Buffer Overflow

memory HIGH

A buffer overflow occurs when a program writes more data to a memory buffer than it was allocated to hold, causing the excess data to spill into adjacent memory regions.

How It Works

A buffer overflow occurs when a program writes more data to a memory buffer than it was allocated to hold, causing the excess data to spill into adjacent memory regions. This overwrites whatever data or code exists there, corrupting program state and potentially giving attackers control over execution flow.

Stack-based overflows are the most common variant. When a function allocates a fixed-size buffer on the stack and then copies user-controlled input without proper bounds checking, attackers can overflow past the buffer to overwrite the function's return address. When the function completes, instead of returning to legitimate code, execution jumps to attacker-specified memory containing malicious shellcode. Heap-based overflows work differently—they corrupt heap metadata like chunk size fields or free list pointers, leading to arbitrary memory writes when the allocator processes the corrupted structures.

Modern exploitation bypasses defensive mechanisms through techniques like Return-Oriented Programming (ROP), which chains together existing code snippets to avoid non-executable memory protections. Attackers may also use heap spraying to reliably position shellcode at predictable addresses, defeating address randomization.

Impact

Remote code execution — attacker gains ability to run arbitrary commands with the privileges of the vulnerable process
Privilege escalation — exploiting kernel or setuid program overflows to gain root/SYSTEM access
Denial of service — crashes and memory corruption that render systems unusable
Information disclosure — reading sensitive data from adjacent memory regions that should be inaccessible
Authentication bypass — overwriting security-critical variables like permission flags or user IDs

Real-World Examples

Fortinet FortiOS suffered a critical buffer overflow (CVE-2025-32756) that allowed unauthenticated remote attackers to execute code as root on firewalls and VPN gateways. Attackers actively exploited this to compromise enterprise network perimeters before patches were available.

The Slammer worm from 2003 exploited a stack overflow in Microsoft SQL Server, spreading to 75,000 hosts in ten minutes by sending a single malformed UDP packet that overwrote the return address with shellcode. No authentication was required.

OpenSSH historically contained a heap overflow in challenge-response authentication that allowed pre-authentication remote root compromise on Unix systems, demonstrating how memory corruption in privileged network services creates maximum impact scenarios.

Mitigation

Memory-safe languages — Rust, Go, and modern managed languages prevent buffer overflows by design through automatic bounds checking
Stack canaries — random values placed before return addresses that detect corruption before control transfer
Address Space Layout Randomization (ASLR) — randomizes memory locations making exploitation less reliable
Data Execution Prevention (DEP/NX) — marks memory regions as non-executable, preventing direct shellcode execution
Bounds checking — validate input sizes before copying, use safe functions like strncpy instead of strcpy
Fuzzing and static analysis — automated testing to discover overflows before deployment

Recent CVEs (36635)

CVE-2022-25460

EPSS 2% CVSS 9.8

CRITICAL POC Act Now

Tenda AC6 v15.03.05.09_multi was discovered to contain a stack overflow via the endip parameter in the SetPptpServerCfg function. Rated critical severity (CVSS 9.8), this vulnerability is remotely exploitable, no authentication required, low attack complexity. Public exploit code available and no vendor patch available.