From 045d169339c5b973f0924269e6ca485e48de3668 Mon Sep 17 00:00:00 2001
From: Guilhem Moulin <guilhem@fripost.org>
Date: Mon, 22 Feb 2021 20:32:33 +0100
Subject: lacme-accountd: Refuse to sign JWS with an invalid Protected Header.
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“The JWS Protected Header is a JSON object” — RFC 7515 sec. 2.

“The JWS Protected Header MUST include the following fields:
    - "alg"
    - "nonce"
    - "url"
    - either "jwk" or "kid"”
 — RFC 8555 sec. 6.2.
---
 Changelog               |  1 +
 lacme-accountd          | 13 +++++++++++++
 tests/accountd-validate | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+)
 create mode 100644 tests/accountd-validate

diff --git a/Changelog b/Changelog
index faf32a8..da34ddc 100644
--- a/Changelog
+++ b/Changelog
@@ -1,6 +1,7 @@
 lacme (0.8.1) upstream;
 
  + lacme-accountd: improve log messages.
+ + lacme-accountd: refuse to sign JWS with an invalid Protected Header.
  - lacme: in the [accountd] config, let lacme-accountd(1) do the
    %-expansion for 'config', not lacme(8) when building the command.
 
diff --git a/lacme-accountd b/lacme-accountd
index 68d0f39..5478cc2 100755
--- a/lacme-accountd
+++ b/lacme-accountd
@@ -256,6 +256,19 @@ sub conn($$$) {
         }
 
         my $req = "header=base64url($header) playload=base64url($payload)";
+
+        eval { $header = JSON::->new->decode($header); };
+        if ($@ or # couldn't decode (parse error)
+                # RFC 7515: not a JSON object
+                !defined($header) or ref($header) ne "HASH" or
+                # RFC 8555 sec. 6.2: the protected Header MUST include all these fields
+                grep !defined, @$header{qw/alg nonce url/} or
+                # RFC 8555 sec. 6.2: the protected header MUST include any of these fields
+                !grep defined, @$header{qw/jwk kid/}) {
+            info("[$id] NOSIGN [invalid JWS Protected Header] ", $req);
+            last;
+        }
+
         my $sig = $SIGN->($data) // panic();
         logmsg(noquiet => "[$id] SIGNED ", $req);
         $out->printflush( encode_base64url($sig), "\r\n" ) or warn "print: $!";
diff --git a/tests/accountd-validate b/tests/accountd-validate
new file mode 100644
index 0000000..d4be5ee
--- /dev/null
+++ b/tests/accountd-validate
@@ -0,0 +1,36 @@
+# JWS Signing Input (RFC 7515) validation
+
+# missing or empty protected header
+printf "\\r\\n" | lacme-accountd --stdio 2>"$STDERR"
+grepstderr -Fq "] NOSIGN [malformed JWS Protected Header]"
+printf ".foo\\r\\n" | lacme-accountd --stdio 2>"$STDERR"
+grepstderr -Fq "] NOSIGN [malformed JWS Protected Header]"
+
+# invalid base64url-encoded protected header
+printf "foo/bar.baz\\r\\n" | lacme-accountd --stdio 2>"$STDERR"
+grepstderr -Fq "] NOSIGN [malformed JWS Protected Header]"
+
+# missing payload
+printf "foo\\r\\n" | lacme-accountd --stdio 2>"$STDERR"
+grepstderr -Fq "] NOSIGN [malformed JWS Payload]"
+
+# invalid base64url-encoded payload
+printf "foo.bar/baz\\r\\n" | lacme-accountd --stdio 2>"$STDERR"
+grepstderr -Fq "] NOSIGN [malformed JWS Payload]"
+
+# invalid JWS Protected Header: not a JSON object; missing fields "alg",
+# "nonce", "url", or either "jwk" or "kid"
+for s in "null" "\"str\"" "{}" "{\"alg\":\"\",\"nonce\":\"\",\"url\":\"\"}" "{\"jwk\":{}}"; do
+    s="$(printf "%s" "$s" | base64 -w0 | sed "s/=*$//" | tr "+/" "-_")"
+    printf "%s.\\r\\n" "$s" | lacme-accountd --stdio 2>"$STDERR"
+    grepstderr -F "] NOSIGN [invalid JWS Protected Header]"
+done
+
+# valid JWS Protected Header and Payload
+h="{\"alg\":\"\",\"nonce\":\"\",\"url\":\"\",\"jwk\":{}}"
+s="$(printf "%s" "$h" | base64 -w0 | sed "s/=*$//" | tr "+/" "-_")"
+p="$(printf "%s" "JWS Payload" | base64 -w0 | sed "s/=*$//" | tr "+/" "-_")"
+printf "%s.%s\\r\\n" "$s" "$p" | lacme-accountd --stdio 2>"$STDERR"
+grepstderr -F "] SIGNED header=base64url($h) playload=base64url(JWS Payload)"
+
+# vim: set filetype=sh :
-- 
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