CVE-2017-8821 Explained : Impact and Mitigation

Tor before versions 0.2.5.16, 0.2.6 through 0.2.8.17, 0.2.9.14, 0.3.0.13, and 0.3.1.9 is vulnerable to a denial of service attack due to a flaw in handling password-protected public keys.

Understanding CVE-2017-8821

This CVE identifies a vulnerability in Tor versions that could lead to an application hang, allowing an attacker to disrupt the service.

What is CVE-2017-8821?

Prior to specific versions of Tor, a flaw exists that enables an attacker to trigger an application hang by manipulating PEM input, indicating a password-protected public key.

The Impact of CVE-2017-8821

The vulnerability allows an attacker to cause a denial of service by exploiting the OpenSSL library's request for a password, leading to an application hang.

Technical Details of CVE-2017-8821

Tor's vulnerability to a denial of service attack due to manipulated PEM input.

Vulnerability Description

Crafted PEM input signaling a password-protected public key triggers an attempt by the OpenSSL library to prompt the user for a password, resulting in an application hang.

Affected Systems and Versions

Tor versions before 0.2.5.16
Tor versions 0.2.6 through 0.2.8 before 0.2.8.17
Tor versions 0.2.9 before 0.2.9.14
Tor versions 0.3.0 before 0.3.0.13
Tor versions 0.3.1 before 0.3.1.9

Exploitation Mechanism

The attacker manipulates PEM input to indicate a password-protected public key, triggering the OpenSSL library to request a password, causing an application hang.

Mitigation and Prevention

Steps to address and prevent the CVE-2017-8821 vulnerability.

Immediate Steps to Take

Update Tor to versions 0.2.5.16, 0.2.6.17, 0.2.9.14, 0.3.0.13, or 0.3.1.9 to mitigate the vulnerability.
Monitor for any unusual application hangs or disruptions.

Long-Term Security Practices

Regularly update Tor and associated libraries to patch known vulnerabilities.
Implement network monitoring and intrusion detection systems to detect and respond to potential attacks.

Patching and Updates

Apply patches provided by Tor to address the vulnerability.
Stay informed about security advisories and updates from Tor to maintain a secure environment.