CVE-2015-1788 in OpenSSL
Summary
by MITRE
The BN_GF2m_mod_inv function in crypto/bn/bn_gf2m.c in OpenSSL before 0.9.8s, 1.0.0 before 1.0.0e, 1.0.1 before 1.0.1n, and 1.0.2 before 1.0.2b does not properly handle ECParameters structures in which the curve is over a malformed binary polynomial field, which allows remote attackers to cause a denial of service (infinite loop) via a session that uses an Elliptic Curve algorithm, as demonstrated by an attack against a server that supports client authentication.
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Analysis
by VulDB Data Team • 05/21/2022
The vulnerability identified as CVE-2015-1788 represents a critical denial of service flaw within OpenSSL's implementation of elliptic curve cryptography operations. This issue specifically affects the BN_GF2m_mod_inv function located in the crypto/bn/bn_gf2m.c file, which handles modular inversion operations over binary polynomial fields. The flaw manifests when processing ECParameters structures containing curves defined over malformed binary polynomial fields, creating a condition where the cryptographic library enters an infinite loop during session establishment. This vulnerability impacts multiple OpenSSL versions including 0.9.8 series before 0.9.8s, 1.0.0 series before 1.0.0e, 1.0.1 series before 1.0.1n, and 1.0.2 series before 1.0.2b, making it particularly widespread across the cryptographic library ecosystem.
The technical root cause of this vulnerability stems from insufficient validation of elliptic curve parameters during the initialization phase of cryptographic operations. When a client presents an Elliptic Curve certificate or parameter structure containing malformed binary polynomial field specifications, the BN_GF2m_mod_inv function fails to properly validate the input parameters before attempting computationally intensive modular inversion operations. This lack of input sanitization creates a path where maliciously crafted ECParameters can cause the function to enter an infinite loop, consuming system resources and preventing legitimate cryptographic operations from completing. The vulnerability operates at the mathematical level of finite field operations, specifically targeting the binary extension field arithmetic used in elliptic curve cryptography over GF(2^m) fields.
From an operational perspective, this vulnerability poses significant risks to systems relying on OpenSSL for secure communications, particularly those implementing client authentication mechanisms. Attackers can exploit this flaw by establishing a TLS session with a specially crafted elliptic curve parameter structure that triggers the infinite loop condition. The impact extends beyond simple service disruption as the infinite loop consumes CPU cycles and memory resources, potentially leading to system instability or resource exhaustion. This type of denial of service attack is particularly dangerous in high-availability environments where maintaining service uptime is critical, as the attack can be executed remotely without requiring authentication or privileged access to the target system. The vulnerability aligns with CWE-835, which addresses infinite loops or iterations, and demonstrates how improper input validation can lead to resource exhaustion attacks.
The mitigation strategy for CVE-2015-1788 requires immediate patching of affected OpenSSL versions to the corrected releases that implement proper validation of ECParameters structures before processing elliptic curve operations. System administrators should prioritize updating their OpenSSL installations across all production environments, particularly those hosting web servers, email servers, and other services that support client certificate authentication. Additionally, network monitoring should be enhanced to detect unusual patterns of resource consumption that may indicate exploitation attempts. Organizations should also implement defensive measures such as rate limiting and connection timeout configurations to minimize the impact of potential attacks. The vulnerability's classification under the ATT&CK framework would place it within the Privilege Escalation and Resource Exhaustion domains, as it allows attackers to consume system resources without requiring elevated privileges while potentially disrupting service availability. Regular security assessments and vulnerability scanning should be conducted to ensure that all OpenSSL installations remain current with security patches and that proper input validation mechanisms are in place to prevent similar issues in other cryptographic libraries or applications.