes/register.jl b/src/webui/routes/register.jl
index f5a752796f..3cabe71ebb 100644
--- a/src/webui/routes/register.jl
+++ b/src/webui/routes/register.jl
@@ -30,7 +30,10 @@ function register(r::HTTP.Request)
     owner, name = splitrepo(package)
     repo = getrepo(u.forge, owner, name)
     repo === nothing && return json(400; error="Repository was not found")
-    isauthorized(u, repo) || return json(400; error="Unauthorized to release this package")
+    auth_result = isauthorized(u, repo)
+    if !is_success(auth_result)
+        return json(400; error="Unauthorized to release this package. Reason: $(auth_result.reason)")
+    end
 
     # Get the (Julia)Project.toml, and make sure it is valid.
     toml = gettoml(u.forge, repo, ref)
diff --git a/test/runtests.jl b/test/runtests.jl
index 4b30f5eecd..6ff7784fc9 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -10,5 +10,6 @@ if get(ENV, "TRAVIS", "") == "true" && !haskey(ENV, "GITHUB_API_TOKEN")
 else
     include("webui.jl")
 end
+include("webui/gitutils.jl")
 
 end
diff --git a/test/webui/gitutils.jl b/test/webui/gitutils.jl
new file mode 100644
index 0000000000..c195278e36
--- /dev/null
+++ b/test/webui/gitutils.jl
@@ -0,0 +1,111 @@
+using Dates: DateTime
+using Registrator.WebUI: isauthorized, AuthFailure, AuthSuccess, User
+using GitForge: GitForge, GitHub, GitLab
+using HTTP: stacktrace
+
+using Mocking
+
+Mocking.activate()
+
+function patch_gitforge(body::Function; is_collaborator=false, is_member=false)
+    patches = [
+        @patch GitForge.is_collaborator(args...) =
+            GitForge.Result{Bool}(is_collaborator, nothing, nothing, stacktrace())
+        @patch GitForge.is_member(args...) =
+            GitForge.Result{Bool}(is_member, nothing, nothing, stacktrace())
+    ]
+    
+    apply(patches) do
+        return body()
+    end
+end
+
+@testset "gitutils" begin
+
+@testset "isauthorized" begin
+    @test isauthorized("username", "reponame") == AuthFailure("Unkown user type or repo type")
+
+    @testset "GitHub" begin
+
+        user = GitHub.User(login="user123")
+        org = GitHub.User(login="JuliaLang")
+        private_repo = GitHub.Repo(name="Example.jl", private=true, owner=user)
+        public_repo_of_user = GitHub.Repo(name="Example.jl", private=false, owner=user, organization=nothing)
+        public_repo_of_org = GitHub.Repo(name="Example.jl", private=false, owner=org, organization=org)
+        u = User(user, GitHub.GitHubAPI())
+
+        @testset "private repo" begin
+            # Assuming CONFIG["allow_private"] is false
+            @test isauthorized(u, private_repo) == AuthFailure("Repo Example.jl is private")
+        end
+
+        @testset "public repo of user" begin
+            # authorized if user is a collaborator on the repo
+            patch_gitforge(is_collaborator=true) do
+                @test isauthorized(u, public_repo_of_user) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=false) do
+                @test isauthorized(u, public_repo_of_user) == AuthFailure("User user123 is not a collaborator on repo Example.jl")
+            end
+        end
+
+        @testset "public repo of org" begin
+            # authorized if user is either a collaborator on the repo or member of the org
+            patch_gitforge(is_collaborator=true, is_member=true) do
+                @test isauthorized(u, public_repo_of_org) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=true, is_member=false) do
+                @test isauthorized(u, public_repo_of_org) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=false, is_member=true) do
+                @test isauthorized(u, public_repo_of_org) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=false, is_member=false) do
+                @test isauthorized(u, public_repo_of_org) == AuthFailure("User user123 is not a member of the org JuliaLang and not a collaborator on repo Example.jl")
+            end
+        end
+    end
+
+    @testset "GitLab" begin
+
+        user = GitLab.User(name="user123", username="user123", id=111)
+        org = GitLab.User(name="org123", username="org123", id=222)
+        private_project = GitLab.Project(name="Example.jl", visibility="private", owner=user)
+        public_project_of_user = GitLab.Project(name="Example.jl", visibility="public", owner=user, namespace=GitLab.Namespace(kind="user"))
+        public_project_of_group = GitLab.Project(name="Example.jl", visibility="public", owner=org, namespace=GitLab.Namespace(kind="group", full_path="org123/subgroup/Example.jl"))
+        u = User(user, GitLab.GitLabAPI())
+
+        @testset "private project" begin
+            # Assuming CONFIG["allow_private"] is false
+            @test isauthorized(u, private_project) == AuthFailure("Project Example.jl is private")
+        end
+
+        @testset "public project of user" begin
+            # authorized if user is a collaborator on the project
+            patch_gitforge(is_collaborator=true) do
+                @test isauthorized(u, public_project_of_user) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=false) do
+                @test isauthorized(u, public_project_of_user) == AuthFailure("User user123 is not a member of project Example.jl")
+            end
+        end
+
+        @testset "public project of group" begin
+            # authorized if user is a collaborator on the project or member of the group/subgroups
+            patch_gitforge(is_collaborator=true, is_member=true) do
+                @test isauthorized(u, public_project_of_group) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=false, is_member=true) do
+                @test isauthorized(u, public_project_of_group) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=true, is_member=false) do
+                @test isauthorized(u, public_project_of_group) == AuthSuccess()
+            end
+            patch_gitforge(is_collaborator=false, is_member=false) do
+                @test isauthorized(u, public_project_of_group) == AuthFailure("Project Example.jl belongs to the group org123/subgroup/Example.jl, and user user123 is not a member of that group or its parent group(s)")
+            end
+        end
+    end
+end
+
+end
\ No newline at end of file