Unicode Normalization in Chi with Mutual Tls

Unicode Normalization in Chi with Mutual Tls can expose inconsistencies in how identifiers are compared after normalization, enabling bypass techniques such as Unicode normalization–based IDOR or BOLA. When a Chi endpoint relies on normalized identifiers (e.g., user IDs or resource handles) but the client certificate’s distinguished name (DN) or subjectAltName is not consistently normalized before comparison, an attacker can use different Unicode representations of the same string to traverse authorization boundaries.

Mutual Tls in Chi typically involves client certificate verification where the server authenticates the client via certificates. If the server extracts a principal (such as a CN or emailAddress field from the client certificate) and normalizes it differently than the normalized form used to look up permissions or associations in the application layer, an attacker can craft a certificate with a deliberately mismatched normalization (for example, using composed vs. decomposed Unicode forms). Because the server-side check might compare a normalized identifier against a differently normalized certificate attribute, the authorization decision may incorrectly evaluate as valid.

Consider a Chi route that resolves a user by a normalized user handle taken from the certificate’s emailAddress field. If the route does not enforce a canonical normalization form (NFC or NFD) for both the certificate input and the stored handle, an attacker with a certificate containing café in NFD (where é is U+00E9) might supply the same string in NFC (where é is U+0065 U+0301). If the comparison layer does not normalize both sides to the same form, the server may treat these as different values and inadvertently grant access to another user’s resource, mapping to BOLA/IDOR or BFLA patterns.

This interaction is especially relevant when OpenAPI/Swagger spec analysis reveals path parameters or headers that are expected to be normalized but are not explicitly validated. Runtime findings from a scan may show that the unauthenticated attack surface includes endpoints where certificate-derived identifiers are used without canonical normalization, increasing the risk of incorrect authorization decisions. The 12 security checks, including Authentication, BOLA/IDOR, and Property Authorization, are designed to surface such inconsistencies, providing prioritized findings with severity and remediation guidance.

To mitigate Unicode Normalization issues in Chi with Mutual Tls, enforce canonical normalization on all certificate-derived identifiers before comparison and ensure consistent normalization across storage, lookup, and validation layers. Below are concrete code examples for a Chi application using Common Lisp libraries to handle certificates and normalization.

First, normalize certificate fields using a Unicode library and validate against stored normalized values:

(ql:quickload '(:cl-ppcre :usocket :babel :unicode))

(defun normalize-utf8 (string)
  ;; Normalize to NFC (Canonical Composition) as the canonical form
  (unicode:nfkc string))

(defun extract-cn-from-cert (cert-pem)
  (let ((cert (sb-ssl:parse-certificate cert-pem)))
    (or (cdr (assoc :common-name (sb-ssl:certificate-subject cert))))))

(defun authenticated-handler (request)
  (let* ((client-cert (find-client-cert request))  ;; obtain cert from request
         (raw-cn (extract-cn-from-cert client-cert))
         (normalized-cn (normalize-utf8 raw-cn)))
    (if (valid-user? normalized-cn)
        (response-ok "Authorized")
        (response-forbidden "Unauthorized"))))

Second, ensure that any user or resource identifiers stored in the application are stored in the same normalized form and compared consistently:

(defun valid-user? (candidate-cn)
  (let ((stored-cn (fetch-normalized-cn-from-db)))  ;; stored in NFC
    (string= candidate-cn stored-cn :end2 (length candidate-cn))))

When integrating with the middleBrick ecosystem, use the CLI to validate your endpoints: scan from terminal with middlebrick scan <url> to detect authentication and authorization issues, including Unicode handling inconsistencies. If you require continuous oversight, the Pro plan offers continuous monitoring and can integrate with GitHub Action to fail builds if security scores drop below your configured threshold.

For teams using AI-assisted development, the MCP Server allows scanning APIs directly from your AI coding assistant, helping to surface normalization and authorization risks during development rather than in post-deployment scans.