ALTCHA Proof-of-Work Vulnerable to Challenge Splicing and Replay
Impact
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.
Patches
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.
Workarounds
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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | altcha-org/altcha | < 1.3.1 | 1.3.1 |
| Java / Kotlin (Maven) | org.altcha:altcha | < 1.3.0 | 1.3.0 |
| npm package | altcha-lib | < 1.4.1 | 1.4.1 |
| Python (PyPI) | altcha | < 1.0.0 | 1.0.0 |
References
- https://github.com/altcha-org/altcha-lib/security/advisories/GHSA-6gvq-jcmp-8959
- https://nvd.nist.gov/vuln/detail/CVE-2025-68113
- https://github.com/altcha-org/altcha-lib-ex/commit/09b2bad466ad0338a5b24245380950ea9918333e
- https://github.com/altcha-org/altcha-lib-go/commit/4a5610745ef79895a67bac858b2e4f291c2614b8
- https://github.com/altcha-org/altcha-lib-java/commit/69277651fdd6418ae10bf3a088901506f9c62114
- https://github.com/altcha-org/altcha-lib-php/commit/9e9e70c864a9db960d071c77c778be0c9ff1a4d0
- https://github.com/altcha-org/altcha-lib-rb/commit/4fd7b64cbbfc713f3ca4e066c2dd466e3b8d359b
- https://github.com/altcha-org/altcha-lib/commit/cb95d83a8d08e273b6be15e48988e7eaf60d5c08
- https://github.com/altcha-org/altcha-lib
- https://github.com/altcha-org/altcha-lib-java/releases/tag/v1.3.0
- https://github.com/altcha-org/altcha-lib-php/releases/tag/v1.3.1
- https://github.com/altcha-org/altcha-lib/releases/tag/1.4.1
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/altcha/CVE-2025-68113.yml
- https://pypi.org/project/altcha
- https://github.com/advisories/GHSA-6gvq-jcmp-8959
- CVSS vector
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- Platform
- Composer, Java / Kotlin (Maven), npm, Python (PyPI)
- Package
altcha-org/altcha, org.altcha:altcha, altcha-lib- Disclosed
- 16.12.2025
- Updated
- 11.10.2026 13:18
Sources and licenses
OSV / GitHub Advisory Database - GHSA-6gvq-jcmp-8959 - CC BY 4.0
OSV / GitHub Advisory Database - PYSEC-2026-1115 - PyPA Advisory Database; CC BY 4.0
Vulnerability history: org.altcha:altcha
All published vulnerabilities of this package (Java / Kotlin (Maven)), newest first. Shows whether this is a one-off slip or a package that needs watching.
This is the only known vulnerability of this package.
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