CVE-2025-68113 identifies a cryptographic semantic binding flaw in the altcha-org altcha-lib, a library used for captcha and bot protection that emphasizes privacy. The vulnerability stems from the way the HMAC signature is computed: it does not clearly separate the challenge parameters from the nonce, leading to ambiguity. Specifically, the HMAC does not bind the expiration parameter unambiguously to the nonce, enabling an attacker to splice challenge payloads and modify the expiration value. This flaw can be exploited to replay previously solved proof-of-work challenges beyond their intended lifetime, depending on how the server handles replayed challenges. Since altcha-lib is used to prevent abuse such as automated bot attacks and rate limiting circumvention, this vulnerability undermines these protections but does not directly affect confidentiality or integrity of data. The flaw is categorized under CWE-115 (Improper Encoding or Escaping of Output) and CWE-347 (Improper Verification of Cryptographic Signature). The vulnerability affects all versions of altcha-lib prior to 1.4.1. The fix involves enforcing explicit semantic separation between challenge parameters and the nonce during HMAC computation, ensuring that parameters like expiration cannot be tampered with without invalidating the signature. As a mitigation, implementations can append a delimiter to the salt value before HMAC computation (e.g., adding a '?' separator before expiration parameters), which prevents ambiguity and is backward-compatible. The vulnerability has a CVSS 3.1 base score of 6.5 (medium severity), with an attack vector of network, low attack complexity, no privileges or user interaction required, and impacts integrity and availability but not confidentiality. No known exploits are currently reported in the wild.

For European organizations, this vulnerability poses a risk primarily to the effectiveness of abuse-prevention mechanisms such as captcha challenges and rate limiting. Attackers could reuse previously solved challenges to bypass bot detection and rate limiting controls, potentially enabling automated attacks, credential stuffing, or denial-of-service attempts. This could lead to increased fraudulent activity, resource exhaustion, and degraded service availability. While the vulnerability does not directly compromise sensitive data confidentiality or integrity, the circumvention of bot protections can indirectly facilitate broader attacks or fraud. Organizations relying on altcha-lib for critical online services, especially in sectors like e-commerce, finance, and public services, may experience increased abuse and operational disruption. The impact is heightened if server-side replay protections are weak or absent. Given the network-based attack vector and lack of required authentication, exploitation can be widespread and automated, increasing risk exposure.

European organizations using altcha-lib should immediately upgrade to version 1.4.1 or later, where the vulnerability is fully addressed by enforcing semantic separation in HMAC computation. Until upgrades are applied, developers should implement the recommended mitigation by appending a delimiter (such as '?') to the salt value before HMAC calculation to prevent parameter ambiguity. Additionally, organizations should review and strengthen server-side replay protections, such as maintaining nonce or challenge usage caches to detect and reject replayed challenges. Monitoring for unusual patterns of challenge reuse or rate limiting bypass attempts can help detect exploitation attempts. Security teams should audit all applications and services using altcha-lib or its language-specific packages (Golang, Ruby, Python, Erlang, npm, Composer, Maven) to ensure no vulnerable versions remain in production. Incorporating multi-layered bot detection techniques beyond altcha-lib can reduce reliance on a single control point. Finally, maintain awareness of updates from altcha-org and apply patches promptly.