diff --git a/.github/workflows/fuzz.yml b/.github/workflows/fuzz.yml
new file mode 100644
index 0000000..1121636
--- /dev/null
+++ b/.github/workflows/fuzz.yml
@@ -0,0 +1,41 @@
+name: "Fuzz"
+
+on:
+  pull_request:
+    branches:
+      - main
+  schedule:
+    # Weekly long-running fuzz pass on Wednesdays at 06:00 UTC.
+    - cron: "0 6 * * 3"
+  workflow_dispatch:
+    inputs:
+      fuzz_time:
+        description: "Per-target fuzz duration (e.g. 60s, 5m, 1h)"
+        required: false
+        default: "60s"
+
+permissions:
+  contents: read
+
+jobs:
+  fuzz:
+    name: Run Go fuzz targets
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    env:
+      # PR runs use a short budget so they don't block merges.
+      # The scheduled weekly run gets a longer budget to actually find bugs.
+      # workflow_dispatch lets a maintainer pick any duration.
+      FUZZ_TIME: ${{ inputs.fuzz_time || (github.event_name == 'schedule' && '10m' || '60s') }}
+    steps:
+      - name: Check out code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+
+      - name: Set up Go 1.x
+        uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c  # v6.4.0
+        with:
+          go-version-file: ./go.mod
+
+      - name: make fuzz
+        run: make fuzz
diff --git a/Makefile b/Makefile
index 4dd6f89..73f97d3 100644
--- a/Makefile
+++ b/Makefile
@@ -167,6 +167,27 @@ test: $(PROJECT_COVERAGE_FILE) go-mod-tidy go-fmt go-vet go-generate ## Run test
 		./... \
 	)
 
+# Default duration each individual fuzz target runs for. Override on the
+# command line, e.g. `FUZZ_TIME=5m make fuzz`. Go's -fuzz flag only accepts
+# one target per invocation, so the loop below iterates over every Fuzz*
+# function discovered by `go test -list`.
+FUZZ_TIME ?= 60s
+
+.PHONY: fuzz
+fuzz: ## Run every Go fuzz target for $$FUZZ_TIME each (default 60s)
+	@printf "👉 Running fuzz tests (FUZZ_TIME=$(FUZZ_TIME))...\n"
+	@set -e; \
+	any=0; \
+	for pkg in $$(go list ./... | grep -v /mocks/); do \
+	  fuzzes=$$(go test -list '^Fuzz' $$pkg 2>/dev/null | grep -E '^Fuzz' || true); \
+	  for fz in $$fuzzes; do \
+	    any=1; \
+	    printf "  🤞 %s :: %s for %s\n" $$pkg $$fz $(FUZZ_TIME); \
+	    go test -run='^$$' -fuzz="^$$fz$$" -fuzztime=$(FUZZ_TIME) $$pkg; \
+	  done; \
+	done; \
+	if [ $$any -eq 0 ]; then printf "  ⚠️  no fuzz targets found\n"; fi
+
 ###############################################################################
 ##@ Build commands
 .PHONY: build
diff --git a/docs/Whats-New.md b/docs/Whats-New.md
index 710b915..46088fc 100644
--- a/docs/Whats-New.md
+++ b/docs/Whats-New.md
@@ -4,6 +4,25 @@ This document tracks notable changes, new features, and bug fixes across release
 
 ## Unreleased
 
+### OpenSSF Scorecard Hardening (Phase 4) — Fuzzing
+
+Closes the **Fuzzing** Scorecard check by adding native Go fuzz targets and a CI workflow that exercises them.
+
+**Targets added:**
+
+* `internal/repository.FuzzDiskRepositoryGetState` — fuzzes JSON state-file loading, the primary untrusted-input surface in the project (state files can be persisted to disk or S3 and re-read).
+* `internal/model.FuzzGroupsResultUnmarshalBinary` — fuzzes gob deserialization, which loops `Items` times calling `dec.Decode`. An attacker who controls the encoded blob can request an enormous number of items; the fuzzer ensures this fails safely rather than panicking or hanging.
+
+**Tooling:**
+
+* New `make fuzz` target enumerates every `Fuzz*` function via `go test -list` and runs each one for `FUZZ_TIME` (default `60s`, override per-invocation).
+* New `.github/workflows/fuzz.yml`:
+  * Runs every PR with a **60s budget per target** (fast enough to not block merges).
+  * Runs weekly on Wednesdays at 06:00 UTC with a **10-minute budget per target** to actually find bugs.
+  * `workflow_dispatch` lets a maintainer pick any duration on demand.
+
+When a fuzz target finds a crashing input, Go saves it under `testdata/fuzz/<FuzzName>/<hash>`. Commit those files: they become permanent regression tests that run as part of `go test ./...`.
+
 ### OpenSSF Scorecard Hardening (Phase 3) — Signed releases + SLSA provenance
 
 Closes the **Signed-Releases** Scorecard check (0/10 → 10/10) by adopting two complementary supply-chain primitives:
diff --git a/internal/model/group_fuzz_test.go b/internal/model/group_fuzz_test.go
new file mode 100644
index 0000000..af0dbd8
--- /dev/null
+++ b/internal/model/group_fuzz_test.go
@@ -0,0 +1,32 @@
+package model
+
+import (
+	"testing"
+)
+
+// FuzzGroupsResultUnmarshalBinary fuzzes the gob deserialization of a
+// GroupsResult. The decoder loops `Items` times calling dec.Decode, so an
+// attacker who controls the encoded blob can ask for an enormous number of
+// items and force the process to allocate aggressively before failing. The
+// fuzzer should surface panics, hangs, or unbounded allocations.
+func FuzzGroupsResultUnmarshalBinary(f *testing.F) {
+	// Seed with a valid round-tripped encoding.
+	seed := GroupsResult{
+		Items: 2,
+		Resources: []*Group{
+			{IPID: "1", Name: "a", Email: "a@example.com"},
+			{IPID: "2", Name: "b", Email: "b@example.com"},
+		},
+	}
+	if data, err := seed.MarshalBinary(); err == nil {
+		f.Add(data)
+	}
+	f.Add([]byte{})
+	f.Add([]byte{0x00})
+	f.Add([]byte{0xff, 0xff, 0xff, 0xff})
+
+	f.Fuzz(func(t *testing.T, data []byte) {
+		var gr GroupsResult
+		_ = gr.UnmarshalBinary(data)
+	})
+}
diff --git a/internal/repository/disk_fuzz_test.go b/internal/repository/disk_fuzz_test.go
new file mode 100644
index 0000000..9f644e6
--- /dev/null
+++ b/internal/repository/disk_fuzz_test.go
@@ -0,0 +1,34 @@
+package repository
+
+import (
+	"bytes"
+	"context"
+	"testing"
+)
+
+// FuzzDiskRepositoryGetState fuzzes the state-file loading path.
+//
+// The state file is the primary untrusted-input surface in this project:
+// users can persist it to disk, S3, or another external store and re-load
+// it later. A malformed state file must never crash, panic, or hang the
+// reconciliation loop — at worst it should return a typed error.
+func FuzzDiskRepositoryGetState(f *testing.F) {
+	// Seed with well-formed and adversarial inputs so the fuzzer has
+	// useful starting points to mutate from.
+	f.Add([]byte(`{}`))
+	f.Add([]byte(`{"schemaVersion":"1.0.0","codeVersion":"v0.0.0","hashCode":"abc","resources":{"groups":{},"users":{},"groupsMembers":{}}}`))
+	f.Add([]byte(``))
+	f.Add([]byte(`{"resources":{`))
+	f.Add([]byte(`{"resources":{"groups":{"items":2147483647,"resources":null}}}`))
+	f.Add([]byte("\x00\x01\x02\x03"))
+
+	f.Fuzz(func(t *testing.T, data []byte) {
+		buf := bytes.NewBuffer(data)
+		dr, err := NewDiskRepository(buf)
+		if err != nil {
+			return
+		}
+		// Errors are acceptable; panics, OOMs, and hangs are not.
+		_, _ = dr.GetState(context.Background())
+	})
+}