ALTCHA Proof-of-Work Vulnerable to Challenge Splicing and Replay

Summary

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.

Timeline

1. Dec 16, 2025, 12:53 AM UTCCVE published
2. Dec 16, 2025, 9:13 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Dec 16, 2025, 9:14 PM UTCRecord last updated

Affected versions

• altcha-org altcha-lib

  fixed in 1.4.1

• altcha-libnpm

  fixed in 1.4.1

• altcha-org/altchaPackagist

  fixed in 1.3.1

• github.com/altcha-org/altcha-lib-goGo

  fixed in 1.0.0

• org.altcha:altchaMaven

  fixed in 1.3.0

• altchaRubyGems

  fixed in 1.0.0

• altchaPyPI

  fixed in 1.0.0

• altchaHex

  fixed in 1.0.0

References 10