CVE-2017-8821

Vulnerability

A denial-of-service vulnerability exists in Tor before versions 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9 [1]. The bug is in the handling of PEM input that indicates a public key requiring a password. When Tor processes such malformed input, it triggers an attempt by the OpenSSL library to read a passphrase from the terminal, causing the application to hang [1]. This issue is tracked as TROVE-2017-011 [1].

Exploitation

An attacker can send crafted PEM input to a Tor instance, for example via a malformed directory object [1]. Tor instances that are run without a terminal (the common case for most Tor packages) are not impacted [1]. Attackers need network access to deliver the crafted input; no authentication is required if the instance processes the input directly.

Impact

Successful exploitation causes a denial of service: the Tor instance pauses while OpenSSL waits for a passphrase that will never come, effectively hanging the application [1]. This disrupts Tor services relying on the affected instance, but does not lead to information disclosure or privilege escalation.

Mitigation

Fixed versions were released on 2017-12-01: 0.3.1.9, 0.3.0.13, 0.2.9.14, 0.2.8.17, and 0.2.5.16 [1]. All users should upgrade to one of these releases or to 0.3.2.6-alpha [1]. Tor instances without a terminal are not vulnerable and require no workaround [1].