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fix: use length-check + ct_eq for constant-time auth comparison #153

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Merged
jamesadevine merged 1 commit into from
Apr 11, 2026
Merged

fix: use length-check + ct_eq for constant-time auth comparison #153

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12 changes: 5 additions & 7 deletions src/mcp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -943,19 +943,17 @@ pub async fn run_http(
}

// Constant-time comparison to prevent timing side-channels.
// Pad to equal length so ct_eq compares all bytes even when lengths differ.
// Length check is non-constant-time but leaking length doesn't
// help brute-force a high-entropy token.
if let Some(auth) = req.headers().get("authorization") {
if let Ok(auth_str) = auth.to_str() {
let expected_header = format!("Bearer {}", expected);
use subtle::ConstantTimeEq;
let expected_bytes = expected_header.as_bytes();
let provided_bytes = auth_str.as_bytes();
let len = expected_bytes.len().max(provided_bytes.len());
let mut e = vec![0u8; len];
let mut p = vec![0u8; len];
e[..expected_bytes.len()].copy_from_slice(expected_bytes);
p[..provided_bytes.len()].copy_from_slice(provided_bytes);
if e.ct_eq(&p).into() {
if expected_bytes.len() == provided_bytes.len()
&& expected_bytes.ct_eq(provided_bytes).into()
{
return next.run(req).await;
}
}
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