CVE-2020-37124: B64dec Buffer Overflow Vulnerability

CVE-2020-37124 Overview

CVE-2020-37124 is a buffer overflow vulnerability in B64dec version 1.1.2, a base64 decoding utility. The flaw allows attackers to execute arbitrary code by overwriting the Structured Exception Handler (SEH) with crafted input. Attackers can leverage an egg hunter technique and carefully constructed payload to inject and execute malicious code during the base64 decoding process.

Critical Impact

Successful exploitation of this buffer overflow vulnerability enables arbitrary code execution through SEH overwrite, potentially allowing complete system compromise when processing maliciously crafted base64 input.

Affected Products

• B64dec 1.1.2

Discovery Timeline

• 2026-02-05 - CVE CVE-2020-37124 published to NVD
• 2026-02-05 - Last updated in NVD database

Technical Details for CVE-2020-37124

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), which occurs when the application writes data beyond the allocated buffer on the stack. In B64dec 1.1.2, the base64 decoding function fails to properly validate the length of input data before copying it into a fixed-size stack buffer.

The local attack vector requires user interaction, meaning an attacker must convince a victim to process a maliciously crafted base64-encoded file or string. When the oversized input is processed, it overwrites critical stack structures including the Structured Exception Handler (SEH) chain.

Root Cause

The root cause of this vulnerability is improper input validation during the base64 decoding process. The application allocates a fixed-size buffer on the stack to store decoded output but does not verify that the input data, when decoded, will fit within the allocated space. This lack of bounds checking allows an attacker to supply input that overflows the buffer and corrupts adjacent memory structures, including the SEH pointers stored on the stack.

Attack Vector

The attack exploits the local code execution path by requiring a user to process malicious input through B64dec. The exploitation technique involves:

1. Buffer Overflow: Supplying oversized base64-encoded data that exceeds the stack buffer allocation
2. SEH Overwrite: The overflow corrupts the Structured Exception Handler chain on the stack
3. Egg Hunter Technique: Due to limited buffer space for shellcode, attackers employ an egg hunter—a small piece of code that searches memory for a larger payload marked with a unique identifier (the "egg")
4. Code Execution: When an exception is triggered, the corrupted SEH pointer redirects execution to attacker-controlled code

The vulnerability requires no special privileges to exploit, though user interaction is necessary to trigger the vulnerable decoding operation. For detailed technical analysis, see the VulnCheck Buffer Overflow Advisory and Exploit-DB #48317.

Detection Methods for CVE-2020-37124

Indicators of Compromise

• Presence of B64dec version 1.1.2 executable on systems
• Crash dumps or application errors from b64dec.exe indicating access violations or stack corruption
• Unusual process spawning or network activity originating from the B64dec process
• Detection of egg hunter shellcode patterns (typically small loops scanning memory) in process memory

Detection Strategies

• Monitor for execution of B64dec 1.1.2 and flag for immediate review or replacement
• Implement endpoint detection rules to identify SEH overwrite patterns and egg hunter behavior
• Deploy application whitelisting to prevent execution of known vulnerable B64dec versions
• Use memory protection technologies (DEP, ASLR) to detect and prevent exploitation attempts

Monitoring Recommendations

• Enable Windows Event logging for application crashes and access violations
• Configure EDR solutions to alert on suspicious memory access patterns associated with SEH manipulation
• Monitor file system access for large or unusually formatted base64-encoded files being processed
• Implement behavioral analysis to detect post-exploitation activities following B64dec execution

How to Mitigate CVE-2020-37124

Immediate Actions Required

• Discontinue use of B64dec version 1.1.2 immediately
• Remove or quarantine the vulnerable application from all systems
• Audit systems for evidence of exploitation or compromise
• Replace B64dec with a secure alternative base64 decoding utility

Patch Information

No official patch information is available from the vendor for this vulnerability. The recommended approach is to stop using the vulnerable B64dec version 1.1.2 and migrate to an alternative, actively maintained base64 decoding tool. Additional technical details can be found on the 4MHz Analysis Page.

Workarounds

• Use built-in operating system utilities or programming language libraries for base64 decoding instead of B64dec
• If B64dec must be used temporarily, run it in an isolated environment with restricted privileges
• Enable Windows exploit protection features (DEP, SEHOP, ASLR) to make exploitation more difficult
• Validate and sanitize all input before processing with any base64 decoding utility