diff --git a/control-plane/src/api/renew.ts b/control-plane/src/api/renew.ts
index 7851967..8d2c5a1 100644
--- a/control-plane/src/api/renew.ts
+++ b/control-plane/src/api/renew.ts
@@ -171,7 +171,15 @@ export function headerPresentedCert(headerName: string = DEFAULT_CLIENT_CERT_HEA
       const value = Array.isArray(raw) ? raw[0] : raw
       if (typeof value !== 'string' || value.length === 0) return null
       try {
-        return new Uint8Array(Buffer.from(value, 'base64'))
+        // The terminator may forward the verified cert as base64 DER, OR as PEM — including nginx's
+        // header-safe `$ssl_client_escaped_cert` (URL-encoded PEM). Normalize all three to raw DER:
+        // URL-decode if escaped, then strip PEM armor + whitespace to recover the base64 DER body.
+        let s = value.includes('%') ? safeDecodeUri(value) : value
+        if (s.includes('BEGIN CERTIFICATE')) {
+          s = s.replace(/-----[^-]+-----/g, '').replace(/\s+/g, '')
+        }
+        const der = Buffer.from(s, 'base64')
+        return der.length > 0 ? new Uint8Array(der) : null
       } catch {
         return null
       }
@@ -179,6 +187,15 @@ export function headerPresentedCert(headerName: string = DEFAULT_CLIENT_CERT_HEA
   }
 }
 
+/** URL-decode `value`, returning it unchanged if it is not valid percent-encoding. */
+function safeDecodeUri(value: string): string {
+  try {
+    return decodeURIComponent(value)
+  } catch {
+    return value
+  }
+}
+
 /**
  * Trust-anchor verification of the presented current cert (defense in depth: the mTLS terminator has
  * already verified it, but this route must not trust a cert the terminator would never have accepted).