6.5

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

Deferred
AliasesGHSA-6gvq-jcmp-8959GO-2025-4239PYSEC-2026-1115
Published 16-12-2025Last modified 16-12-2025

Vulnerability summary

CVE-2025-68113 is a medium (CVSS 6.5) Misinterpretation of Input vulnerability in altcha-org altcha-lib. EPSS estimates a 0.46% chance of exploitation in the next 30 days, higher than 40.1% of scored vulnerabilities.

Priority
Routine

Not severe (CVSS 6.5, network access); no exploitation reported by the 5 checked sources

Source freshness unavailable

How is this decided?
CVSS score
6.5/10
010
Medium · v3.1 (CVE.org)
EPSS score
0.46%
0%100%
Low · 0.00 pp in 30 days
KEVNot listedNot listed by any tracked catalog.
RansomwareNot flaggedNo ransomware use recorded.
Public exploits0No public exploits recorded

Advisory lineage

Upstream 0 · Downstream 2 · Related 3
OPENSUSE-SU-2026:21483-1SUSE-SU-2026:0037-1CGA-PVW9-4R3W-243M

Timeline

  1. 16-12-2025
    PublishedVulnerability first disclosed.
  2. 16-12-2025
    Last modifiedVulnerability information updated.

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.

CVSS metrics

8 scores · 8 sources · 1 version

EPSS trend

Probability

0.46%

Chance of exploitation in the next 30 days

Percentile

40%

Versus all other CVEs

30-day change

No change 0.00 pp

From 0.46% on 04-10-2026

04-10-202603-11-2026

Known exploited

Compare KEV catalogs
CISA KEV

Not in the CISA KEV catalog.

ENISA CNW

Not in the ENISA CNW catalog.

CIRCL

Not in the CIRCL catalog.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Version check

Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.

Altcha-org2 products
Chainguard2 products
  • listmonk< 5.1.0-r2
  • listmonk-compat< 5.1.0-r2
Github.com/altcha-org1 product
  • altcha-lib-go< 1.0.0
Hex1 product
  • altcha< 1.0.0
Npm1 product
  • altcha-lib< 1.4.1
Org.altcha1 product
  • altcha< 1.3.0
PyPI1 product
  • altcha< 1.0.0
RubyGems1 product
  • altcha< 1.0.0

8 vendors · 10 products

References

16 links
Show all 16 references