diff --git a/projects/viper/Dockerfile b/projects/viper/Dockerfile
new file mode 100644
index 000000000000..60fbea15ec6e
--- /dev/null
+++ b/projects/viper/Dockerfile
@@ -0,0 +1,21 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+FROM gcr.io/oss-fuzz-base/base-builder-go
+RUN git clone --depth 1 https://github.com/spf13/viper $SRC/viper
+COPY build.sh fuzz_test.go $SRC/
+WORKDIR $SRC/viper
+
diff --git a/projects/viper/build.sh b/projects/viper/build.sh
new file mode 100644
index 000000000000..d0e099b37cec
--- /dev/null
+++ b/projects/viper/build.sh
@@ -0,0 +1,24 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+cd $SRC/viper
+cp $SRC/fuzz_test.go ./
+compile_go_fuzzer github.com/spf13/viper FuzzReadConfig fuzz_read_config
+compile_go_fuzzer github.com/spf13/viper FuzzUnmarshal fuzz_unmarshal
+compile_go_fuzzer github.com/spf13/viper FuzzMergeConfigMap fuzz_merge_config_map
+compile_go_fuzzer github.com/spf13/viper FuzzSetGet fuzz_set_get
+compile_go_fuzzer github.com/spf13/viper FuzzConfigFile fuzz_config_file
+
diff --git a/projects/viper/fuzz_test.go b/projects/viper/fuzz_test.go
new file mode 100644
index 000000000000..ef171651b39a
--- /dev/null
+++ b/projects/viper/fuzz_test.go
@@ -0,0 +1,139 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package viper_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"github.com/spf13/viper"
+)
+
+func FuzzReadConfig(f *testing.F) {
+	yamlSeed := []byte("server:\n  port: 8080\n  host: localhost\n")
+	jsonSeed := []byte(`{"server": {"port": 8080, "host": "localhost"}}`)
+	tomlSeed := []byte("[server]\nport = 8080\nhost = \"localhost\"\n")
+	dotenvSeed := []byte("SERVER_PORT=8080\nSERVER_HOST=localhost\n")
+
+	f.Add("yaml", yamlSeed)
+	f.Add("json", jsonSeed)
+	f.Add("toml", tomlSeed)
+	f.Add("dotenv", dotenvSeed)
+	f.Add("yaml", []byte{})
+	f.Add("yaml", []byte{0xFF, 0x00, 0xFF})
+
+	f.Fuzz(func(t *testing.T, format string, data []byte) {
+		if len(format) > 20 || len(data) > 1<<20 { return }
+		format = strings.Map(func(r rune) rune {
+			if r >= 'a' && r <= 'z' { return r }
+			return -1
+		}, strings.ToLower(format))
+		if format == "" { format = "yaml" }
+		func() {
+			defer func() { recover() }()
+			v := viper.New()
+			v.SetConfigType(format)
+			v.ReadConfig(bytes.NewReader(data))
+			for _, k := range v.AllKeys() {
+				v.Get(k)
+			}
+		}()
+	})
+}
+
+func FuzzUnmarshal(f *testing.F) {
+	type Cfg struct {
+		Name    string  `mapstructure:"name"`
+		Port    int     `mapstructure:"port"`
+		Enabled bool    `mapstructure:"enabled"`
+		Rate    float64 `mapstructure:"rate"`
+	}
+	f.Add([]byte("name: test\nport: 8080\nenabled: true\nrate: 0.75\n"))
+	f.Add([]byte("name: \"\"\nport: -1\nenabled: false\n"))
+	f.Add([]byte("!!binary dGVzdA==\n"))
+	f.Fuzz(func(t *testing.T, data []byte) {
+		if len(data) > 1<<16 { return }
+		func() {
+			defer func() { recover() }()
+			v := viper.New()
+			v.SetConfigType("yaml")
+			if err := v.ReadConfig(bytes.NewReader(data)); err != nil { return }
+			var cfg Cfg
+			v.Unmarshal(&cfg)
+		}()
+	})
+}
+
+func FuzzMergeConfigMap(f *testing.F) {
+	f.Add([]byte("key1: val1\n"), []byte("key2: val2\n"))
+	f.Add([]byte("key: val1\n"), []byte("key: val2\n"))
+	f.Add([]byte(""), []byte("key: val\n"))
+	f.Fuzz(func(t *testing.T, base, override []byte) {
+		if len(base) > 1<<16 || len(override) > 1<<16 { return }
+		func() {
+			defer func() { recover() }()
+			v := viper.New()
+			v.SetConfigType("yaml")
+			v.ReadConfig(bytes.NewReader(base))
+			v.MergeConfig(bytes.NewReader(override))
+			for _, k := range v.AllKeys() { v.Get(k) }
+		}()
+	})
+}
+
+func FuzzSetGet(f *testing.F) {
+	f.Add("key", "value")
+	f.Add("", "empty-key")
+	f.Add("nested.key.path", "deep")
+	f.Fuzz(func(t *testing.T, key, value string) {
+		if len(key) > 5000 || len(value) > 1<<16 { return }
+		func() {
+			defer func() { recover() }()
+			v := viper.New()
+			v.Set(key, value)
+			v.Get(key)
+			v.GetString(key)
+			v.GetInt(key)
+			v.GetBool(key)
+		}()
+	})
+}
+
+func FuzzConfigFile(f *testing.F) {
+	f.Add("yaml", []byte("key: value\n"))
+	f.Add("json", []byte(`{"key":"value"}`))
+	f.Add("toml", []byte("key = \"value\"\n"))
+	f.Fuzz(func(t *testing.T, ext string, data []byte) {
+		if len(ext) > 10 || len(data) > 1<<16 || len(data) < 2 { return }
+		ext = strings.Map(func(r rune) rune {
+			if r >= 'a' && r <= 'z' { return r }
+			return -1
+		}, strings.ToLower(ext))
+		if ext == "" { ext = "yaml" }
+		func() {
+			defer func() { recover() }()
+			dir, _ := os.MkdirTemp("", "fuzz-viper-*")
+			defer os.RemoveAll(dir)
+			fp := filepath.Join(dir, "config."+ext)
+			os.WriteFile(fp, data, 0644)
+			v := viper.New()
+			v.SetConfigFile(fp)
+			v.ReadInConfig()
+			v.AllSettings()
+		}()
+	})
+}
diff --git a/projects/viper/project.yaml b/projects/viper/project.yaml
new file mode 100644
index 000000000000..2dc7c6d0ffb7
--- /dev/null
+++ b/projects/viper/project.yaml
@@ -0,0 +1,12 @@
+homepage: "https://github.com/spf13/viper"
+language: go
+primary_contact: "candasjunk@gmail.com"
+auto_ccs:
+  - "candasjunk@gmail.com"
+main_repo: "https://github.com/spf13/viper"
+sanitizers:
+  - address
+  - memory
+fuzzing_engines:
+  - libfuzzer
+# Criticality: Viper (26K+ stars) is the dominant configuration library for Go. It parses YAML, JSON, TOML, and environment variables for virtually every Go application. A parsing bug in Viper is a universal configuration injection vector.