Format String in Actix with Jwt Tokens
Format String in Actix with Jwt Tokens — how this specific combination creates or exposes the vulnerability
In Actix web applications, a format string vulnerability can occur when user-controlled input is passed directly into logging, error messages, or response formatting functions without proper sanitization or parameterization. When this happens in the context of JSON Web Token (Jwt) handling, the risk expands into authentication and authorization logic. For example, if an Actix handler builds log entries or HTTP responses using functions like format! or println! and incorporates values derived from Jwt claims—such as subject (sub), issuer (iss), or scopes—an attacker may be able to inject format specifiers like %s, %x, or %n into those strings. This can lead to information disclosure or, in some environments, arbitrary memory read or write depending on the underlying language runtime and logging configuration.
Consider an Actix service that decodes a Jwt using a library such as jsonwebtoken in Rust. If the developer logs the claims using a formatted string that includes unchecked user input, the application might expose sensitive details in logs or through error responses. A typical vulnerable pattern looks like this:
use actix_web::{web, HttpResponse};
use jsonwebtoken::{decode, DecodingKey, Validation};
async fn handle_token(user_input: web::Json<serde_json::Value>) -> HttpResponse {
let token = user_input["token"].as_str().unwrap_or("");
// Vulnerable: directly including token or claims in formatted output
let claims_str = format!("Processing token for: {}", token);
println!(claims_str); // format string risk if token contains format specifiers
match decode::If an attacker sends a token containing strings like %s or %x, the println! or format! call may attempt to read from the stack, potentially leaking memory contents or causing panics. In the context of Jwt processing, this can expose parts of the runtime state or error details that aid further exploitation. Because Actix routes often handle authentication and authorization based on Jwt claims, information leaks from format strings can undermine trust in the entire token validation pipeline.
Another angle involves dynamically building responses or validation messages using unchecked claim values. For instance, using user-influenced claim fields such as sub or custom data in formatted strings without explicit sanitization can expose the application to injection-style behaviors in structured outputs like JSON or HTML. Even if the runtime does not exhibit classic memory corruption, the resulting malformed logs or responses can confuse downstream systems and obscure legitimate security events.
It is important to note that the vulnerability here is not in Jwt parsing itself, but in how the application handles and formats data derived from the token. The combination of Actix web routing, Rust formatting macros, and Jwt-derived inputs creates a pathway for unintended behavior when security best practices like input validation and output encoding are omitted.
Jwt Tokens-Specific Remediation in Actix — concrete code fixes
To remediate format string risks when working with Jwt tokens in Actix, ensure all logging, error reporting, and response construction use proper formatting APIs that treat user input strictly as data, not as format instructions. In Rust, prefer using the log crate with structured logging, or at minimum ensure that any string built with format! does not directly incorporate untrusted input in the formatting position.
Below is a secure version of the earlier example, where the token and its claims are handled safely:
use actix_web::{web, HttpResponse};
use jsonwebtoken::{decode, DecodingKey, Validation};
use serde::de::IgnoredAny;
use log::{info, error};
async fn handle_token(user_input: web::Json<serde_json::Value>) -> HttpResponse {
let token = match user_input["token"].as_str() {
Some(t) => t,
None => return HttpResponse::BadRequest().body("Missing token"),
};
// Safe: structured logging avoids format string injection
info!(token_sub="unknown", "Processing Jwt");
match decode::Key practices include:
- Never pass raw, user-controlled strings directly into
println!,format!, or logging macros that interpret format syntax. - Use structured logging with the
logcrate so that log entries are built from named fields rather than interpolated strings. - Validate and sanitize any claim values before using them in any output context, including HTTP responses.
- Ensure that error messages returned to clients do not reflect raw formatting strings or internal state that could be leveraged through format injection.
For continuous protection, integrating the middleBrick CLI or GitHub Action can help detect format string patterns and other insecure coding practices during development. The middleBrick scanner runs 12 security checks in parallel on the unauthenticated attack surface and can identify risky formatting patterns alongside other API issues. Teams using the Pro plan gain continuous monitoring, so any regression involving Jwt handling or general format string risks can be caught before deployment.