Add fuzz testing for query parser and token validator

Implements cargo-fuzz based fuzzing for security-critical components:
- Query parser (URL and path-based request parsing)
- Token validator (SSRF token validation and file:// handling)

Changes:
- Add fuzz/ directory with two fuzz targets
- Add GitHub Actions workflow to run fuzz tests on PRs (2 min per target)
- Expose internal types via lib.rs for fuzzing
- Add fuzz/README.md with usage instructions
- Update main README with fuzz testing section

The fuzz tests run automatically on every PR to catch parsing vulnerabilities,
edge cases, and potential crashes before they reach production.

Author: reyhankoyun

.github/workflows/fuzz.yml

@@ -0,0 +1,43 @@
+name: Fuzz Tests
+
+on:
+  pull_request:
+    branches: ["main"]
+  workflow_dispatch:
+
+env:
+  CARGO_TERM_COLOR: always
+
+permissions:
+  contents: read
+
+jobs:
+  fuzz:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      
+      - name: Install Rust nightly
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+      
+      - name: Install cargo-fuzz
+        run: cargo install cargo-fuzz
+      
+      - name: Run fuzz_query_parser (2 min)
+        run: cargo +nightly fuzz run fuzz_query_parser -- -max_total_time=120
+        continue-on-error: false
+      
+      - name: Run fuzz_token_validator (2 min)
+        run: cargo +nightly fuzz run fuzz_token_validator -- -max_total_time=120
+        continue-on-error: false
+      
+      - name: Upload crash artifacts
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: fuzz-crashes-${{ github.sha }}
+          path: fuzz/artifacts/
+          retention-days: 30

Cargo.toml

@@ -2,3 +2,4 @@
 resolver = "2"
 
 members = ["aws_secretsmanager_agent", "aws_secretsmanager_caching", "integration-tests"]
+exclude = ["fuzz"]

README.md

@@ -489,6 +489,12 @@ For an agent architecture, the domain of trust is where the agent endpoint and S
 
 Security conscious applications that are not already using an agent solution with the Secrets Manager credentials locked down to the application should consider using the language\-specific AWS SDKs or caching solutions\. For more information, see [Get secrets](https://docs.aws.amazon.com/secretsmanager/latest/userguide/retrieving-secrets.html)\.
 
+## Fuzz Testing<a name="fuzz-testing"></a>
+
+The AWS Secrets Manager Agent includes fuzz tests to discover security vulnerabilities and edge cases by feeding malformed inputs to critical components. Fuzz tests run automatically on every pull request.
+
+For detailed instructions on running fuzz tests locally, reproducing crashes, and managing the corpus, see [fuzz/README.md](fuzz/README.md).
+
 ## Running Integration Tests Locally<a name="integration-tests-local"></a>
 
 The AWS Secrets Manager Agent includes a comprehensive integration test suite that validates functionality against real AWS Secrets Manager. These tests cover caching behavior, security features, configuration options, version management, and error handling scenarios.

aws_secretsmanager_agent/src/error.rs

@@ -1,5 +1,5 @@
 #[derive(Debug)]
-pub(crate) struct HttpError(pub u16, pub String);
+pub struct HttpError(pub u16, pub String);
 
 impl From<url::ParseError> for HttpError {
     fn from(e: url::ParseError) -> Self {

aws_secretsmanager_agent/src/lib.rs

@@ -0,0 +1,15 @@
+// Library interface for the AWS Secrets Manager Agent
+// This exposes internal modules for testing and fuzzing purposes
+
+pub mod cache_manager;
+pub mod config;
+pub mod constants;
+pub mod error;
+pub mod logging;
+pub mod parse;
+pub mod server;
+pub mod utils;
+
+// Re-export key types and functions for fuzzing
+pub use parse::GSVQuery;
+pub use utils::get_token;

aws_secretsmanager_agent/src/parse.rs

@@ -5,7 +5,7 @@ use url::Url;
 use crate::error::HttpError;
 
 #[derive(Debug)]
-pub(crate) struct GSVQuery {
+pub struct GSVQuery {
     pub secret_id: String,
     pub version_id: Option<String>,
     pub version_stage: Option<String>,
@@ -23,7 +23,7 @@ impl GSVQuery {
         }
     }
 
-    pub(crate) fn try_from_query(s: &str) -> Result<Self, HttpError> {
+    pub fn try_from_query(s: &str) -> Result<Self, HttpError> {
         // url library can only parse complete URIs. The host/port/scheme used is irrelevant since it is not used
         let complete_uri = format!("http://localhost{}", s);
 
@@ -53,7 +53,7 @@ impl GSVQuery {
         Ok(query)
     }
 
-    pub(crate) fn try_from_path_query(s: &str, path_prefix: &str) -> Result<Self, HttpError> {
+    pub fn try_from_path_query(s: &str, path_prefix: &str) -> Result<Self, HttpError> {
         // url library can only parse complete URIs. The host/port/scheme used is irrelevant since it gets stripped
         let complete_uri = format!("http://localhost{}", s);
 

aws_secretsmanager_agent/src/utils.rs

@@ -59,7 +59,6 @@ pub fn err_response(err_code: &str, msg: &str) -> String {
 ///
 /// * `Ok(String)` - The SSRF token value.
 /// * `Err(Error)` - Error indicating that the variable is not set or could not be read.
-#[doc(hidden)]
 pub fn get_token(config: &Config) -> Result<String, Box<dyn std::error::Error>> {
     // Iterate through the env name list looking for the first variable set
     #[allow(clippy::redundant_closure)]

fuzz/.gitignore

@@ -0,0 +1,6 @@
+target
+artifacts
+coverage
+
+# Ignore generated corpus files (40-char SHA-1 hashes, keep seed corpus)
+corpus/*/[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]