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Stack-based Buffer OverflowWeakness · CWE-121

CVE-2026-56766

HIGH · 8.8 CVSS v3.1 Published 2026-06-25
Patch available
A vendor patch is available. No clean upgrade release — apply the published patch.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges Patch available 8 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Hydra through 9.7, fixed in commit 9cc84c2, contains a stack buffer overflow in NTLM authentication across SMTP, POP3, IMAP, NNTP, HTTP, HTTP-Proxy, and HTTP-Proxy-Urlenum modules when processing malicious NTLM Type-2 challenges. A malicious server can send a crafted NTLM Type-2 challenge with an excessively long domain string, causing base64-encoded response data to overflow a 500-byte stack buffer by 18 to 330 bytes, enabling remote code execution on systems without stack protection.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis
How this class of weakness works · CWE-121

Data overflows a fixed-size buffer allocated on the stack, overwriting adjacent stack memory — including saved return addresses — which is the classic route to redirecting execution into attacker-supplied code. Crafted input is all it takes. Remediation is strict length checks, safe bounded string and memory functions, and modern stack-protection mitigations.

General guidance for the stack-based buffer overflow class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Patch available Apply the vendor patch
Vendor patch github.com →
Recommended fix Moderate confidence

Upgrade to the latest thc-hydra release (version 9.8 or later) that incorporates commit 9cc84c2, or manually apply patch commit 9cc84c20e75f5fef6bb1790bb9ada2afad2204e2

  1. Check current thc-hydra version: hydra -version
  2. If running version 9.7 or earlier, upgrade to the latest version of thc-hydra from the official repository: git clone https://github.com/vanhauser-thc/thc-hydra.git
  3. Navigate to the cloned repository: cd thc-hydra
  4. Fetch the latest changes: git fetch origin
  5. Checkout the fixed commit directly for immediate patch: git checkout 9cc84c20e75f5fef6bb1790bb9ada2afad2204e2
  6. Rebuild thc-hydra: ./configure && make
  7. Install the fixed version: make install
  8. Verify the fix is applied by checking the NTLM authentication code in the source for the patched domain handling logic
Caveat Minimal risk - this is a security fix for a buffer overflow; ensure any custom login modules or authentication workflows are tested after upgrade

Generated from the published advisory — verify against the referenced sources before acting.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-56766 in production — separate from our analysis above.

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