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The vulnerability in ALTCHA libraries arises from insufficient semantic separation in HMAC computation for proof-of-work challenges. When a challenge is generated, parameters like salt, expiration time, and nonce are concatenated into a string before HMAC signing. The HMAC signature does not explicitly bind the nonce to other parameters, creating ambiguity. An attacker can intercept a valid challenge payload and splice it by altering the expiration value without changing the HMAC. This is possible because the HMAC treats the entire string as a single entity, and the nonce can be misinterpreted as part of the parameter string. For example, if the salt is “salt?expires=time&”, the nonce may be appended without a clear delimiter. By manipulating the expiration, an attacker can reuse a solved challenge with a new expiration time, as the HMAC remains valid. This exploits the lack of delineation between parameters and nonce during signing. The vulnerability allows replay attacks if server-side handling does not enforce strict binding, potentially bypassing rate limiting or bot mitigation mechanisms. It does not affect data confidentiality or integrity directly, but compromises abuse-prevention controls by extending challenge lifetime beyond intended limits.
DailyCVE Form:
Platform: ALTCHA libraries
Version: All versions
Vulnerability: Cryptographic binding flaw
Severity: Medium
Date: Unspecified 2024
Prediction: Expected patch 2024-07
What Undercode Say:
Analytics:
Example vulnerable HMAC generation salt="challenge_salt" expires="1234567890" nonce="solved_nonce" message="$salt?expires=$expires&$nonce" hmac=$(echo -n "$message" | openssl dgst -sha256 -hmac "secret_key")
Python code demonstrating payload splicing
import hmac
import hashlib
def compute_hmac(key, message):
return hmac.new(key.encode(), message.encode(), hashlib.sha256).hexdigest()
salt = "salt123"
expires = "1000000000"
nonce = "nonce567"
message = salt + "?expires=" + expires + "&" + nonce
hmac_original = compute_hmac("secret", message)
expires_new = "2000000000"
message_spliced = salt + "?expires=" + expires_new + "&" + nonce
hmac_spliced = compute_hmac("secret", message_spliced)
If no delimiter, HMAC may match, enabling replay
How Exploit:
Intercept valid challenge. Modify expiration parameter. Re-submit spliced payload. Bypass replay protection.
Protection from this CVE:
Upgrade patched versions. Append delimiter salt. Strict server validation.
Impact:
Affects abuse-prevention mechanisms. Enables replay attacks. No data compromise.
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Sources:
Reported By: github.com
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