CVE-2015-8790 (published 2016-01-29T19:59Z)

The EbmlUnicodeString::UpdateFromUTF8 function in libEBML before 1.3.3 allows context-dependent attackers to obtain sensitive information from process heap memory via a crafted UTF-8 string, which triggers an invalid memory access.

Severity: MEDIUM
CVSS v2 base score: 4.3 (AV:N/AC:M/Au:N/C:P/I:N/A:N)
CVSS v3 base score: 4.3, MEDIUM (CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N)

Weakness information:
cwe_id: CWE-200
cwe_name: Exposure of Sensitive Information to an Unauthorized Actor
description: The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.
extended_description: 
	   There are many different kinds of mistakes that introduce information exposures. The severity of the error can range widely, depending on the context in which the product operates, the type of sensitive information that is revealed, and the benefits it may provide to an attacker.  Some kinds of sensitive information include:
       
         private, personal information, such as personal messages, financial data, health records, geographic location, or contact details
         system status and environment, such as the operating system and installed packages
		 business secrets and intellectual property
		 network status and configuration
		 the product's own code or internal state
         metadata, e.g. logging of connections or message headers
		 indirect information, such as a discrepancy between two internal operations that can be observed by an outsider
       
	    Information might be sensitive to different parties, each of which may have their own expectations for whether the information should be protected.  These parties include:
               
                  the product's own users
                  people or organizations whose information is created or used by the product, even if they are not direct product users
		  the product's administrators, including the admins of the system(s) and/or networks on which the product operates
                  the developer
               
	    Information exposures can occur in different ways:
               
                  the code explicitly inserts sensitive information into resources or messages that are intentionally made accessible to unauthorized actors, but should not contain the information - i.e., the information should have been "scrubbed" or "sanitized"
                  a different weakness or mistake indirectly inserts the sensitive information into resources, such as a web script error revealing the full system path of the program.
                  the code manages resources that intentionally contain sensitive information, but the resources are unintentionally made accessible to unauthorized actors. In this case, the information exposure is resultant - i.e., a different weakness enabled the access to the information in the first place.
               
	    It is common practice to describe any loss of confidentiality as an "information exposure," but this can lead to overuse of CWE-200 in CWE mapping. From the CWE perspective, loss of confidentiality is a technical impact that can arise from dozens of different weaknesses, such as insecure file permissions or out-of-bounds read.  CWE-200 and its lower-level descendants are intended to cover the mistakes that occur in behaviors that explicitly manage, store, transfer, or cleanse sensitive information.
         
         

Fix information:
repo_url: https://github.com/Matroska-Org/libebml
commit_details: repo_url: https://github.com/Matroska-Org/libebml
author: Moritz Bunkus
author_date: 2015-10-20 11:27:52+02:00
msg: EbmlUnicodeString: don't read beyond end of string

The conversion from an UTF-8 encoded string into a wchar_t one was
reading from beyond the end of the source buffer if the length indicated
by a UTF-8 character's first byte exceeds the number of bytes actually
present afterwards.

Fixes the issue reported as Cisco TALOS-CAN-0036.
repository: repo_url: https://github.com/Matroska-Org/libebml
repo_name: Matroska-Org/libebml
description: a C++ library to parse EBML files
repo_language: C++
owner: Matroska-Org
file_changes: filename: ChangeLog
change_type: ModificationType.MODIFY
diff: @@ -1,3 +1,12 @@
+2015-10-20  Moritz Bunkus  <moritz@bunkus.org>
+
+        * EbmlUnicodeString::UpdateFromUTF8(): Fixed an invalid memory
+        access. When reading from a UTF-8 string in which the length
+        indicated by a UTF-8 character's first byte exceeds the string's
+        actual number of bytes the parser would access beyond the end of
+        the string resulting in a heap information leak. Fixes the issue
+        reported as Cisco TALOS-CAN-0036.
+
 2015-10-17  Moritz Bunkus  <moritz@bunkus.org>
 
         * Released v1.3.2.

programming_language: Markdown
filename: EbmlUnicodeString.cpp
change_type: ModificationType.MODIFY
diff: @@ -47,6 +47,21 @@ START_LIBEBML_NAMESPACE
 
 // ===================== UTFstring class ===================
 
+static unsigned int UTFCharLength(uint8 lead)
+{
+  if (lead < 0x80)
+    return 1;
+  else if ((lead >> 5) == 0x6)
+    return 2;
+  else if ((lead >> 4) == 0xe)
+    return 3;
+  else if ((lead >> 3) == 0x1e)
+    return 4;
+  else
+    // Invalid size?
+    return 0;
+}
+
 UTFstring::UTFstring()
   :_Length(0)
   ,_Data(NULL)
@@ -143,39 +158,39 @@ void UTFstring::UpdateFromUTF8()
   delete [] _Data;
   // find the size of the final UCS-2 string
   size_t i;
-  for (_Length=0, i=0; i<UTF8string.length(); _Length++) {
-    uint8 lead = static_cast<uint8>(UTF8string[i]);
-    if (lead < 0x80)
-      i++;
-    else if ((lead >> 5) == 0x6)
-      i += 2;
-    else if ((lead >> 4) == 0xe)
-      i += 3;
-    else if ((lead >> 3) == 0x1e)
-      i += 4;
+  const size_t SrcLength = UTF8string.length();
+  for (_Length=0, i=0; i<SrcLength; _Length++) {
+    const unsigned int CharLength = UTFCharLength(static_cast<uint8>(UTF8string[i]));
+    if ((CharLength >= 1) && (CharLength <= 4))
+      i += CharLength;
     else
       // Invalid size?
       break;
   }
   _Data = new wchar_t[_Length+1];
   size_t j;
-  for (j=0, i=0; i<UTF8string.length(); j++) {
-    uint8 lead = static_cast<uint8>(UTF8string[i]);
-    if (lead < 0x80) {
+  for (j=0, i=0; i<SrcLength; j++) {
+    const uint8 lead              = static_cast<uint8>(UTF8string[i]);
+    const unsigned int CharLength = UTFCharLength(lead);
+    if ((CharLength < 1) || (CharLength > 4))
+      // Invalid char?
+      break;
+
+    if ((i + CharLength) > SrcLength)
+      // Guard against invalid memory access beyond the end of the
+      // source buffer.
+      break;
+
+    if (CharLength == 1)
       _Data[j] = lead;
-      i++;
-    } else if ((lead >> 5) == 0x6) {
+    else if (CharLength == 2)
       _Data[j] = ((lead & 0x1F) << 6) + (UTF8string[i+1] & 0x3F);
-      i += 2;
-    } else if ((lead >> 4) == 0xe) {
+    else if (CharLength == 3)
       _Data[j] = ((lead & 0x0F) << 12) + ((UTF8string[i+1] & 0x3F) << 6) + (UTF8string[i+2] & 0x3F);
-      i += 3;
-    } else if ((lead >> 3) == 0x1e) {
+    else if (CharLength == 4)
       _Data[j] = ((lead & 0x07) << 18) + ((UTF8string[i+1] & 0x3F) << 12) + ((UTF8string[i+2] & 0x3F) << 6) + (UTF8string[i+3] & 0x3F);
-      i += 4;
-    } else
-      // Invalid char?
-      break;
+
+    i += CharLength;
   }
   _Data[j] = 0;
 }

programming_language: C++