ALTCHA CVE-2025-68113

Exploitation requires the attacker to first obtain a single legitimately solved ALTCHA proof-of-work payload (its valid HMAC signature and salt), then resubmit it with an altered 'expires' value exploiting the ambiguous binding between challenge parameters and the nonce. The critical enabling condition is a server-side deployment that validates the signed payload's embedded expiration without independently tracking already-consumed challenges or enforcing its own replay/nonce state - the vendor explicitly conditions impact on 'server-side replay handling and deployment assumptions.' Limiting factors: impact is confined to abuse-prevention mechanisms (rate limiting, bot mitigation) with no effect on data confidentiality or integrity; deployments that enforce one-time challenge consumption server-side, or that have upgraded to the patched library versions or applied the backward-compatible salt-delimiter mitigation, are not exploitable.

This is a genuine but moderate, narrowly-scoped weakness rather than a top-tier emergency. The CVSS 3.1 vector (6.5, AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L) is internally consistent with the description: the attack is remote and low-complexity (merely editing the expiration value in a submitted payload) and requires no privileges or user interaction, but its impact is deliberately capped at Low integrity and Low availability with zero confidentiality - matching the CWE-115 (misinterpretation of input) root cause and the vendor's explicit statement that data confidentiality and integrity are not directly affected. The exploitation signals are mild and mutually reinforcing: EPSS is 0.46% (38th percentile), there is no CISA KEV listing, and no public weaponized exploit was identified - only vendor-authored regression tests demonstrating salt-splicing exist in the patch commits, which validate the fix rather than constitute a drop-in attack tool. A key caveat: real-world exploitability is conditional on 'server-side replay handling and deployment assumptions,' so a deployment that already tracks consumed challenges or enforces independent nonce/replay state may be unaffected despite the library flaw. Net: prioritize patching for anyone relying on ALTCHA for rate-limiting or bot mitigation at scale, but this does not warrant out-of-band emergency response.

An attacker legitimately solves one ALTCHA proof-of-work challenge, then captures and resubmits that valid payload while editing the expiration (expires) parameter, because the HMAC signature does not unambiguously bind the challenge parameters to the nonce. On servers that trust the signed payload's embedded expiry without independent replay tracking, the single solved challenge can be reused far beyond its intended lifetime to defeat rate limiting and bot mitigation. No public exploit tool exists, but vendor regression tests in the patch commits concretely demonstrate the salt-splicing technique.