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Fix CAPTCHA hostname verification when providers return parent domains #5376

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Fix CAPTCHA hostname verification when providers return parent domains #5376

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35b86a1
Implement hostname normalization and matching for CAPTCHA responses
imnotevelyn Dec 26, 2025
0a13b33
Format crates/handlers/src/captcha.rs
imnotevelyn Dec 26, 2025
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55 changes: 54 additions & 1 deletion crates/handlers/src/captcha.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,29 @@ pub enum Error {
RequestFailed(#[from] reqwest::Error),
}

fn normalize_hostname(hostname: &str) -> String {
hostname.trim_end_matches('.').to_ascii_lowercase()
}

/// Checks whether the CAPTCHA response hostname matches the site hostname.
///
/// Some CAPTCHA providers (notably hCaptcha) may return only the registrable
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@reivilibre reivilibre Jan 21, 2026

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do you maybe have any documentation reference from hCaptcha about this behaviour?

/// domain in the verification response even if the site is hosted on a
/// subdomain. In that case, the response hostname is a parent domain of the
/// site hostname and should be accepted.
fn hostname_matches_site(site_hostname: &str, response_hostname: &str) -> bool {
let site = normalize_hostname(site_hostname);
let response = normalize_hostname(response_hostname);

if site == response {
return true;
}

// Accept the case where the response contains the parent domain of the site,
// e.g. site: "mauth.example.com"; response: "example.com".
site.strip_suffix(&format!(".{response}")).is_some()
}

#[allow(clippy::struct_field_names)]
#[derive(Debug, Deserialize, Default)]
#[serde(rename_all = "kebab-case")]
Expand Down Expand Up @@ -245,7 +268,7 @@ impl Form {
span.record("captcha.hostname", &hostname);
span.record("captcha.challenge_ts", &challenge_ts);

if hostname != site_hostname {
if !hostname_matches_site(site_hostname, &hostname) {
return Err(Error::HostnameMismatch {
expected: site_hostname.to_owned(),
got: hostname,
Expand All @@ -255,3 +278,33 @@ impl Form {
Ok(())
}
}

#[cfg(test)]
mod tests {
use super::hostname_matches_site;

#[test]
fn hostname_matches_exact() {
assert!(hostname_matches_site(
"mauth.example.com",
"mauth.example.com"
));
}

#[test]
fn hostname_matches_parent_domain() {
assert!(hostname_matches_site("mauth.example.com", "example.com"));
}

#[test]
fn hostname_does_not_match_similar_suffix() {
assert!(!hostname_matches_site("badexample.com", "example.com"));
assert!(!hostname_matches_site("mauth.example.com", "example.com"));
}

#[test]
fn hostname_matching_is_case_insensitive_and_allows_trailing_dot() {
assert!(hostname_matches_site("MAuth.Example.Com.", "example.com"));
assert!(hostname_matches_site("mauth.example.com", "EXAMPLE.COM."));
}
}