ALTCHA Proof-of-Work Vulnerable to Challenge Splicing and Replay
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Security bypass
- Confidentiality · integrity · availability
- None · Low · Low
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-347.
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| altcha-org | altcha-lib | before 1.4.1 | 1.4.1 | Patch available | CNA |
| — | altcha-lib (npm) | before 1.4.1 | 1.4.1 | Patch available | GHSA-6gvq-jcmp-8959 |
| — | altcha-org/altcha (Packagist) | before 1.3.1 | 1.3.1 | Patch available | GHSA-6gvq-jcmp-8959 |
| — | github.com/altcha-org/altcha-lib-go (Go) | before 1.0.0 | 1.0.0 | Patch available | GHSA-6gvq-jcmp-8959 |
| — | org.altcha:altcha (Maven) | before 1.3.0 | 1.3.0 | Patch available | GHSA-6gvq-jcmp-8959 |
| — | altcha (RubyGems) | before 1.0.0 | 1.0.0 | Patch available | GHSA-6gvq-jcmp-8959 |
| — | altcha (PyPI) | before 1.0.0 | 1.0.0 | Patch available | GHSA-6gvq-jcmp-8959 |
| — | altcha (Hex) | before 1.0.0 | 1.0.0 | Patch available | GHSA-6gvq-jcmp-8959 |
- altcha-lib Vendoraltcha-org Affected before 1.4.1 Fixed in 1.4.1 Patch available CNA
- altcha-lib (npm) Vendor— Affected before 1.4.1 Fixed in 1.4.1 Patch available GHSA-6gvq-jcmp-8959
- altcha-org/altcha (Packagist) Vendor— Affected before 1.3.1 Fixed in 1.3.1 Patch available GHSA-6gvq-jcmp-8959
- github.com/altcha-org/altcha-lib-go (Go) Vendor— Affected before 1.0.0 Fixed in 1.0.0 Patch available GHSA-6gvq-jcmp-8959
- org.altcha:altcha (Maven) Vendor— Affected before 1.3.0 Fixed in 1.3.0 Patch available GHSA-6gvq-jcmp-8959
- altcha (RubyGems) Vendor— Affected before 1.0.0 Fixed in 1.0.0 Patch available GHSA-6gvq-jcmp-8959
- altcha (PyPI) Vendor— Affected before 1.0.0 Fixed in 1.0.0 Patch available GHSA-6gvq-jcmp-8959
- altcha (Hex) Vendor— Affected before 1.0.0 Fixed in 1.0.0 Patch available GHSA-6gvq-jcmp-8959
Remediation
- Fix · CNA altcha-lib (altcha-org) Fixed in 1.4.1
- Fix · GHSA-6gvq-jcmp-8959 altcha-lib (npm) Fixed in 1.4.1 Advisory →
- Fix · GHSA-6gvq-jcmp-8959 altcha-org/altcha (Packagist) Fixed in 1.3.1 Advisory →
- Fix · GHSA-6gvq-jcmp-8959 github.com/altcha-org/altcha-lib-go (Go) Fixed in 1.0.0 Advisory →
- Fix · GHSA-6gvq-jcmp-8959 org.altcha:altcha (Maven) Fixed in 1.3.0 Advisory →
- Fix · GHSA-6gvq-jcmp-8959 altcha (RubyGems) Fixed in 1.0.0 Advisory →
- Fix · GHSA-6gvq-jcmp-8959 altcha (PyPI) Fixed in 1.0.0 Advisory →
- Fix · GHSA-6gvq-jcmp-8959 altcha (Hex) Fixed in 1.0.0 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2025-68113
- 2026-10-03 Alias added: GHSA-6gvq-jcmp-8959
References
Other 12
- https://github.com/altcha-org/altcha-lib/security/advisories/GHSA-6gvq-jcmp-8959
- 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-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/altcha-org/altcha-lib
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/altcha/CVE-2025-68113.yml
Sources
- CVE-2025-68113 · CVE Program
- CVE-2025-68113 · NVD
- FIRST EPSS
- GHSA-6gvq-jcmp-8959 · GitHub Advisory Database · CC BY 4.0
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