BusyBox TLS Montgomery Reduction Heap Buffer Overflow

Executive Summary

CVE-2026-88830 is a unit confusion vulnerability in BusyBox's TLS implementation. It occurs during Montgomery reduction buffer allocation where the code incorrectly treats a digit count as a byte count. This causes memory to be allocated 4 to 8 times smaller than required, leading to a pre-authentication heap buffer overflow when processing a crafted ClientKeyExchange message.

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Detection Guidance

Detecting this vulnerability requires checking if your BusyBox TLS implementation is vulnerable. Inspect the BusyBox version and configuration to see if the ssl_server applet is enabled. Monitor network traffic for malformed TLS ClientKeyExchange messages, particularly those with all-zeros payloads. Use tools like tcpdump or Wireshark to capture and analyze TLS handshake packets for anomalies.

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Impact Analysis

The primary impact is a denial of service where the BusyBox TLS server crashes. Remote code execution is theoretically possible but unlikely due to system mitigations like ASLR, PIE, full RELRO, and SELinux. Attackers can exploit this by sending a specially crafted ClientKeyExchange message with an all-zeros payload.

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Mitigation Strategies

Disable the BusyBox ssl_server applet if it is enabled in your environment. Replace it with a well-maintained TLS implementation like OpenSSL or GnuTLS. Ensure system-level mitigations such as ASLR, PIE, full RELRO, and SELinux are enabled. Regularly update BusyBox and related packages to the latest versions.

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