Executive Summary

CVE-2025-71263 is a buffer overflow vulnerability in the UNIX Fourth Research Edition (v4) su command. The su program, which is used to switch user identity and gain root privileges, reads the password input into a fixed-size buffer of 100 bytes without proper bounds checking. This means that if a local user inputs more than 100 characters as a password, the buffer can overflow, potentially overwriting adjacent memory.

This overflow can cause the program to crash or behave unpredictably, and it can be exploited by a local user to gain root privileges. The vulnerability arises because the password variable has a fixed size and the input reading loop does not limit the number of characters read.

The vulnerability was discovered by examining the only known UNIX v4 copy from 1973, and a patch was created by adding a counter to limit input length and prevent overflow.

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Impact Analysis

If you are running UNIX Fourth Research Edition (v4), this vulnerability allows a local user to exploit the su command to gain root privileges by overflowing the password buffer.

This means an attacker with local access could escalate their privileges to root, gaining full control over the system.

However, it is very unlikely that UNIX v4 is running in any production environment today, as it is mostly found only in a few lab or archival settings.

The impact includes potential unauthorized root access, system crashes, and unpredictable program behavior.

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Compliance Impact

I don't know

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Detection Guidance

The vulnerability exists in the UNIX Fourth Edition (v4) su command due to a buffer overflow in the password input handling. Detection involves verifying if the su binary is the vulnerable version and checking for abnormal behavior such as crashes or unexpected root privilege escalations when su is used.

Since UNIX v4 is unlikely to be running outside of very limited lab environments, detection commands would focus on inspecting the su binary and its source code for the vulnerable input handling.

Suggested commands include:

• Checking the su binary permissions and version: `ls -l /bin/su` and comparing the binary timestamp or hash against known patched versions.
• Reviewing the su source code (if available) for the presence or absence of the input length check counter variable `i` in the password reading loop.
• Monitoring system logs for crashes or abnormal terminations of the su command.
• Testing su with input longer than 100 characters in a controlled environment to observe if a buffer overflow or crash occurs.

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Mitigation Strategies

Immediate mitigation involves patching the vulnerable su program to prevent buffer overflow by adding input length checks.

Specifically, the fix introduced an integer counter variable `i` to track the number of characters read during password input, aborting input if the count reaches or exceeds 100 bytes.

After patching the source code, the su program should be recompiled using the original UNIX v4 C compiler and the new binary installed with proper setuid permissions (chmod 4755) to maintain functionality.

Additional recommended steps include:

• Restrict local user access to the system until the patch is applied, as the vulnerability allows local privilege escalation.
• Restore TTY echo mode upon error detection to maintain terminal usability after input errors.
• Audit system logs and user activity for any signs of exploitation.

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