CVE-2020-27840 Overview
A heap-based out-of-bounds write vulnerability was discovered in Samba's handling of domain name (DN) strings. The flaw occurs when spaces are used around a domain name in LDAP requests. While these spaces should be ignored during processing, malformed DN strings containing spaces can trigger a condition where a zero-byte is written into out-of-bounds memory. This memory corruption results in a crash of the Samba service, causing a denial of service condition.
Critical Impact
This vulnerability allows unauthenticated remote attackers to crash Samba servers by sending specially crafted LDAP requests, causing service disruption to enterprise file sharing and Active Directory domain services.
Affected Products
- Samba (multiple versions)
- Debian Linux 9.0, 10.0
- Fedora 32, 33, 34
Discovery Timeline
- 2021-05-12 - CVE-2020-27840 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-27840
Vulnerability Analysis
This vulnerability is classified as CWE-125 (Out-of-Bounds Read), though the actual impact involves an out-of-bounds write condition. The flaw resides in Samba's LDAP server component, specifically in the code responsible for parsing Distinguished Name (DN) strings used in LDAP operations.
The vulnerability occurs when the DN parser encounters strings with spaces around domain name components. The parsing logic should normalize these strings by ignoring extraneous whitespace, but a boundary condition error causes the parser to miscalculate buffer boundaries. When processing malformed input, the code writes a null byte (zero-byte) beyond the allocated memory region, corrupting heap memory and causing the Samba daemon to crash.
The attack requires no authentication and can be executed remotely over the network. An attacker needs only network access to the Samba LDAP service (typically port 389 or 636 for LDAPS) to exploit this vulnerability. While the vulnerability does not allow code execution or information disclosure, repeated exploitation can effectively deny service to legitimate users relying on Samba for file sharing or Active Directory services.
Root Cause
The root cause is improper boundary checking in the DN string parsing routines when handling whitespace characters. The code fails to properly validate buffer boundaries when normalizing DN strings that contain spaces around domain components, resulting in an out-of-bounds memory write when processing malformed input.
Attack Vector
The attack vector is network-based and requires no user interaction or authentication. An attacker can exploit this vulnerability by sending specially crafted LDAP requests containing malformed DN strings with strategic placement of space characters around domain name components. The malformed DN causes the parser to write outside allocated memory boundaries, triggering a crash.
The vulnerability can be exploited by crafting LDAP bind or search requests with DN values containing spaces that trigger the parsing flaw. For detailed technical information, refer to the Samba CVE-2020-27840 Security Notice.
Detection Methods for CVE-2020-27840
Indicators of Compromise
- Samba daemon (smbd, nmbd, or samba-ad-dc) crashes or unexpected restarts
- LDAP service unavailability or intermittent connectivity issues
- Segmentation fault entries in Samba logs associated with LDAP operations
- Unusual LDAP requests containing malformed DN strings with excessive whitespace
Detection Strategies
- Monitor Samba process stability and implement alerting for unexpected daemon restarts
- Analyze LDAP traffic for malformed DN strings containing unusual whitespace patterns
- Deploy intrusion detection rules to identify LDAP requests with suspicious DN formatting
- Review system logs for segmentation fault signals related to Samba processes
Monitoring Recommendations
- Enable verbose logging for Samba LDAP operations to capture potential exploitation attempts
- Implement network monitoring for LDAP traffic anomalies on ports 389 and 636
- Configure process monitoring to alert on Samba daemon crashes
- Establish baseline metrics for LDAP service availability to detect denial of service conditions
How to Mitigate CVE-2020-27840
Immediate Actions Required
- Update Samba to the latest patched version available for your distribution
- Apply security updates from your Linux distribution's package repository
- Review firewall rules to restrict LDAP access to trusted networks only
- Consider temporarily disabling LDAP services if immediate patching is not possible
Patch Information
Samba has released security patches to address this vulnerability. Users should update to patched versions as specified in the Samba CVE-2020-27840 Security Notice. Distribution-specific patches are available:
- Debian: DSA-4884 and Debian LTS Announcement
- Fedora: Updates available via Fedora package announcements for Fedora 32, 33, and 34
- Gentoo: GLSA 202105-22
- NetApp: Security Advisory ntap-20210326-0007
Workarounds
- Implement network segmentation to limit LDAP service exposure to trusted clients only
- Deploy a firewall or access control list to restrict access to Samba LDAP ports (389/636)
- Consider using VPN or other secure channels for LDAP access if external access is required
- Monitor for exploitation attempts while patches are being deployed
# Restrict LDAP access using iptables
# Allow LDAP only from trusted network (example: 10.0.0.0/8)
iptables -A INPUT -p tcp --dport 389 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 636 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 389 -j DROP
iptables -A INPUT -p tcp --dport 636 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
