CVE-2026-56968

Executive Summary

CVE-2026-56968 is a memory disclosure vulnerability in GNU SASL (libgsasl) versions prior to 2.2.4. The flaw resides in the NTLM client implementation, specifically in the _gsasl_ntlm_client_step function. When a malicious server sends a crafted short challenge, the client fails to properly sanitize the input, potentially leaking sensitive memory contents to the attacker. This vulnerability is classified as a low-severity information disclosure (CVSS 3.7) due to the high attack complexity and limited impact on confidentiality. No public proof-of-concept or exploit code is currently available, and the EPSS score is 0.0%, indicating a very low probability of exploitation in the wild.

Supply Chain & Package Security

This vulnerability affects the GNU SASL library, which is a core component in many Linux distributions and embedded systems. Organizations using package managers (e.g., apt, yum, dnf) should verify their installed version of libgsasl or gsasl and update to 2.2.4 or later. Supply chain risk is moderate, as the library is widely distributed and integrated into authentication workflows.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attacker must be able to send network traffic to the vulnerable client.
• Attack Complexity (AC): High (H) – The attacker must craft a specific short NTLM challenge and control a server that the client connects to.
• Privileges Required (PR): None (N) – No authentication required.
• User Interaction (UI): None (N) – The client automatically processes the challenge.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): Low (L) – Limited memory disclosure possible.
• Integrity (I): None (N) – No data modification.
• Availability (A): None (N) – No denial of service.

Threat Level: Low. Exploitation requires a man-in-the-middle or rogue server scenario, and the disclosed memory content is limited.

Deep Dive & Mechanics

The vulnerability exists in the NTLM authentication handshake. In NTLM, the server sends an 8-byte challenge to the client. The _gsasl_ntlm_client_step function in src/ntlm.c (or similar) processes this challenge. When the challenge length is shorter than expected (e.g., less than 8 bytes), the function fails to validate the length before copying it into a fixed-size buffer. This leads to a buffer over-read, where uninitialized or residual memory from the heap or stack is copied into the response, potentially leaking sensitive data.

The core issue is a missing length check:

// Vulnerable code (simplified)
void _gsasl_ntlm_client_step(gsasl_session_t sctx, const char *challenge, size_t challenge_len) {
    char buffer[8];
    // No check if challenge_len < 8
    memcpy(buffer, challenge, challenge_len); // Over-read if challenge_len < 8
    // ... use buffer
}

In the patched version (2.2.4), the code validates the challenge length:

// Patched code (simplified)
void _gsasl_ntlm_client_step(gsasl_session_t sctx, const char *challenge, size_t challenge_len) {
    char buffer[8] = {0};
    if (challenge_len < 8) {
        // Handle short challenge (e.g., pad with zeros or return error)
        memcpy(buffer, challenge, challenge_len);
        // Remaining bytes stay zeroed
    } else {
        memcpy(buffer, challenge, 8);
    }
    // ... use buffer
}

Vulnerable vs. Patched Code

Vulnerable (pre-2.2.4):

// No length validation
memcpy(buffer, challenge, challenge_len);

Patched (2.2.4+):

// Length validation added
if (challenge_len < 8) {
    memset(buffer, 0, 8);
    memcpy(buffer, challenge, challenge_len);
} else {
    memcpy(buffer, challenge, 8);
}

Exploitation & Lab Setup

Red Team

No public exploit code is available. To simulate the attack:

1. Set up a rogue NTLM server that sends a challenge of length 1-7 bytes.
2. Use a tool like scapy or a custom Python script to craft the NTLM challenge packet.
3. The vulnerable client will process the short challenge and include heap/stack memory in its response.
4. Capture the response and extract leaked data.

Example Python script (conceptual):

import socket

def rogue_ntlm_server():
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind(('0.0.0.0', 445))
    server.listen(1)
    conn, addr = server.accept()
    # Send NTLM challenge with 4 bytes instead of 8
    challenge = b'\x01\x02\x03\x04'  # Short challenge
    conn.send(challenge)
    response = conn.recv(1024)
    print("Leaked data:", response.hex())
    conn.close()

Blue Team

• Monitor for NTLM authentication attempts with unusually short challenges (length < 8 bytes).
• Use network intrusion detection systems (NIDS) to flag NTLM challenge packets with invalid lengths.
• Ensure all systems are updated to GNU SASL 2.2.4 or later.

Indicators of Compromise (IOCs)

• Network: NTLM challenge packets with length field < 8 bytes.
• Host: Unusual memory patterns in NTLM responses (e.g., non-random data).
• Logs: Authentication failures or anomalies in NTLM handshake logs.

Detection Engineering

YARA Rule

rule CVE_2026_56968_NTLM_Short_Challenge {
    meta:
        description = "Detects NTLM challenge packets with length less than 8 bytes"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-56968"
    strings:
        $ntlm_header = "NTLMSSP"
        $short_challenge = { 00 00 00 00 00 00 00 00 } // Placeholder for short length
    condition:
        $ntlm_header at 0 and #short_challenge < 8
}

Sigma Rule

title: NTLM Short Challenge Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects NTLM authentication attempts with challenge length less than 8 bytes
author: Valters IT
logsource:
    category: network
    product: suricata
detection:
    selection:
        app_protocol: ntlm
        challenge_length|lt: 8
    condition: selection
falsepositives:
    - Legitimate NTLM implementations with non-standard challenges (rare)
level: medium

Suricata Rule

alert tcp any any -> any 445 (msg:"CVE-2026-56968 NTLM Short Challenge"; flow:to_server; content:"|4e 54 4c 4d 53 53 50|"; offset:0; depth:8; content:"|00|"; distance:8; within:1; byte_test:1,<,8,12; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade GNU SASL to version 2.2.4 or later.
2. Compensating Controls: If upgrade is not possible, restrict NTLM authentication to trusted networks only.
3. Monitoring: Enable logging for NTLM authentication and alert on anomalies.
4. Long-term: Implement input validation for all protocol fields in authentication libraries.

Valters IT
Securing Digital Frontiers