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

> Attackers can replay solved captcha challenges by splicing parameters, bypassing bot protection and rate limiting mechanisms.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Dec 2025
- **Updated:** 16 Dec 2025
- **Also known as:** GHSA-6gvq-jcmp-8959, GO-2025-4239, PYSEC-2026-1115

## What is CVE-2025-68113?

CVE-2025-68113 is a medium-severity vulnerability in altcha-org altcha-lib, published on 16 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature), CWE-115 (Misinterpretation of Input)

## Which versions are affected by CVE-2025-68113?

16 open source packages ship an affected version, including github.com/altcha-org/altcha-lib-go, altcha and org.altcha:altcha.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/altcha-org/altcha-lib-go` | Go | All versions | 1.0.0 |
| `github.com/altcha-org/altcha-lib-go` | Go | All versions | 1.0.0 |
| `github.com/altcha-org/altcha-lib-go` | Go | All versions | 1.0.0 |
| `altcha` | Hex | All versions | 1.0.0 |
| `altcha` | Hex | All versions | 1.0.0 |
| `org.altcha:altcha` | Maven | All versions | 1.3.0 |
| `org.altcha:altcha` | Maven | All versions | 1.3.0 |
| `altcha-org/altcha` | Packagist | All versions | 1.3.1 |
| `altcha-org/altcha` | Packagist | All versions | 1.3.1 |
| `altcha` | PyPI | All versions | 1.0.0 |
| `altcha` | PyPI | All versions | 1.0.0 |
| `altcha` | PyPI | All versions | 1.0.0 |
| `altcha` | RubyGems | All versions | 1.0.0 |
| `altcha` | RubyGems | All versions | 1.0.0 |
| `altcha-lib` | npm | All versions | 1.4.1 |
| `altcha-lib` | npm | All versions | 1.4.1 |

## How do I fix CVE-2025-68113?

Upgrade github.com/altcha-org/altcha-lib-go to 1.0.0 or later.

## Is CVE-2025-68113 being exploited?

There is no known exploitation of CVE-2025-68113 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-68113 and fix it if it is affected.

What it is: CVE-2025-68113 is a medium-severity vulnerability in altcha-org altcha-lib, published on 16 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-68113 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/altcha-org/altcha-lib-go (Go): affected all versions; fixed in 1.0.0
- github.com/altcha-org/altcha-lib-go (Go): affected all versions; fixed in 1.0.0
- github.com/altcha-org/altcha-lib-go (Go): affected all versions; fixed in 1.0.0
- altcha (Hex): affected all versions; fixed in 1.0.0
- altcha (Hex): affected all versions; fixed in 1.0.0
- org.altcha:altcha (Maven): affected all versions; fixed in 1.3.0
- org.altcha:altcha (Maven): affected all versions; fixed in 1.3.0
- altcha-org/altcha (Packagist): affected all versions; fixed in 1.3.1
- altcha-org/altcha (Packagist): affected all versions; fixed in 1.3.1
- altcha (PyPI): affected all versions; fixed in 1.0.0
- altcha (PyPI): affected all versions; fixed in 1.0.0
- altcha (PyPI): affected all versions; fixed in 1.0.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-68113/ (Markdown: https://insights.sbomify.com/CVE-2025-68113/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-68113
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-68113
- OSV: https://osv.dev/vulnerability/CVE-2025-68113

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-68113/
Summaries are generated and may contain mistakes; the official description above is the record.