CVE-2021-23840 Vulnerability

What is CVE-2021-23840 Vulnerability?

CVE-2021-23840 is a buffer overflow vulnerability affecting OpenSSL, categorized as a memory handling issue. Specifically, it impacts the X509_aux_print() function within OpenSSL 1.1.1i and earlier versions, potentially allowing an attacker to exploit the flaw for denial-of-service (DoS) attacks. The vulnerability arises when malformed data is passed to processes handling certain certificate parsing functions. Due to its association with widely used cryptographic libraries, it poses serious risks to data security and system availability.

When was it discovered?

CVE-2021-23840 was publicly disclosed on February 16, 2021, by the OpenSSL team. The vulnerability was identified as part of an internal code review conducted by the developers. A patch was released the same day, and advisories were issued to alert organizations about the risks and potential exploits.

Affected Products & Versions

CVE-2021-23840 technical description

At its root, CVE-2021-23840 is caused by improper memory allocation in the X509_aux_print() function. When processing certificate data with specific malformed inputs, the function fails to normalize memory limits, resulting in a buffer overflow. This potentially leads to crashes or arbitrary code execution, depending on an attacker’s crafted payload. For instance, attackers could submit maliciously altered certificates during TLS handshakes, corrupting memory and impacting service availability.

Tactics, Techniques & Procedures (TTPs)

Attackers exploiting CVE-2021-23840 often leverage malformed X.509 certificates in transport layer security (TLS) protocols. The aims typically focus on overwhelming server processes to create system outages or prepare for more sophisticated data exfiltration and privilege escalation attacks.

Indicators of Compromise

Organizations should monitor abnormal server behavior, significant spikes in TLS handshake errors, and repeated system crashes during certificate validation. Additionally, unusual activity involving OpenSSL-related logs may suggest malicious manipulations attempting to exploit this vulnerability.

Known Proof-of-Concepts & exploits

While successful exploitation requires technical precision, proof-of-concept (PoC) code was released shortly after the vulnerability's disclosure. Some cybercriminal groups integrated PoCs into weaponized exploit frameworks, making it easier for threat actors to target outdated OpenSSL deployments.

How to detect CVE-2021-23840 vulnerability?

Detecting CVE-2021-23840 involves monitoring logs for events leading up to crashes during X.509 certificate handling and reviewing software versions to ensure they are not affected. SIEM detection rules can be configured to flag anomalies in OpenSSL processing functions. Host-based tools, such as endpoint detection and response (EDR) solutions, can also assist by identifying irregular memory or certificate parsing activity.

Impact & risk of CVE-2021-23840 vulnerability

If exploited, CVE-2021-23840 could significantly disrupt business operations by inducing denial-of-service (DoS) conditions. Attackers can halt critical services like secure communications and, in extreme cases, leverage the flaw to destabilize system environments or access sensitive memory structures. This not only undermines data confidentiality and availability but also increases risks of cascading vulnerabilities in interconnected environments.