GNU SASL Memory Disclosure via NTLM Challenge

Executive Summary

CVE-2026-56968 is a vulnerability in GNU gsasl version 2.2.3 when used with the NTLM client. The issue occurs in the function _gsasl_ntlm_client_step(), which processes a challenge message from an NTLM server. If the server sends a challenge shorter than expected, the function allocates memory without initializing it properly, leaving some memory uninitialized.

This uninitialized memory is then read and included in the NTLM authentication response sent back to the server. Because the server can be attacker-controlled, this leads to a heap memory disclosure, potentially leaking sensitive data from the client's memory.

The flaw arises because the code uses malloc() instead of calloc(), and it only rejects challenges that are too long, not those that are too short. The vulnerability was independently reported and verified using MemorySanitizer.

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Impact Analysis

This vulnerability can lead to the disclosure of heap memory contents from the client system to an attacker-controlled NTLM server. An attacker exploiting this flaw can receive uninitialized memory data, which might contain sensitive information.

While the CVSS score indicates a medium severity, the impact is primarily the potential exposure of sensitive data residing in memory, which could be used for further attacks or information gathering.

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Detection Guidance

This vulnerability occurs in GNU gsasl version 2.2.3 when linked against libntlm, specifically in the NTLM client implementation. Detection involves identifying if your system is running this vulnerable version and if NTLM authentication is used.

Since the vulnerability involves memory disclosure during NTLM challenge-response exchanges, network detection can focus on monitoring NTLM authentication traffic for unusual or malformed Type-2 (challenge) messages from servers.

Suggested commands to detect the vulnerable version and monitor NTLM traffic include:

• Check GNU gsasl version installed: `gsasl --version` or check package manager info.
• Use packet capture tools like `tcpdump` or `Wireshark` to filter NTLM traffic, for example: `tcpdump -i <interface> -w ntlm_traffic.pcap 'tcp port 445 or tcp port 139'`.
• Analyze captured NTLM packets in Wireshark to inspect Type-2 challenge messages for irregular lengths or malformed data.

Additionally, memory analysis tools like MemorySanitizer were used to verify the flaw, but such tools require source-level debugging and are not typically used in production detection.

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Mitigation Strategies

The primary mitigation step is to upgrade GNU gsasl to version 2.2.4 or later, where the vulnerability has been fixed by using zero-initialized buffers and adding validation checks.

If upgrading immediately is not possible, consider disabling or avoiding the use of NTLM authentication in your environment to prevent exposure to malicious NTLM servers.

Additionally, monitor and restrict network access to trusted servers only, reducing the risk of connecting to attacker-controlled NTLM servers.

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Compliance Impact

The vulnerability in GNU SASL before 2.2.4 allows a malicious NTLM server to cause heap memory disclosure by leaking uninitialized memory contents in the NTLM response. This could potentially expose sensitive data to an attacker.

Such memory disclosure vulnerabilities can impact compliance with data protection regulations like GDPR and HIPAA, which require protection of personal and sensitive information from unauthorized access or disclosure.

However, the CVE description and resources do not explicitly discuss compliance implications or specific regulatory impacts.

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