CVE-2021-34558 in Google
Summary
by MITRE • 07/15/2021
The crypto/tls package of Go through 1.16.5 does not properly assert that the type of public key in an X.509 certificate matches the expected type when doing a RSA based key exchange, allowing a malicious TLS server to cause a TLS client to panic.
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Analysis
by VulDB Data Team • 05/20/2025
The vulnerability identified as CVE-2021-34558 affects the crypto/tls package in Go versions through 1.16.5, representing a critical flaw in the implementation of Transport Layer Security protocols. This issue specifically targets the validation mechanisms that occur during RSA-based key exchanges within the TLS handshake process, creating a scenario where a malicious server can manipulate certificate contents to trigger unexpected behavior in compliant clients. The flaw stems from insufficient type validation during certificate processing, allowing attackers to craft certificates that contain mismatched public key types while maintaining valid cryptographic signatures. This vulnerability operates at the intersection of cryptographic protocol implementation and software robustness, where the absence of proper type assertion creates an attack surface that can be exploited to disrupt TLS communications.
The technical implementation of this vulnerability involves a specific failure in the certificate validation logic within Go's TLS package. During the TLS handshake process, when a client encounters an X.509 certificate during an RSA key exchange, the system should verify that the public key contained within the certificate matches the expected RSA key type. However, the Go implementation fails to enforce this critical type assertion, allowing a malicious server to present a certificate with an RSA key exchange mechanism but containing a public key of a different type, such as an ECDSA key. When the client processes this malformed certificate, it attempts to perform operations that are incompatible with the actual key type, resulting in a panic condition that terminates the TLS connection attempt. This represents a classic case of improper input validation and type checking within cryptographic software components.
The operational impact of CVE-2021-34558 extends beyond simple service disruption, as it can be leveraged to create denial-of-service conditions that affect any application relying on Go's standard TLS implementation. Any system using Go versions prior to 1.16.6 that engages in TLS communications with potentially malicious endpoints becomes vulnerable to this attack vector. The vulnerability is particularly concerning in environments where automated clients or services establish connections to untrusted servers, as these systems can be exploited to cause cascading failures across networked applications. The panic condition generated by this flaw means that affected applications may crash or become unresponsive, potentially affecting availability of services and creating opportunities for further exploitation. Organizations using Go-based applications in production environments face significant risk if they have not updated to patched versions, as this vulnerability can be exploited without requiring any special privileges or authentication from the attacker.
Mitigation strategies for CVE-2021-34558 center on immediate software updates to Go versions 1.16.6 and later, which contain the necessary fixes to properly validate public key types during certificate processing. Organizations should conduct comprehensive vulnerability assessments to identify all systems running affected Go versions and prioritize patching efforts accordingly. Additionally, network administrators can implement monitoring solutions to detect anomalous TLS handshake behavior that might indicate exploitation attempts, though the primary defense remains software update. The vulnerability aligns with CWE-242, which addresses "Use of Inherently Dangerous Functions" and specifically relates to improper validation of cryptographic parameters in TLS implementations. From an ATT&CK framework perspective, this vulnerability maps to T1566.001, "Phishing: Spearphishing Attachment", as attackers could potentially craft malicious certificates to exploit this weakness, and T1499.004, "Toggle Switch: Network Denial of Service", due to its potential for causing service disruption. Organizations should also consider implementing certificate pinning mechanisms as an additional defensive layer, though the most effective solution remains the immediate application of vendor-provided patches to eliminate the underlying vulnerability in the cryptographic library implementation.