scripts/ddos_protection_challenge.README.md

@@ -207,6 +207,8 @@ pow_difficulty = 4  -- Difficulty levels:
 
 This creates real computational cost for attackers - a bot making 1000 requests/sec would need to spend 1000-3000 seconds of CPU time with difficulty 4.
 
+**Security**: The server verifies the proof-of-work by computing `SHA-256(challenge + nonce)` and checking that it has the required leading zeros. Bots cannot bypass this by submitting random nonces.
+
 ## Path-Based Protection
 
 You can configure the module to protect only specific paths, which is useful for:

scripts/ddos_protection_challenge.lua

@@ -468,14 +468,24 @@ local function challengeCallback(cfg, state)
                 local challenge_exists = state.tokens_dict:get("pow:" .. challenge)
 
                 if challenge_exists then
-                    -- Verify the hash using resty.sha256 or just trust client for now
+                    -- Verify the proof-of-work server-side
-                    -- In production, you'd want to verify: sha256(challenge + nonce) starts with N zeros
+                    local resty_sha256 = require("resty.sha256")
-                    -- For simplicity, we'll trust the client solved it correctly
+                    local str = require("resty.string")
-                    -- A more secure implementation would verify server-side
-                    challenge_passed = true
 
+                    local sha256 = resty_sha256:new()
+                    sha256:update(challenge .. nonce)
+                    local digest = sha256:final()
+                    local hash_hex = str.to_hex(digest)
+
+                    -- Check if hash starts with required number of zeros
+                    local required_zeros = string.rep("0", state.pow_difficulty)
+                    if hash_hex:sub(1, state.pow_difficulty) == required_zeros then
+                        challenge_passed = true
                         -- Clean up the challenge
                         state.tokens_dict:delete("pow:" .. challenge)
+                    else
+                        ngx.log(ngx.WARN, "PoW verification failed: hash doesn't have enough leading zeros")
+                    end
                 end
             end
 

tests/ddos_protection_challenge.lua

@@ -493,6 +493,26 @@ function TestDDoSProtectionChallengeQuestion:teardown()
     teardownNgx()
 end
 
+-- Helper function to compute a valid PoW nonce for testing
+local function computeValidNonce(challenge, difficulty)
+    local resty_sha256 = require("resty.sha256")
+    local str = require("resty.string")
+    local required_zeros = string.rep("0", difficulty)
+
+    for nonce = 0, 1000000 do
+        local sha256 = resty_sha256:new()
+        sha256:update(challenge .. tostring(nonce))
+        local digest = sha256:final()
+        local hash_hex = str.to_hex(digest)
+
+        if hash_hex:sub(1, difficulty) == required_zeros then
+            return tostring(nonce)
+        end
+    end
+
+    error("Could not find valid nonce after 1M attempts")
+end
+
 -- Tests for proof-of-work challenge type
 TestDDoSProtectionChallengePow = {}
 
@@ -562,13 +582,16 @@ function TestDDoSProtectionChallengePow:testValidPowPassesChallenge()
     local test_challenge = 'test_pow_challenge'
     ngx.shared.aproxy_tokens:set('pow:' .. test_challenge, true, 300)
 
+    -- Compute a valid nonce that satisfies the PoW requirement
+    local valid_nonce = computeValidNonce(test_challenge, 4)
+
     setupFakeRequest('/__aproxy_challenge_verify', {})
     ngx.var.remote_addr = '192.168.4.2'
     ngx.var.request_method = 'POST'
     ngx._post_args = {
         return_to = '/api/test',
         challenge = test_challenge,
-        nonce = '12345'  -- In reality, this would be computed
+        nonce = valid_nonce
     }
 
     onRequest()