GnuTLS versions prior to 2.8.2 contain a vulnerability that could allow an unauthenticated, remote attacker to bypass security restrictions and launch spoofing attacks.
The vulnerability is due to an implementation error in the affected software when performing domain name validation for a signed certificate. An unauthenticated, remote attacker could exploit this vulnerability through a crafted X.509 digital certificate. GnuTLS would validate such certificates and consider the certificates trustworthy. This exploit could allow the attacker to bypass security restrictions and conduct man-in-the-middle attacks.
The vendor has confirmed the vulnerability and released updated software.
Indicators of Compromise
GnuTLS versions prior to 2.8.2 are vulnerable.
The vulnerability is due to improper processing of NULL byte characters within X.509 certificate Common Names (CN) or Subject Alternative Names (SAN) in the domain name field. When processing certificate domain names, the vulnerable library, libgnutls, may incorrectly interpret a domain name that contains a NULL byte. The application would then incorrectly consider the certificate origin as the character string preceding the NULL byte. This error could allow an attacker to impersonate another host through a crafted certificate that contains a NULL byte in the domain name followed by a malicious domain name.
An unauthenticated, remote attacker could exploit this vulnerability by constructing a malicious certificate and convincing a user or application to access a site that uses the certificate. Additionally, the attacker could use the certificate to conduct man-in-the-middle attacks and defeat the protection mechanism of X.509 certificates.
An attacker may rely on user interaction to exploit this vulnerability. The attacker could convince a user to visit a malicious website using a certificate that may be displayed as valid by sending the user an e-mail message that contains a link to the malicious site.
The attacker could also leverage the vulnerability to conduct a man-in-the-middle attack. If the attacker can monitor an ongoing SSL session, the attacker could substitute a malicious certificate with a malformed name that the client may accept as valid. A successful exploit could allow the attacker to monitor ongoing communications, possibly allowing the attacker to capture sensitive information.
Administrators are advised to apply the appropriate updates.
Users are advised not to visit websites or follow links that have suspicious characteristics or cannot be verified as safe.
Users are advised not to open e-mail messages from suspicious or unrecognized sources. If users cannot verify that links or attachments included in e-mail messages are safe, they are advised not to open them.
Administrators are advised to monitor affected systems.
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Cisco Multivendor Vulnerability Alerts respond to vulnerabilities identified in third-party vendors' products. These alerts contain information compiled from diverse sources and provide comprehensive technical descriptions, objective analytical assessments, workarounds and practical safeguards, and links to vendor advisories and patches.