CVE-2017-20228: Flat Assembler Buffer Overflow Vulnerability

CVE-2017-20228 Overview

CVE-2017-20228 is a stack-based buffer overflow vulnerability affecting Flat Assembler (FASM) version 1.71.21. This local code execution vulnerability allows attackers to execute arbitrary code by supplying oversized input to the application. By crafting malicious assembly input exceeding 5895 bytes, an attacker can overwrite the instruction pointer and execute return-oriented programming (ROP) chains for shell command execution.

Critical Impact

Local attackers can achieve arbitrary code execution through crafted input, potentially leading to full system compromise on affected development workstations.

Affected Products

• Flatassembler Flat Assembler version 1.71.21
• Systems using vulnerable FASM builds for assembly development
• Development environments integrating affected FASM versions

Discovery Timeline

• 2026-03-28 - CVE-2017-20228 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2017-20228

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), manifesting as a classic stack-based buffer overflow in Flat Assembler's input handling routines. When processing assembly source files, FASM fails to properly validate the length of input data before copying it to a fixed-size stack buffer. This lack of bounds checking allows an attacker to craft input that exceeds the allocated buffer space.

The overflow condition enables attackers to corrupt adjacent stack memory, including saved return addresses and other critical program state. With sufficient control over the overwritten data, attackers can redirect program execution to arbitrary memory locations, enabling return-oriented programming (ROP) chain execution for shellcode delivery.

Root Cause

The root cause of CVE-2017-20228 lies in improper input validation within the assembly source file parsing routines. Flat Assembler allocates a fixed-size buffer on the stack for processing input data but does not enforce proper boundary checks when copying user-supplied content. Input exceeding approximately 5895 bytes can overflow this buffer, enabling memory corruption attacks.

Attack Vector

This vulnerability requires local access to the target system. An attacker must be able to supply a crafted assembly source file to the FASM application. The attack vector involves creating a malicious .asm file with carefully constructed input that triggers the buffer overflow condition. When FASM attempts to assemble this file, the oversized input overwrites the instruction pointer, allowing the attacker to redirect execution flow.

The local attack vector means this vulnerability is primarily a concern for development environments where FASM is used to compile untrusted or externally-sourced assembly code. Successful exploitation can lead to arbitrary code execution with the privileges of the user running FASM.

Detection Methods for CVE-2017-20228

Indicators of Compromise

• Unusually large assembly source files (exceeding 5895 bytes of specific input patterns)
• Unexpected crashes or segmentation faults when running FASM
• Process execution anomalies originating from FASM processes

Detection Strategies

• Monitor FASM process behavior for signs of memory corruption or unexpected child process spawning
• Implement file integrity monitoring on development systems to detect suspicious assembly files
• Use application whitelisting to control what FASM can execute post-compilation

Monitoring Recommendations

• Enable crash dump collection for FASM to capture exploitation attempts
• Monitor system call patterns from FASM processes for anomalous shell execution
• Implement endpoint detection and response (EDR) solutions to detect ROP chain execution patterns

How to Mitigate CVE-2017-20228

Immediate Actions Required

• Avoid processing untrusted assembly source files with affected FASM versions
• Isolate FASM execution environments using sandboxing or containerization
• Upgrade to a patched version of Flat Assembler if available from the Flat Assembler Official Site

Patch Information

Users should check the Flat Assembler Official Site for updated versions that address this vulnerability. Additional technical details about the vulnerability and exploitation techniques can be found in Exploit-DB #42265 and the VulnCheck Security Advisory.

Workarounds

• Run FASM in a sandboxed environment with restricted permissions
• Implement input validation to reject assembly files exceeding expected size thresholds
• Use memory protection mechanisms like ASLR and DEP/NX to increase exploitation difficulty

# Example: Running FASM in a restricted sandbox using firejail
firejail --private --net=none --caps.drop=all fasm input.asm output.bin

# Limit input file size before processing
MAX_SIZE=5000
if [ $(stat -c%s "input.asm") -gt $MAX_SIZE ]; then
    echo "Input file exceeds safe size limit"
    exit 1
fi