CVE-2011-4354 in OpenSSL
Summary
by MITRE
crypto/bn/bn_nist.c in OpenSSL before 0.9.8h on 32-bit platforms, as used in stunnel and other products, in certain circumstances involving ECDH or ECDHE cipher suites, uses an incorrect modular reduction algorithm in its implementation of the P-256 and P-384 NIST elliptic curves, which allows remote attackers to obtain the private key of a TLS server via multiple handshake attempts.
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Analysis
by VulDB Data Team • 11/29/2021
The vulnerability described in CVE-2011-4354 represents a critical cryptographic flaw affecting OpenSSL implementations on 32-bit platforms. This issue specifically impacts the elliptic curve cryptography operations within the crypto/bn/bn_nist.c component of OpenSSL versions prior to 0.9.8h. The flaw manifests when ECDH or ECDHE cipher suites are utilized in TLS connections, creating a pathway for remote attackers to systematically extract private keys through repeated handshake attempts. The vulnerability's impact extends beyond OpenSSL itself to affect products like stunnel that rely on OpenSSL's cryptographic libraries, making it a widespread concern across numerous applications and systems.
The technical root cause of this vulnerability lies in an incorrect modular reduction algorithm implementation within the NIST elliptic curve operations. When processing P-256 and P-384 curves, the algorithm fails to properly handle the modular arithmetic required for elliptic curve point multiplication, leading to mathematical inconsistencies that can be exploited. This particular flaw is specific to 32-bit platform architectures, where the implementation's handling of large integer operations creates predictable patterns that attackers can leverage. The vulnerability operates through a side-channel attack vector where multiple handshake attempts allow an attacker to gather sufficient information to reconstruct the private key, with each failed handshake providing incremental data about the cryptographic parameters.
The operational impact of this vulnerability is severe and potentially devastating for systems relying on TLS with elliptic curve cryptography. Attackers can systematically compromise TLS server private keys without requiring direct access to the target system, making this a particularly dangerous vulnerability for web servers, email servers, and other services that depend on elliptic curve cryptography for secure communications. The attack requires multiple handshake attempts but can be automated, making it feasible for adversaries to exploit without significant technical expertise. This vulnerability directly violates the fundamental security guarantees of public key cryptography, as it allows attackers to derive private keys from public information obtained through legitimate TLS connections.
Mitigation strategies for CVE-2011-4354 primarily focus on immediate software updates and cryptographic configuration changes. Organizations must upgrade to OpenSSL 0.9.8h or later versions that contain the corrected modular reduction algorithm implementation. System administrators should also consider disabling vulnerable ECDH and ECDHE cipher suites in their configurations, particularly when using P-256 and P-384 curves. The vulnerability aligns with CWE-310, which addresses cryptographic issues related to improper implementation of cryptographic algorithms. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access and privilege escalation through cryptographic attacks, specifically targeting the compromise of private keys used in TLS implementations. Additionally, organizations should implement monitoring for unusual handshake patterns that might indicate exploitation attempts, and consider rotating affected cryptographic keys proactively as part of their incident response protocols.