fix: enforce !-free cidPrefix invariant at both concatenation sites

The cid: namespacing scheme (cid:<prefix>!<id>) is only unambiguous
because the prefix half is guaranteed free of '!': the first '!' in the
result is always the separator, so an attacker-controlled Content-ID
containing '!' only extends the id half rather than colliding with a
different prefix. That invariant previously existed only as a comment.

Add Q_ASSERT_X at both independent call sites that perform this
concatenation (CidSchemeHandler::namespacedKey and
HtmlBuilder::namespaceCids) so a future prefix generator that violates it
traps in debug builds, per Task 5's precedent of not letting one unit's
correctness depend silently on another's future behaviour. Since
Q_ASSERT compiles out in release, pin the property that actually matters
release builds too test: distinct (prefix, id) pairs across a documented
"m<index>" prefix set and hostile Content-IDs (containing '!', percent-
encoded '!', empty, leading/trailing '!') never collide, and the key
always splits at its first '!' back to the exact original prefix.

1 files changed, 20 insertions, 1 deletions

diff --git a/src/cidschemehandler.h b/src/cidschemehandler.h
index 56e6ea1..1231fc6 100644
--- a/src/cidschemehandler.h
+++ b/src/cidschemehandler.h
@@ -20,8 +20,27 @@ public:
     void setParts(const QHash<QString, InlinePart> &parts) { m_parts = parts; }
 
     /// Builds the namespaced key HtmlBuilder's rewritten URLs will request.
+    ///
+    /// The '!' separator disambiguates a hostile Content-ID from the prefix
+    /// only because prefix is guaranteed '!'-free: the FIRST '!' in the
+    /// result is always the separator, so an attacker-controlled contentId
+    /// containing '!' (even several) cannot make one message's key collide
+    /// with another's, it only extends the id half after that first '!'.
+    /// This is asserted here rather than merely documented, since two call
+    /// sites (this one and HtmlBuilder::namespaceCids) perform the same
+    /// concatenation independently and neither should trust the other to
+    /// have checked it. Q_ASSERT is compiled out in release builds; the
+    /// property that matters there (distinct pairs never collide, and the
+    /// key always splits at its first '!' back to the original prefix) is
+    /// pinned by a test instead, since it holds unconditionally regardless
+    /// of whether this assertion fires.
     static QString namespacedKey(const QString &prefix, const QString &contentId)
-    { return prefix + QLatin1Char('!') + contentId; }
+    {
+        Q_ASSERT_X(!prefix.contains(QLatin1Char('!')), "CidSchemeHandler::namespacedKey",
+                   "cidPrefix must never contain '!': it is the separator, and a "
+                   "prefix containing one would make the split ambiguous");
+        return prefix + QLatin1Char('!') + contentId;
+    }
 
     void requestStarted(QWebEngineUrlRequestJob *job) override;