CVE-2025-68113
MediumCVSS 6.5Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
ALTCHA libraries contain a cryptographic semantic binding flaw that allows challenge payload splicing, potentially enabling replay attacks. The HMAC signature does not unambiguously bind challenge parameters to the nonce, allowing an attacker to reinterpret a valid proof-of-work submission with a modified expiration value. This may allow previously solved challenges to be reused beyond their intended lifetime. The issue has been addressed by enforcing explicit semantic separation between challenge parameters and the nonce during HMAC computation.
Risk Assessment
The vulnerability weakens abuse-prevention mechanisms such as rate limiting and bot mitigation, as solved challenges may be replayed beyond their intended lifetime. It does not directly affect data confidentiality or integrity.
Recommendation
Upgrade ALTCHA libraries to patched versions (including altcha Golang 1.0.0, altcha Rubygem 1.0.0, altcha pip 1.0.0, altcha Erlang 1.0.0, altcha-lib npm 1.4.1, altcha-org/altcha Composer 1.3.1, org.altcha:altcha Maven 1.3.0). As a mitigation, append a delimiter to the end of the salt value before HMAC computation (e.g., `<salt>?expires=<time>&`).
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Original NVD description (English source)
ALTCHA is privacy-first software for captcha and bot protection. A cryptographic semantic binding flaw in ALTCHA libraries allows challenge payload splicing, which may enable replay attacks. The HMAC signature does not unambiguously bind challenge parameters to the nonce, allowing an attacker to reinterpret a valid proof-of-work submission with a modified expiration value. This may allow previously solved challenges to be reused beyond their intended lifetime, depending on server-side replay handling and deployment assumptions. The vulnerability primarily impacts abuse-prevention mechanisms such as rate limiting and bot mitigation. It does not directly affect data confidentiality or integrity. This issue has been addressed by enforcing explicit semantic separation between challenge parameters and the nonce during HMAC computation. Users are advised to upgrade to patched versions, which include version 1.0.0 of the altcha Golang package, version 1.0.0 of the altcha Rubygem, version 1.0.0 of the altcha pip package, version 1.0.0 of the altcha Erlang package, version 1.4.1 of the altcha-lib npm package, version 1.3.1 of the altcha-org/altcha Composer package, and version 1.3.0 of the org.altcha:altcha Maven package. As a mitigation, implementations may append a delimiter to the end of the `salt` value prior to HMAC computation (for example, `<salt>?expires=<time>&`). This prevents ambiguity between parameters and the nonce and is backward-compatible with existing implementations, as the delimiter is treated as a standard URL parameter separator.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS