CVE-2025-65849: Altcha Proof-of-Work Info Disclosure

CVE-2025-65849 Overview

A cryptanalytic break has been identified in Altcha Proof-of-Work obfuscation mode version 0.8.0 and later. This vulnerability allows remote visitors to recover the Proof-of-Work nonce in constant time via mathematical deduction, effectively bypassing the intended obfuscation mechanism. The weakness is classified under CWE-327 (Use of a Broken or Risky Cryptographic Algorithm).

Note: This vulnerability is disputed by the supplier. According to Altcha, the product's objective is "to discourage automated scraping / bots, not guarantee resistance to determined attackers." The documentation explicitly states that "the goal is not to provide a secure cryptographic algorithm but to use a proof-of-work mechanism that allows any capable device to decrypt the hidden data."

Critical Impact

Remote attackers can mathematically deduce the Proof-of-Work nonce in constant time, completely circumventing the obfuscation protection intended to deter automated scraping and bot activity.

Affected Products

• Altcha Proof-of-Work obfuscation mode version 0.8.0 and later

Discovery Timeline

• 2025-12-08 - CVE-2025-65849 published to NVD
• 2025-12-11 - Last updated in NVD database

Technical Details for CVE-2025-65849

Vulnerability Analysis

This vulnerability represents a cryptographic weakness in Altcha's Proof-of-Work obfuscation implementation. The core issue lies in the predictability of the cryptographic operations used to generate the Proof-of-Work nonce. Rather than requiring computational effort to solve the challenge (which is the fundamental purpose of a Proof-of-Work system), the implementation allows the nonce to be derived through direct mathematical calculation in constant time.

The vulnerability is network-accessible and requires no authentication or user interaction to exploit. An attacker can remotely recover the nonce without performing the intended computational work, effectively negating the bot deterrence mechanism that the obfuscation layer was designed to provide.

Root Cause

The root cause is classified as CWE-327: Use of a Broken or Risky Cryptographic Algorithm. The obfuscation algorithm used in Altcha version 0.8.0 and later does not provide sufficient cryptographic strength to prevent mathematical deduction of the nonce. The algorithm appears to have been designed for basic deterrence rather than cryptographic security, as acknowledged by the vendor in their documentation.

The implementation in the helpers.ts source file contains the vulnerable obfuscation logic that can be reversed through cryptanalysis.

Attack Vector

The attack is conducted remotely over the network. An attacker targeting a website or application protected by Altcha's obfuscation mode can:

1. Intercept the Proof-of-Work challenge presented by the Altcha widget
2. Apply mathematical deduction techniques to extract the nonce directly
3. Submit the solved challenge without performing the intended computational work
4. Bypass the bot deterrence mechanism to perform automated scraping or other unwanted activities

The altcha-deobfs project on GitHub demonstrates techniques for deobfuscating the Altcha challenges. For technical details on the obfuscation mechanism, refer to the Altcha Obfuscation Documentation.

Detection Methods for CVE-2025-65849

Indicators of Compromise

• Unusually rapid completion of Proof-of-Work challenges (constant time instead of variable computational time)
• High volume of successful challenge completions from single IP addresses or user agents
• Automated request patterns that successfully bypass the Altcha obfuscation layer
• Evidence of known deobfuscation tools or libraries in request headers or traffic patterns

Detection Strategies

• Monitor time-to-solve metrics for Proof-of-Work challenges; legitimate solutions should show variable completion times based on computational difficulty
• Implement server-side logging to track challenge completion times and flag anomalously fast responses
• Analyze traffic patterns for signs of automated scraping that successfully bypasses bot deterrence
• Review application logs for high-frequency requests from sources that consistently solve challenges instantly

Monitoring Recommendations

• Establish baseline metrics for legitimate Proof-of-Work solution times and alert on statistical anomalies
• Deploy network monitoring to identify known deobfuscation tool signatures
• Implement rate limiting and behavioral analysis as secondary defense layers
• Consider additional bot detection mechanisms beyond Proof-of-Work obfuscation

How to Mitigate CVE-2025-65849

Immediate Actions Required

• Assess whether your use case requires cryptographic-strength protection versus basic bot deterrence
• Implement additional bot detection and rate limiting mechanisms alongside Altcha
• Monitor for exploitation attempts using the detection strategies outlined above
• Consider alternative CAPTCHA or bot mitigation solutions if cryptographic resistance is required

Patch Information

As of the last update on 2025-12-11, no vendor patch has been released. The vendor disputes this CVE, stating that the obfuscation feature was never intended to provide cryptographic security against determined attackers. According to the Altcha documentation, the goal is deterrence rather than guaranteed protection.

Organizations relying on Altcha for critical bot protection should evaluate whether the disputed design goals align with their security requirements.

Workarounds

• Layer Altcha with additional bot detection mechanisms such as behavioral analysis, device fingerprinting, or traditional CAPTCHAs
• Implement server-side rate limiting to mitigate automated abuse even if challenges are solved instantly
• Use IP reputation services and threat intelligence feeds to block known malicious sources
• Consider migrating to alternative Proof-of-Work or CAPTCHA solutions that provide stronger cryptographic guarantees if required by your threat model