CVE-2025-67125: docopt.cpp Auth Bypass Vulnerability

CVE-2025-67125 Overview

A signed integer overflow vulnerability exists in docopt.cpp v0.6.2, specifically in the LeafPattern::match function within docopt_private.h. The vulnerability is triggered when merging occurrence counters, such as when a default value of LONG_MAX is combined with user-provided flags like -v/--verbose. This can cause counter wrap-around resulting in negative or unbounded semantics, leading to logic and policy bypass in applications that rely on occurrence-based limits, rate-gating, or safety toggles. In hardened builds compiled with UBSan or -ftrapv, the overflow may also result in process abort, causing a Denial of Service (DoS) condition.

Critical Impact
Applications using docopt.cpp for command-line argument parsing may experience logic bypass when occurrence counters wrap due to integer overflow, potentially allowing attackers to circumvent rate limits and safety mechanisms.

Affected Products

docopt.cpp v0.6.2
Applications utilizing docopt.cpp for command-line argument parsing
Hardened builds with UBSan or -ftrapv enabled (susceptible to DoS)

Discovery Timeline

2026-01-23 - CVE CVE-2025-67125 published to NVD
2026-01-26 - Last updated in NVD database

Technical Details for CVE-2025-67125

Vulnerability Analysis

This vulnerability falls under CWE-190 (Integer Overflow or Wraparound). The root issue lies in how docopt.cpp handles occurrence counting for command-line options. When an application initializes a counter with the maximum signed long integer value (LONG_MAX) and a user provides additional occurrences of a flag, the counter arithmetic causes a signed integer overflow.

The impact is twofold depending on the build configuration. In standard builds, the overflow wraps the counter to a negative value or causes undefined behavior, which can be exploited to bypass application logic that depends on these counters for security decisions like rate limiting or feature toggles. In hardened builds that enable undefined behavior sanitizers or arithmetic trap flags, the overflow triggers an immediate process termination.

Root Cause

The root cause is insufficient validation of arithmetic operations on occurrence counters in the LeafPattern::match function within docopt_private.h. The code does not check for potential overflow conditions before incrementing or merging counter values, allowing attackers to craft command-line inputs that trigger the overflow condition.

Attack Vector

The attack requires local access to the target system with the ability to execute the vulnerable application with crafted command-line arguments. An attacker would need to:

Identify an application using docopt.cpp v0.6.2 that relies on occurrence counting for security-relevant logic
Determine the initial counter value or default configuration
Provide sufficient repeated flag occurrences (e.g., multiple -v flags) to trigger the integer overflow
Exploit the resulting logic bypass or DoS condition

The vulnerability mechanism involves the merging of occurrence counters in the command-line parsing logic. When a default value near LONG_MAX is set and additional user-provided occurrences are added, the signed integer arithmetic overflows, causing the counter to wrap to a negative value or trigger undefined behavior. Technical details and a proof-of-concept are available in the GitHub Gist PoC.

Detection Methods for CVE-2025-67125

Indicators of Compromise

Unexpected application crashes or terminations when processing command-line arguments
Abnormal behavior in rate-limited or occurrence-gated application features
Log entries showing unusually high or negative counter values for command-line options

Detection Strategies

Monitor for process crashes associated with UBSan or arithmetic trap violations
Implement runtime checks for command-line argument parsing anomalies
Deploy application-level logging to track occurrence counter values before and after parsing
Use static analysis tools to identify docopt.cpp v0.6.2 dependencies in your codebase

Monitoring Recommendations

Enable verbose logging in applications using docopt.cpp to capture argument parsing details
Configure crash monitoring to alert on UBSan-related terminations
Implement anomaly detection for applications with occurrence-based security controls
Regularly audit third-party library versions in development pipelines

How to Mitigate CVE-2025-67125

Immediate Actions Required

Audit all applications in your environment for docopt.cpp v0.6.2 usage
Implement input validation to cap the maximum number of flag occurrences before passing to docopt
Consider adding bounds checking wrappers around docopt counter values
Monitor for abnormal application behavior that may indicate exploitation attempts

Patch Information

No official patch information is currently available from the vendor. Users should monitor the docopt.cpp GitHub repository for security updates. As an interim measure, consider implementing application-level mitigations or evaluating alternative command-line parsing libraries.

Workarounds

Implement pre-parsing validation to reject command-line inputs with excessive flag repetitions
Avoid using LONG_MAX or similarly large default values for occurrence counters
Consider compiling with overflow-safe arithmetic libraries or using unsigned integers with explicit bounds checking
For critical applications, implement custom occurrence counting logic with overflow protection

bash

# Configuration example - Limit flag occurrences at application level
# Add argument validation before docopt parsing
MAX_VERBOSE_FLAGS=10
verbose_count=$(echo "$@" | grep -o '\-v\|--verbose' | wc -l)
if [ "$verbose_count" -gt "$MAX_VERBOSE_FLAGS" ]; then
    echo "Error: Too many verbose flags specified (max: $MAX_VERBOSE_FLAGS)"
    exit 1
fi