CVE-2016-1494 in RSA Package for Python
Summary
by MITRE
The verify function in the RSA package for Python (Python-RSA) before 3.3 allows attackers to spoof signatures with a small public exponent via crafted signature padding, aka a BERserk attack.
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Analysis
by VulDB Data Team • 10/14/2024
The vulnerability identified as CVE-2016-1494 resides within the Python-RSA library, specifically in the verify function implementation. This flaw represents a critical security weakness that undermines the integrity of digital signature verification processes. The vulnerability affects versions of the library prior to 3.3, making it a persistent threat in systems that have not been updated to address this specific weakness. The issue stems from improper handling of signature padding when utilizing small public exponents, creating a pathway for malicious actors to forge valid signatures without possessing the corresponding private key.
The technical root cause of this vulnerability can be traced to the BERserk attack vector, which exploits weaknesses in the RSA signature verification algorithm when small public exponents are employed. The verify function fails to properly validate the structure of signature padding, allowing attackers to craft malicious signatures that appear legitimate to the verification process. This occurs because the implementation does not adequately check for proper ASN.1 BER encoding structures that should be present in valid RSA signatures. The vulnerability specifically targets the way the library processes signature padding, enabling attackers to manipulate the padding values to create forged signatures that pass validation checks.
From an operational perspective, this vulnerability poses significant risks to systems relying on RSA-based digital signatures for authentication, data integrity, and non-repudiation. Attackers can exploit this weakness to impersonate legitimate entities, bypass authentication mechanisms, and compromise the security of applications that depend on signature verification for trust establishment. The impact extends beyond simple forgery, potentially enabling man-in-the-middle attacks, code injection scenarios, and unauthorized access to protected resources. Systems using Python-RSA for secure communications, software distribution, or digital certificate validation are particularly vulnerable to exploitation.
The security implications of CVE-2016-1494 align with CWE-327, which addresses the use of weak encryption algorithms and improper implementation of cryptographic functions. This vulnerability also maps to ATT&CK technique T1556.004, which covers credential access through the exploitation of cryptographic weaknesses in signature verification processes. Organizations implementing RSA-based security solutions must recognize that this vulnerability can be exploited to undermine the fundamental security assumptions of public key infrastructure. The attack vector is particularly concerning because it requires minimal privileges to execute and can be automated, making it attractive to both opportunistic and targeted attackers.
Mitigation strategies for this vulnerability include immediate upgrading to Python-RSA version 3.3 or later, where the verification function has been properly implemented to validate signature padding structures. Security administrators should also implement additional signature validation checks and monitor for suspicious signature patterns that might indicate exploitation attempts. Organizations should consider implementing multiple layers of security verification beyond simple signature checks, including certificate pinning, hardware security modules, and regular cryptographic audits. The fix addresses the core implementation issue by ensuring proper BER encoding validation and robust padding structure verification, preventing attackers from crafting valid signatures through manipulated padding values.