Medium 6.5Published Dec 15, 2025

CVE-2025-68113ALTCHA Proof-of-Work Vulnerable to Challenge Splicing and Replay

Updated under an hour ago (Oct 27, 2026, 3:30 PM ET)

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Every CVE for altcha-org altcha-lib

Exploited in the last 30 days

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.

CVSS: 6.5 MEDIUM (v3.1, CNA)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
Weakness (CWE): CWE-115, CWE-347
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: GitHub_M
NVD analysis: Deferred
Record last updated: Dec 16, 2025
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

altcha-org altcha-lib altcha_org:altcha_lib (from the name)	before 1.4.1	CNA record

References

References

https://github.com/altcha-org/altcha-lib/security/advisories/GHSA-6gvq-jcmp-8959	x-refsource-confirm
https://github.com/altcha-org/altcha-lib-ex/commit/09b2bad466ad0338a5b24245380950ea9918333e	x-refsource-misc
https://github.com/altcha-org/altcha-lib-go/commit/4a5610745ef79895a67bac858b2e4f291c2614b8	x-refsource-misc
https://github.com/altcha-org/altcha-lib-java/commit/69277651fdd6418ae10bf3a088901506f9c62114	x-refsource-misc
https://github.com/altcha-org/altcha-lib-php/commit/9e9e70c864a9db960d071c77c778be0c9ff1a4d0	x-refsource-misc
https://github.com/altcha-org/altcha-lib-rb/commit/4fd7b64cbbfc713f3ca4e066c2dd466e3b8d359b	x-refsource-misc
https://github.com/altcha-org/altcha-lib/commit/cb95d83a8d08e273b6be15e48988e7eaf60d5c08	x-refsource-misc
https://github.com/altcha-org/altcha-lib-java/releases/tag/v1.3.0	x-refsource-misc
https://github.com/altcha-org/altcha-lib-php/releases/tag/v1.3.1	x-refsource-misc
https://github.com/altcha-org/altcha-lib/releases/tag/1.4.1	x-refsource-misc

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.