Header Injection in Axum with Mongodb

Header injection in Axum with MongoDB occurs when user-controlled data from HTTP headers is directly used to construct MongoDB operations without validation or sanitization. Axum, a Rust web framework, encourages strongly typed handlers, but developers can still pass header values into MongoDB queries or update documents. If header values are concatenated into filter or update documents, they may alter query logic in unintended ways, potentially bypassing intended access controls or exposing more data than intended.

For example, a header such as x-user-id might be read and used to filter documents by user_id. If the header is assumed to be safe because it is not user input in the request body, developers may skip normalization or allowlist checks. In MongoDB, a query like { user_id: header_value } can be manipulated if the header contains operators such as {"$ne": ""} or {"$in": [...]}, effectively changing the scope of the query. This can lead to IDOR or unauthorized data access, which middleBrick checks under BOLA/IDOR and Property Authorization categories.

Another risk vector involves using headers to influence MongoDB update operators. An attacker-supplied header could provide keys like $set or $unset if the application merges user input into an update document without strict schema enforcement. This technique can escalate privileges by modifying roles or permissions stored in user documents, aligning with BFLA/Privilege Escalation checks. middleBrick’s Property Authorization tests validate whether header-derived inputs can modify fields that should be immutable, such as admin flags or ownership attributes.

Because Axum does not enforce a schema on runtime data, developers must enforce strict validation when headers feed into MongoDB pipelines or commands. middleBrick’s 12 security checks run in parallel, including Input Validation and Property Authorization, to detect whether headers can influence query structure, bypass authentication, or expose sensitive records. These checks reference the OWASP API Top 10 and map findings to remediation guidance, helping teams understand how to safely bind header values to strongly typed parameters instead of dynamic document construction.

To mitigate header injection in Axum with MongoDB, treat all HTTP headers as untrusted input. Use Axum extractors to parse headers into known, validated types, and avoid passing raw header strings directly into MongoDB filters or updates. Prefer using strongly typed structs and explicit field selection rather than building BSON documents from header values.

Below are concrete code examples for safe handling.

// Safe: parse and validate header before using in MongoDB filter
use axum::headers::HeaderMap;
use mongodb::bson::doc;

async fn get_user_by_header(headers: &HeaderMap, user_collection: &Collection) -> Result, Error> {
    // Extract a known header and validate format
    let user_id = match headers.get("x-user-id") {
        Some(h) => std::str::from_utf8(h.as_bytes()).map_err(|_| Error::InvalidHeader)?,
        None => return Ok(None),
    };
    // Validate format (e.g., UUID or ObjectId pattern)
    if !is_valid_user_id(user_id) {
        return Err(Error::InvalidHeader);
    }
    // Use a parameterized filter with a typed value, not concatenated strings
    let filter = doc! { "user_id": user_id };
    let result = user_collection.find_one(filter, None).await?;
    Ok(result)
}

For updates, avoid merging raw header fields into update operators. Instead, explicitly map allowed fields.

// Safe: explicit mapping, no direct header-to-operator injection
use mongodb::bson::doc;
use axum::extract::HeaderMap;

async fn update_user_role(headers: HeaderMap, update_collection: &Collection) -> Result<(), Error> {
    let user_id = headers.get("x-user-id").ok_or(Error::MissingHeader)?;
    let role = headers.get("x-role").ok_or(Error::MissingHeader)?;
    // Validate both values before use
    let user_id = validate_user_id(user_id)?;
    let role = validate_role(role)?;
    let update = doc! { "$set": { "role": role } };
    let _ = update_collection.update_one(doc! { "user_id": user_id }, update, None).await?;
    Ok(())
}

Additionally, enforce schema validation at the database layer where possible and use allowlists for header values. middleBrick’s Input Validation checks confirm whether header-derived inputs could introduce operators or malformed data into MongoDB queries. By combining runtime validation with typed bindings, you reduce the risk of header-driven query manipulation and align with secure coding practices.