CVE-2016-9594

Vulnerability

The vulnerability resides in a new internal function introduced in libcurl 7.52.0 that is intended to return a strong 32-bit random value. Due to an implementation error, the function overwrites the pointer to the output buffer instead of writing the random value into the buffer [2]. This results in an uninitialized random variable being used. The random value is used for generating nonces in Digest and NTLM authentication, boundary strings in HTTP formposts, and similar operations. Only libcurl version 7.52.0 is affected; versions before 7.52.0 do not contain this function, and versions from 7.52.1 onward have the fix [2].

Exploitation

An attacker can exploit this vulnerability by interacting with an application that uses the affected libcurl version for Digest or NTLM authentication, or for generating formpost boundaries. Since the random value is effectively uninitialized, it may be predictable or always the same. This could allow an attacker to predict authentication nonces or boundary strings, facilitating man-in-the-middle attacks, session hijacking, or bypass of authentication mechanisms. No special privileges are required, but the attacker must be able to observe or influence the network communication [2][3].

Impact

Successful exploitation weakens the security of cryptographic operations that rely on strong randomness. An attacker may be able to predict or replay authentication nonces, leading to unauthorized access or impersonation. In the context of Digest and NTLM authentication, this could allow authentication bypass or credential theft. The impact is limited to information disclosure and potential session hijacking; direct remote code execution is not indicated for this specific vulnerability [2][3].

Mitigation

The issue is fixed in curl version 7.52.1 [2]. Users should upgrade libcurl to 7.52.1 or later. For systems where immediate upgrade is not possible, applying the patch available from the curl advisory [2] to version 7.52.0 is recommended [2][4]. No other workarounds are known. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.