BaseFortify

Executive Summary

CVE-2026-15548 is a stack-based buffer overflow vulnerability found in the Tomato firmware, specifically in the DNS list rendering function sub_407220 within the /usr/sbin/httpd file.

The vulnerable function allocates a fixed 128-byte buffer on the stack and uses unsafe sprintf operations to append DNS entries without verifying if the buffer size is sufficient.

When there are 8 DNS entries, the generated string exceeds the buffer size by 19 bytes, causing a stack overflow.

This overflow can be exploited remotely to crash the httpd management service or potentially corrupt adjacent stack memory.

This vulnerability is distinct from other Tomato vulnerabilities because it involves an accumulated sprintf stack overflow rather than issues related to system() or popen() calls.

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Impact Analysis

This vulnerability can be exploited remotely to cause a stack-based buffer overflow in the httpd management service of affected Tomato firmware devices.

The immediate impact includes crashing the httpd service, which may lead to denial of service (DoS) conditions.

More severe impacts could involve corruption of adjacent stack memory, potentially allowing an attacker to execute arbitrary code or gain elevated privileges.

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Detection Guidance

This vulnerability can be detected by monitoring the behavior of the httpd management service on affected Tomato firmware devices, particularly looking for crashes or abnormal terminations caused by stack-based buffer overflow in the DNS list rendering function.

Since the vulnerability arises from unsafe sprintf operations when rendering DNS entries, detection can involve checking the number of DNS entries configured. Specifically, having 8 or more DNS entries may trigger the overflow.

A practical detection approach is to attempt to reproduce the overflow condition by configuring 8 or more DNS entries and observing if the httpd service crashes.

Commands to check the number of DNS entries or to monitor the httpd process logs might include:

• Use router or device CLI to list DNS entries (command varies by device).
• Monitor httpd process status: `ps aux | grep httpd`
• Check system logs for httpd crashes: `dmesg | grep httpd` or `logread | grep httpd`
• Attempt to add 8 or more DNS entries and observe if the httpd service crashes or restarts.

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Mitigation Strategies

Immediate mitigation steps include avoiding the use of 8 or more DNS entries in the affected Tomato firmware to prevent triggering the stack-based buffer overflow.

Since the vulnerability affects the httpd management service, monitoring and restarting the service if it crashes can help maintain availability.

Upgrading to a firmware version that is not affected, such as switching from Shibby Tomato to the superseding FreshTomato project, is recommended for a long-term fix.

If upgrading is not immediately possible, consider limiting remote access to the httpd management interface to trusted networks or IP addresses to reduce exposure.

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