ALTCHA Proof-of-Work Vulnerable to Challenge Splicing and Replay

published 2025-12-16 10 mo ago modified 2026-10-07 32 d ago also GHSA-6gvq-jcmp-8959 , GO-2025-4239 , PYSEC-2026-1115 , EUVD-2025-203484

cvss: 6.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.5%; EPSS 0.5% chance of exploitation in 30 days, percentile 40%
public exploits: 0; none known

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.

Patch / fix

fixed in 7 distros 2 links 9

npm altcha-lib 1.4.1
packagist altcha-org/altcha 1.3.1
go github.com/altcha-org/altcha-lib-go 1.0.0
maven org.altcha:altcha 1.3.0
rubygems altcha 1.0.0
pypi altcha 1.0.0
hex altcha 1.0.0

suse opensuse

govulncheck-vulndb openSUSE Leap 15.6 0.0.20251230T014957-150000.1.134.1

suse advisories SUSE-SU-2026:0037-1

govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 10 mo ago (2025-12-16)

Exploit prediction

0.46% chance of exploitation in the next 30 days, higher than 40% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
GitHub	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
OSV	3.1	6.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

Affected packages

ecosystem	package	affected	fixed in
npm	altcha-lib	<1.4.1	1.4.1
packagist	altcha-org/altcha	<1.3.1	1.3.1
go	github.com/altcha-org/altcha-lib-go	<1.0.0	1.0.0
maven	org.altcha:altcha	<1.3.0	1.3.0
rubygems	altcha	<1.0.0	1.0.0
pypi	altcha	<1.0.0	1.0.0
hex	altcha	<1.0.0	1.0.0

Affected products

altcha-org altcha-lib

Classification

Weakness

CWE-115 Misinterpretation of Input
CWE-347 Improper Verification of Cryptographic Signature

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary