Buffer overflow in Vixie Cron library up to version 3.0 allows local users to obtain root access via a long environmental variable.
Vixie Cron, a widely used scheduling utility, is vulnerable to a buffer overflow that allows attackers to gain root privileges. Exploiting a flaw in how the cron daemon handles environmental variables, an attacker can overwrite critical memory regions, leading to complete system compromise. This vulnerability, dating back to the late 90s, remains a serious threat due to the prevalence of affected systems and the ease of exploitation.
Step 1: Payload Preparation: The attacker crafts a malicious environmental variable. This variable is designed to be significantly longer than the buffer allocated by the cron daemon for storing environmental variables. The malicious variable also contains shellcode, or it is crafted to overwrite the return address with the address of shellcode already present in memory (e.g., in the environment itself).
Step 2: Cron Job Submission: The attacker submits a cron job that utilizes the malicious environmental variable. This can be done directly by editing the crontab file or by exploiting other vulnerabilities to inject the cron job.
Step 3: Cron Daemon Execution: The cron daemon, when executing the scheduled job, attempts to load the malicious environmental variable.
Step 4: Buffer Overflow: The cron daemon copies the oversized environmental variable into a fixed-size buffer, causing a buffer overflow. This overwrites adjacent memory, including potentially the return address or other critical data.
Step 5: Shellcode Execution: When the cron job completes, the overwritten return address directs execution to the attacker's shellcode, granting root access.
The vulnerability lies within the Vixie Cron library, specifically in how it processes environmental variables passed to scheduled tasks. The cron daemon fails to properly validate the size of environmental variables before copying them into a fixed-size buffer. When a user submits a cron job with an overly long environmental variable, a buffer overflow occurs. This overwrites adjacent memory, potentially including critical data structures like the return address or function pointers. By carefully crafting the oversized environmental variable, an attacker can overwrite the return address with the address of a malicious code (shellcode) injected into the environment or other memory regions. When the cron job executes, the overwritten return address directs execution to the attacker's shellcode, granting them root access. The root cause is a lack of bounds checking on the size of environmental variables before copying them into a fixed-size buffer, leading to the buffer overflow condition.