Executive Summary

Hydra versions up to 9.7 contain a stack buffer overflow vulnerability in the NTLM authentication handler affecting multiple protocols such as SMTP, POP3, IMAP, NNTP, HTTP, HTTP-Proxy, and HTTP-Proxy-Urlenum.

The vulnerability occurs when processing a malicious NTLM Type-2 challenge that includes an excessively long domain string. This causes the base64-encoded response data to overflow a 500-byte stack buffer by 18 to 330 bytes.

This overflow can enable remote code execution on systems that do not have stack protection mechanisms in place.

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Impact Analysis

This vulnerability can allow a malicious server to execute arbitrary code remotely on a system running vulnerable versions of Hydra without stack protection.

Successful exploitation could lead to full compromise of the affected system, including unauthorized access, data manipulation, or disruption of services.

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Detection Guidance

This vulnerability involves a stack buffer overflow triggered by malicious NTLM Type-2 challenges containing excessively long domain strings during authentication across multiple protocols such as SMTP, POP3, IMAP, NNTP, HTTP, and HTTP-Proxy.

To detect this vulnerability on your network or system, you should monitor NTLM authentication traffic for unusually long or malformed NTLM Type-2 challenge messages, especially those with abnormally long domain strings.

While specific commands are not provided in the resources, general detection steps include capturing network traffic with tools like Wireshark or tcpdump and filtering for NTLM authentication packets. For example:

• Use tcpdump to capture NTLM traffic on relevant ports (e.g., SMTP port 25, IMAP port 143, HTTP port 80): tcpdump -i <interface> port 25 or port 143 or port 80 -w ntlm_capture.pcap
• Analyze the capture with Wireshark, applying a filter for NTLM Type-2 messages: ntlmssp.type == 2
• Look for NTLM Type-2 challenge messages with unusually long domain strings or base64-encoded data exceeding typical lengths (greater than 500 bytes).

Additionally, review application or system logs for error messages related to malformed NTLM authentication attempts, as the fixed version includes logging for such events.

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Mitigation Strategies

The primary mitigation step is to upgrade Hydra to a version that includes the fix implemented in commit 9cc84c2, which addresses the stack buffer overflow by adding length checks, dynamic buffer resizing, and refusal of oversized NTLM responses.

• Update Hydra to a version later than 9.7 that contains the security fix.
• If immediate upgrade is not possible, consider restricting or monitoring NTLM authentication traffic from untrusted or unknown servers to reduce exposure.
• Enable logging and alerting for malformed or suspicious NTLM authentication attempts to detect potential exploitation attempts.

These steps help prevent exploitation of the vulnerability by ensuring that malicious NTLM Type-2 challenges with oversized domain strings are properly handled or blocked.

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Compliance Impact

The provided information does not specify how the CVE-2026-56766 vulnerability affects compliance with common standards and regulations such as GDPR or HIPAA.

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