CVE-2021-4044 in OpenSSL
Summary
by MITRE • 12/14/2021
Internally libssl in OpenSSL calls X509_verify_cert() on the client side to verify a certificate supplied by a server. That function may return a negative return value to indicate an internal error (for example out of memory). Such a negative return value is mishandled by OpenSSL and will cause an IO function (such as SSL_connect() or SSL_do_handshake()) to not indicate success and a subsequent call to SSL_get_error() to return the value SSL_ERROR_WANT_RETRY_VERIFY. This return value is only supposed to be returned by OpenSSL if the application has previously called SSL_CTX_set_cert_verify_callback(). Since most applications do not do this the SSL_ERROR_WANT_RETRY_VERIFY return value from SSL_get_error() will be totally unexpected and applications may not behave correctly as a result. The exact behaviour will depend on the application but it could result in crashes, infinite loops or other similar incorrect responses. This issue is made more serious in combination with a separate bug in OpenSSL 3.0 that will cause X509_verify_cert() to indicate an internal error when processing a certificate chain. This will occur where a certificate does not include the Subject Alternative Name extension but where a Certificate Authority has enforced name constraints. This issue can occur even with valid chains. By combining the two issues an attacker could induce incorrect, application dependent behaviour. Fixed in OpenSSL 3.0.1 (Affected 3.0.0).
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Analysis
by VulDB Data Team • 11/26/2024
The vulnerability described in CVE-2021-4044 represents a critical flaw in OpenSSL's certificate verification process that stems from improper error handling within the X509_verify_cert() function. This issue specifically affects OpenSSL version 3.0.0 and demonstrates how internal error propagation can lead to unpredictable application behavior. The root cause lies in the mishandling of negative return values from X509_verify_cert() during client-side certificate validation, where OpenSSL fails to properly process these error conditions, resulting in incorrect SSL_ERROR_WANT_RETRY_VERIFY return codes.
The technical implementation of this vulnerability occurs when OpenSSL's SSL_connect() or SSL_do_handshake() functions encounter internal errors during certificate verification. When X509_verify_cert() returns a negative value indicating an internal error such as memory exhaustion, OpenSSL's error handling mechanism fails to correctly interpret this condition. Instead of properly managing the error state, the system returns SSL_ERROR_WANT_RETRY_VERIFY, which is specifically designed for use with custom certificate verification callbacks through SSL_CTX_set_cert_verify_callback(). This mismatch creates a scenario where applications receive unexpected error codes that they cannot properly handle, leading to undefined behavior patterns.
The operational impact of this vulnerability extends beyond simple error handling failures to potentially cause application crashes, infinite loops, or other critical malfunctions. This behavior directly violates the principle of predictable error handling that security applications must maintain, as described in CWE-248, which addresses "Uncaught Exception." The vulnerability becomes particularly dangerous when combined with a separate issue in OpenSSL 3.0 where X509_verify_cert() incorrectly reports internal errors when processing certificate chains that lack Subject Alternative Name extensions but are subject to name constraints enforced by Certificate Authorities. This combination creates a scenario where legitimate certificate chains can trigger the flawed error handling mechanism, making the vulnerability exploitable even with valid certificates.
From an attack perspective, this vulnerability aligns with techniques described in the MITRE ATT&CK framework under T1059 Command and Scripting Interpreter and T1211 Lateral Tool Transfer, as it can be leveraged to cause application instability and potentially facilitate further exploitation. The issue affects applications that rely on OpenSSL for secure communication, making it particularly concerning for web servers, email clients, and any software implementing TLS/SSL connections. The vulnerability's exploitation potential is amplified because it can be triggered without requiring malicious certificate content, simply by crafting certificate chains that exercise the specific error conditions within OpenSSL's certificate validation logic. This makes it a significant concern for systems that process certificates from untrusted sources or that may encounter certificates with specific extension configurations that trigger the underlying error condition. The fix implemented in OpenSSL 3.0.1 addresses the core issue by properly handling negative return values from X509_verify_cert() and ensuring that SSL_ERROR_WANT_RETRY_VERIFY is only returned in appropriate contexts, thereby restoring expected application behavior and eliminating the potential for crashes or infinite loops.