How to fix XSS in API Responses
in Poem
Executive Summary
Poem is a high-performance Rust web framework, but like any web toolkit, it's susceptible to XSS if you're reflecting unsanitized input into HTML responses. The core issue arises when handlers return raw strings or manually constructed HTML templates without proper contextual encoding. If an attacker can inject a script tag into a parameter that gets rendered in the response body, you've got a reflected XSS vulnerability.
The Vulnerable Pattern
use poem::{handler, web::Query, Route};
use serde::Deserialize;
#[derive(Deserialize)]
struct Params {
name: String,
}
#[handler]
fn hello(Query(params): Query) -> String {
// VULNERABLE: Direct string interpolation into HTML without escaping.
// If name is , it executes in the browser.
format!(“
Hello, {}!
”, params.name)
}The Secure Implementation
The vulnerable example returns a raw String which, depending on headers, the browser may interpret as HTML. Even if the framework defaults to text/plain, many developers explicitly set Content-Type to text/html for custom templates. The fix involves two steps: 1) Use a library like 'html-escape' to perform entity encoding on user-controlled data (converting < to <, etc.). 2) Use Poem's 'Html' responder type to ensure the 'Content-Type' header is correctly set to 'text/html; charset=utf-8'. For larger projects, use a templating engine like Askama or Maud which provides auto-escaping by default.
use poem::{handler, web::Query, web::Html};
use serde::Deserialize;
use html_escape::encode_safe;
#[derive(Deserialize)]
struct Params {
name: String,
}
#[handler]
fn hello(Query(params): Query) -> Html {
// SECURE: Use html_escape crate to encode the input and wrap in Html type.
let escaped_name = encode_safe(¶ms.name);
Html(format!(“
Hello, {}!
”, escaped_name))
}
Your API Responses API
might be exposed to XSS
74% of API Responses apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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