Xpath Injection in Axum with Api Keys

XPath Injection occurs when an attacker can influence the structure or evaluation of an XPath expression used to query XML or HTML documents. In Axum, a Rust web framework, this typically arises when constructing XPath strings from user-controlled input such as API keys. If an API key is embedded directly into an XPath expression without proper escaping or parameterization, an attacker can manipulate the logic of the query.

Consider a scenario where an API key is used to locate a user configuration node in an XML document. A developer might write code that concatenates the key into the path, for example:

let api_key = "user_supplied_value";
let xpath_expr = format!("//config[api_key='{}']/setting", api_key);
let nodes = document.select_nodes(&xpath_expr);

If the API key contains characters such as a single quote ('), the expression can be altered. An API key like ' or '1'='1 changes the predicate logic, potentially returning all nodes or bypassing intended authorization checks. This is analogous to SQL injection but for XPath, and it violates the principle of separating data from code.

Because Axum often handles authentication and authorization logic, an exploited XPath Injection can lead to unauthorized data access or privilege escalation. For instance, an attacker might extract sensitive configuration data, modify user permissions, or infer the structure of underlying XML stores. The risk is compounded when API keys are treated as implicit trust boundaries, assuming they are opaque and non-manipulable.

The 12 security checks in middleBrick run in parallel and specifically test for input validation issues like XPath Injection during black-box scanning. When an OpenAPI spec is provided, middleBrick cross-references definitions with runtime probes to identify whether API keys flow into dynamic XPath constructions. This helps detect whether user-controlled data reaches XML querying logic without proper sanitization or use of compiled expressions.

Remediation focuses on avoiding string concatenation for XPath construction and using language-native mechanisms that treat user input as data, not executable path segments. In Rust, prefer using dedicated XML libraries that support parameterized queries or explicit node traversal rather than raw string building.

Instead of building an XPath with format!, use a library that supports compiled expressions or explicit node filtering. For example, with the roxmltree crate, you can iterate and match safely:

use roxmltree::Document;

let doc = Document::parse(&xml_data).unwrap();
let api_key = "user_supplied_value";

let node = doc.descendants()
    .find(|n| n.has_tag_name("config") 
        && n.children().any(|c| c.has_tag_name("api_key") && c.text() == Some(api_key))
        && n.children().any(|c| c.has_tag_name("setting")));

if let Some(config) = node {
    // process config
}

This approach eliminates XPath string building entirely. The API key is compared as a discrete string value rather than interpolated into a path expression, neutralizing injection vectors.

If your workflow depends on an XML library that supports XPath 1.0 or 2.0, use prepared expressions or binding mechanisms where available. For example, with xdg or similar crates that expose XPath evaluation, bind the API key as a parameter:

// Hypothetical binding API, library-dependent
let compiled = xpath.compile("//config[api_key=$key]/setting")?;
let result = compiled.evaluate_with_param(&[("key", api_key)])?;

In Axum handlers, ensure API keys are validated before use. Apply strict allowlists for key format, and avoid using keys in security-sensitive selectors unless they are verified to be safe. Combine this with runtime scanning using middleBrick’s CLI to verify that no XPath Injection surfaces in your deployed endpoints:

middlebrick scan https://api.example.com

For continuous assurance, the Pro plan enables scheduled scans and GitHub Action integration to fail builds if regressions appear. The MCP Server allows you to trigger checks directly from AI coding assistants, embedding security validation into development workflows.