CVE-1999-0131 in Sendmail
Summary
by MITRE
buffer overflow and denial of service in sendmail 8.7.5 and earlier through gecos field gives root access to local users.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 10/27/2025
The vulnerability described in CVE-1999-0131 represents a critical buffer overflow flaw in sendmail versions 8.7.5 and earlier that specifically targets the gecos field processing functionality. This issue occurs when sendmail processes user information from the gecos field, which typically contains user identification data including full names and other descriptive information. The buffer overflow vulnerability arises from inadequate input validation and bounds checking within the sendmail daemon's handling of this particular field, allowing attackers to exploit memory corruption through malformed gecos data. The flaw is particularly dangerous because it can be exploited by local users who have access to the system, potentially enabling privilege escalation to root level access.
The technical implementation of this vulnerability stems from the improper handling of string data within the sendmail configuration processing. When the sendmail daemon encounters a gecos field containing excessively long input data, it fails to properly validate the length of the input before copying it into fixed-size buffers. This classic buffer overflow condition allows attackers to overwrite adjacent memory locations, potentially corrupting the program's execution flow. The vulnerability is classified under CWE-121 as a stack-based buffer overflow, where the attacker can manipulate the program's control flow by overwriting return addresses and function pointers in memory. The gecos field specifically represents user information that is often populated through system user databases, making it a common entry point for exploitation.
The operational impact of this vulnerability extends beyond simple denial of service to represent a serious privilege escalation threat for local attackers. Local users who can submit crafted gecos field data can potentially execute arbitrary code with root privileges, effectively compromising the entire system. This represents a significant security risk in multi-user environments where local access might be available to untrusted users. The vulnerability allows attackers to bypass normal access controls and gain elevated system privileges, which could enable them to install backdoors, modify system files, or extract sensitive information. The denial of service aspect occurs when the buffer overflow causes the sendmail process to crash, disrupting email services and potentially providing a vector for further attacks.
Mitigation strategies for this vulnerability should include immediate patching of sendmail installations to versions that properly validate gecos field inputs and implement proper bounds checking. System administrators should also implement additional security measures such as restricting local user access to sendmail configuration files and monitoring for unusual gecos field data patterns. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the T1068 - Exploitation for Privilege Escalation tactic. Organizations should also consider implementing network segmentation to limit local access to systems running sendmail and establishing robust monitoring for suspicious buffer overflow patterns in system logs. Regular security audits should verify that all sendmail installations have been updated to patched versions and that proper input validation mechanisms are in place to prevent similar vulnerabilities from occurring in other components of the email infrastructure.