Deserialization Attack in Axum
How Deserialization Attack Manifests in Axum
Deserialization attacks in Axum applications typically arise when user-controlled input is passed directly to unsafe deserialization functions without proper validation. Axum's async, type-safe design does not eliminate risk if developers use crates like serde_json or bincode on untrusted data in request handlers. A common pattern involves extracting raw bytes from axum::extract::Json or axum::extract::Bytes and feeding them into serde_json::from_slice or similar without restricting types. For example, an endpoint accepting JSON payloads might inadvertently deserialize into a type that implements Deserialize with dangerous side effects, such as invoking std::process::Command via a custom Deserialize impl (though rare in practice) or more commonly, enabling object injection that leads to remote code execution when combined with gadget chains in dependencies.
In Axum, this often appears in middleware or route handlers where Extension or State is used to pass deserialized data across layers. If the deserialization occurs early in the request pipeline (e.g., in a custom extractor) and the resulting type is trusted without validation, an attacker can craft payloads that exploit known gadget chains in widely used crates like serde_json (though serde_json itself is generally safe, misuse with Value or custom types can still lead to issues). Real-world parallels include CVE-2020-25613 in actix-web (similar async framework) where improper deserialization led to RCE, and similar risks exist in Axum if developers mirror unsafe patterns from lower-level frameworks.
Attackers probe for deserialization endpoints by sending serialized payloads (e.g., JSON with __proto__ or constructor properties in JavaScript-like contexts, though Axum is Rust-based, the risk shifts to type confusion or unintended enum variant instantiation). The impact ranges from data tampering (e.g., elevating privileges by flipping a boolean field) to potential RCE if the application uses deserialization in contexts involving dynamic code loading or plugin systems—though such patterns are less common in idiomatic Axum code.
Axum-Specific Detection
Identifying deserialization flaws in Axum requires analyzing request handlers for points where user input is converted from raw bytes to structured types without adequate safeguards. middleBrick detects these issues during its black-box scan by sending payloads designed to trigger unsafe deserialization behaviors, such as deeply nested objects, unexpected types, or known gadget-like structures, and observing for anomalies in response behavior, error messages, or side effects (e.g., delayed responses indicating resource consumption). Since middleBrick does not require agents or source code, it focuses on the unauthenticated attack surface: any endpoint accepting POST, PUT, or PATCH with body content is a candidate.
Specifically, middleBrick’s Input Validation and Property Authorization checks include tests for deserialization risks. For example, it may send a payload with an excessive number of nested objects to test for stack exhaustion or resource abuse, or inject unexpected field names to see if the application deserializes into unintended struct variants (if using enums with #[serde(untagged)]). It also checks for error messages that leak stack traces or validation details, which can aid attackers in refining exploits. If an Axum endpoint returns 500 errors with traces pointing to serde_json::from_str or similar when given malformed input, it indicates a lack of proper error handling—a precursor to exploitation.
Developers can self-assess by reviewing route handlers: look for uses of axum::extract::Json where T is a complex type with custom Deserialize logic, or direct calls to serde_json::from_slice on Bytes extractors. middleBrick’s report will flag such endpoints under ‘Input Validation’ with severity based on exploitability and potential impact, providing remediation guidance without needing to inspect internal code.
Axum-Specific Remediation
Mitigating deserialization risks in Axum centers on strict input validation, limiting deserialized types, and avoiding unsafe patterns. First, never deserialize untrusted data into generic types like serde_json::Value or std::collections::HashMap without subsequent validation—use strongly typed structs with explicit fields and enable #[serde(deny_unknown_fields)] to reject unexpected properties. For example:
use serde::{Deserialize, Serialize};
#[derive(Debug, Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
struct UserUpdate {
username: String,
email: String,
// age is optional; if omitted, defaults to None
age: Option,
}
async fn update_user(
axum::extract::Json(payload): axum::extract::Json,
) -> impl axum::IntoResponse {
// payload is guaranteed to have only username, email, age
// unknown fields cause deserialization to fail
axum::Json(json!({
"status": "updated",
"user": payload,
}))
} Second, avoid custom Deserialize implementations unless absolutely necessary, and if used, rigorously audit for side effects. Prefer deriving Deserialize where possible. Third, validate deserialized data post-construction: even with strict deserialization, apply business logic checks (e.g., ensuring a role field is not set to ‘admin’ unless authorized).
Fourth, use Axum’s extractors wisely: axum::extract::Json already uses serde_json internally and returns an error on failure—handle this error explicitly rather than unwrapping. Use axum::extract::rejection::JsonRejection to return 400 responses for malformed JSON, preventing error leakage. Finally, consider limiting payload size with middleware like axum::middleware::from_fn to prevent resource exhaustion via large nested objects. middleBrick’s remediation guidance will align with these practices, emphasizing type safety and error handling as core defenses in Axum applications.
Frequently Asked Questions
Does using Axum's built-in JSON extractor prevent deserialization attacks?
axum::extract::Json extractor relies on serde_json for deserialization and will return an error on malformed JSON or type mismatches, but it does not prevent unsafe deserialization if the target type permits dangerous patterns (e.g., custom Deserialize impls with side effects) or if unknown fields are allowed. Always pair it with #[serde(deny_unknown_fields)] and validate the resulting struct.