CVE-2016-6298 (published 2016-09-01T23:59Z)

The _Rsa15 class in the RSA 1.5 algorithm implementation in jwa.py in jwcrypto before 0.3.2 lacks the Random Filling protection mechanism, which makes it easier for remote attackers to obtain cleartext data via a Million Message Attack (MMA).

Severity: MEDIUM
CVSS v2 base score: 4.3 (AV:N/AC:M/Au:N/C:P/I:N/A:N)
CVSS v3 base score: 5.3, MEDIUM (CVSS:3.0/AV:N/AC:H/PR:L/UI:N/S:U/C:H/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/latchset/jwcrypto
commit_details: repo_url: https://github.com/latchset/jwcrypto
author: Simo Sorce
author_date: 2016-08-29 12:23:19-04:00
msg: CVE-2016-6298: Million Messages Attack mitigation

RFC 3218 describes an oracle attack called Million Messages Attack
against RSA with PKCS1 v1.5 padding.

Depending on how JWEs are used a server may become an Oracle, and the
mitigation presecribed in RFC 3218 2.3.2 need to be implemented.

Many thanks to Dennis Detering for his responsible disclosure and help
verifying the mitigation approach.

Resolves #65
Signed-off-by: Simo Sorce <simo@redhat.com>
Closes #66
repository: repo_url: https://github.com/latchset/jwcrypto
repo_name: latchset/jwcrypto
description: Implements JWK,JWS,JWE specifications using python-cryptography
repo_language: Python
owner: latchset
file_changes: filename: jwa.py
change_type: ModificationType.MODIFY
diff: @@ -379,6 +379,23 @@ class _Rsa15(_RSA, JWAAlgorithm):
     def __init__(self):
         super(_Rsa15, self).__init__(padding.PKCS1v15())
 
+    def unwrap(self, key, bitsize, ek, headers):
+        self._check_key(key)
+        # Address MMA attack by implementing RFC 3218 - 2.3.2. Random Filling
+        # provides a random cek that will cause the decryption engine to
+        # run to the end, but will fail decryption later.
+
+        # always generate a random cek so we spend roughly the
+        # same time as in the exception side of the branch
+        cek = _randombits(bitsize)
+        try:
+            cek = super(_Rsa15, self).unwrap(key, bitsize, ek, headers)
+            # always raise so we always run through the exception handling
+            # code in all cases
+            raise Exception('Dummy')
+        except Exception:  # pylint: disable=broad-except
+            return cek
+
 
 class _RsaOaep(_RSA, JWAAlgorithm):
 

programming_language: Python