Abuse-prevention bypass via challenge payload splicing
Published Dec 16, 2025 · Updated Dec 16, 2025
Challenge verification in ALTCHA JS Library before 1.4.1 allows remote attackers to reuse solved proofs with altered expiration values. The HMAC input concatenates URL-encoded salt parameters directly with the proof nonce, so a parameter suffix can be reinterpreted as part of the nonce without invalidating the signature. Exploitation requires a deployment that accepts the spliced payload and lacks server-side replay rejection; successful reuse bypasses rate limiting or bot mitigation without exposing or modifying application data.
Summary
What happened
Challenge verification in ALTCHA JS Library before 1.4.1 allows remote attackers to reuse solved proofs with altered expiration values. The HMAC input concatenates URL-encoded salt parameters directly with the proof nonce, so a parameter suffix can be reinterpreted as part of the nonce without invalidating the signature. Exploitation requires a deployment that accepts the spliced payload and lacks server-side replay rejection; successful reuse bypasses rate limiting or bot mitigation without exposing or modifying application data.
The record
- CVE
- CVE-2025-68113
- Published
- Dec 16, 2025
- Updated
- Dec 16, 2025
- Vendor
- ALTCHA
- Product
- ALTCHA JS Library
- Classifications
- CWE-115, CWE-347, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Dec 16, 2025CVE publishedPublication date reported by the CVE source.
- Dec 16, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 1.4.1
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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