CVE-2025-68113 identifies a cryptographic semantic binding vulnerability in the altcha-org altcha-lib, a privacy-focused software library used for captcha and bot protection. The core issue stems from improper binding of challenge parameters to the nonce within the HMAC signature computation. Specifically, the HMAC does not unambiguously associate the challenge parameters, such as expiration timestamps, with the nonce, allowing an attacker to splice or reinterpret a valid proof-of-work submission by modifying the expiration value. This enables replay attacks where previously solved challenges can be reused beyond their intended lifetime, depending on how the server handles replay detection and the deployment environment. The vulnerability primarily affects abuse-prevention features like rate limiting and bot mitigation, potentially allowing attackers to bypass these controls and automate abusive behaviors. Importantly, the flaw does not directly impact confidentiality or integrity of user data. The vulnerability is tracked under CWE-115 (Improper Encoding or Escaping of Output) and CWE-347 (Improper Verification of Cryptographic Signature). It affects altcha-lib versions prior to 1.4.1 and corresponding versions of altcha packages in Golang, Ruby, Python, Erlang, JavaScript (npm), PHP (Composer), and Java (Maven). The vendor has addressed the issue by enforcing explicit semantic separation between challenge parameters and the nonce during HMAC computation. As an interim mitigation, implementations can append a delimiter (e.g., "?expires=<time>&") to the salt value before HMAC calculation to prevent ambiguity. Users are strongly advised to upgrade to the patched versions to fully remediate the vulnerability. The CVSS v3.1 score is 6.5 (medium), reflecting network exploitability without privileges or user interaction, with no confidentiality impact but some integrity and availability impact due to replay attacks. No known exploits have been reported in the wild as of publication.

For European organizations, this vulnerability poses a risk primarily to the effectiveness of bot mitigation and abuse-prevention systems that rely on altcha-lib or its language-specific implementations. Attackers exploiting this flaw can replay previously solved captcha challenges beyond their expiration, potentially enabling automated abuse such as credential stuffing, scraping, or denial-of-service attempts that bypass rate limiting. This can lead to increased fraudulent transactions, account takeover attempts, or service degradation. While the vulnerability does not directly expose sensitive data or allow unauthorized data modification, the circumvention of bot defenses can indirectly facilitate broader attacks. Organizations in sectors with high online interaction—such as e-commerce, banking, telecommunications, and government services—are particularly at risk. Additionally, services relying on altcha-lib for privacy-first captcha solutions may face reputational damage if abuse escalates. The impact depends on the deployment specifics and server-side replay protections in place, but the potential for undermining automated abuse controls is significant.

European organizations using altcha-lib or its language-specific packages should immediately upgrade to the patched versions: altcha Golang package 1.0.0, Rubygem 1.0.0, pip 1.0.0, Erlang 1.0.0, npm 1.4.1, Composer 1.3.1, and Maven 1.3.0 or later. Until upgrades are applied, implement the recommended mitigation by appending a delimiter (e.g., "?expires=<time>&") to the salt value before HMAC computation to enforce unambiguous parameter binding and prevent payload splicing. Review server-side replay detection and handling mechanisms to ensure they effectively reject reused challenges beyond expiration. Conduct thorough testing to verify that bot mitigation controls are functioning as intended post-mitigation. Additionally, monitor logs for unusual patterns indicative of replay attacks or automated abuse. Incorporate this vulnerability into risk assessments and incident response plans. Finally, maintain awareness of altcha-org security advisories for any further updates or exploit reports.