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CVE-2006-5051: CVE-2006-5051: The Zombie Signal Handler That Ate OpenSSH

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CVE-2006-5051: The Zombie Signal Handler That Ate OpenSSH

Vulnerability ID: CVE-2006-5051
CVSS Score: 8.1
Published: 2006-09-27

A deep-dive technical analysis of the historic OpenSSH signal handler race condition. Originally patched in 2006, this vulnerability demonstrates the catastrophic risks of calling async-signal-unsafe functions like syslog() within interrupt handlers. It serves as the genetic ancestor and direct cause of the 2024 'regreSSHion' (CVE-2024-6387) vulnerability.

TL;DR

In 2006, Mark Dowd found a race condition in OpenSSH's SIGALRM handler. The handler called syslog()—which uses malloc()—creating a reentrancy bug if the signal interrupted the main thread's memory operations. This led to heap corruption and potential root RCE. It was patched, then accidentally reverted in 2020, leading to the 2024 'regreSSHion' crisis.

⚠️ Exploit Status: POC

Technical Details

  • CWE: CWE-362 (Race Condition)
  • CWE Secondary: CWE-479 (Signal Handler Use of Non-Reentrant Function)
  • Attack Vector: Network
  • CVSS v3.1: 8.1 (High)
  • Privileges Required: None
  • Impact: Remote Code Execution (Root)
  • Exploit Status: PoC Available (Complex)

Affected Systems

  • OpenSSH Server (sshd) < 4.4
  • OpenSSH Server (sshd) 8.5p1 through 9.7p1 (Regression)
  • OpenSSH: < 4.4 (Fixed in: 4.4)
  • OpenSSH: 8.5p1 - 9.7p1 (Fixed in: 9.8p1)

Exploit Details

  • Qualys Security Advisory: Detailed technical analysis of the regression and exploitation of the signal handler race condition.

Mitigation Strategies

  • Update OpenSSH binaries immediately.
  • Disable LoginGraceTime (Set to 0) as a temporary measure.
  • Restrict Port 22 access via VPN or Allow-List.

Remediation Steps:

  1. Check current version: ssh -V
  2. If version is < 4.4 (Historic) or between 8.5p1 and 9.7p1 (Modern), apply patch.
  3. Edit /etc/ssh/sshd_config.
  4. Set LoginGraceTime 0 if patching is impossible (Risk: DoS).
  5. Restart sshd service: systemctl restart sshd.

References

Read the full report for CVE-2006-5051 on our website for more details including interactive diagrams and full exploit analysis.

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