diff --git a/docs/scripts/snmp-usernames.xml b/docs/scripts/snmp-usernames.xml
new file mode 100644
index 000000000..e69de29bb
diff --git a/scripts/script.db b/scripts/script.db
index 4d750cfbc..15121f183 100644
--- a/scripts/script.db
+++ b/scripts/script.db
@@ -527,6 +527,7 @@ Entry { filename = "snmp-ios-config.nse", categories = { "intrusive", } }
 Entry { filename = "snmp-netstat.nse", categories = { "default", "discovery", "safe", } }
 Entry { filename = "snmp-processes.nse", categories = { "default", "discovery", "safe", } }
 Entry { filename = "snmp-sysdescr.nse", categories = { "default", "discovery", "safe", } }
+Entry { filename = "snmp-usernames.nse", categories = { "brute", "discovery", } }
 Entry { filename = "snmp-win32-services.nse", categories = { "default", "discovery", "safe", } }
 Entry { filename = "snmp-win32-shares.nse", categories = { "default", "discovery", "safe", } }
 Entry { filename = "snmp-win32-software.nse", categories = { "default", "discovery", "safe", } }
diff --git a/scripts/snmp-usernames.nse b/scripts/snmp-usernames.nse
new file mode 100644
index 000000000..ac4b3e5df
--- /dev/null
+++ b/scripts/snmp-usernames.nse
@@ -0,0 +1,374 @@
+local nmap      = require "nmap"
+local shortport = require "shortport"
+local stdnse    = require "stdnse"
+local creds     = require "creds"
+local unpwdb    = require "unpwdb"
+local io        = require "io"
+
+description = [[
+Enumerates valid SNMPv3 usernames by exploiting the difference in Report PDUs
+returned by SNMPv3 agents when a probe is sent with noAuthNoPriv security level.
+
+When an UNKNOWN username is used, the agent returns a Report PDU containing:
+  1.3.6.1.6.3.15.1.1.3.0  (usmStatsUnknownUserNames)
+
+When a VALID username is used (but auth fails because we send no key), the
+agent returns a Report PDU containing:
+  1.3.6.1.6.3.15.1.1.5.0  (usmStatsWrongDigests)
+
+Some permissive agents reply with a normal GetResponse-PDU (tag 0xA2) for
+noAuthNoPriv-configured users — those are detected as valid too.
+
+The SNMPv3 wire packet is built from scratch in pure Lua using BER/ASN.1
+primitives, because Nmap's bundled snmp.lua library only supports v1/v2c.
+Valid usernames are saved to Nmap's credentials database with state USERNAME
+so scripts like snmp-brute can automatically chain password attacks on them.
+
+References:
+  * RFC 3412 - Message Processing and Dispatching for SNMP
+  * RFC 3414 - User-based Security Model (USM) for SNMPv3
+  * https://github.com/hatlord/snmpwn
+]]
+
+---
+-- @usage
+-- nmap -sU -p 161 --script snmp-usernames <target>
+-- nmap -sU -p 161 --script snmp-usernames \
+--      --script-args snmp-usernames.userdb=/path/to/users.txt <target>
+-- nmap -sU -p 161 --script snmp-usernames \
+--      --script-args snmp-usernames.maxusers=50 <target>
+--
+-- @args snmp-usernames.userdb   Path to a file with one username per line.
+--                               Lines starting with # are treated as comments.
+--                               Defaults to Nmap's built-in usernames.lst.
+-- @args snmp-usernames.maxusers Maximum number of usernames to try (default: unlimited).
+--                               Must be a positive integer; invalid values are ignored.
+--
+-- @output
+-- PORT    STATE SERVICE
+-- 161/udp open  snmp
+-- | snmp-usernames:
+-- |   admin    - Valid SNMPv3 username
+-- |   monitor  - Valid SNMPv3 username
+-- |_  readonly - Valid SNMPv3 username
+
+author     = "Sachin"
+license    = "Same as Nmap--See https://nmap.org/book/man-legal.html"
+categories = {"discovery", "brute"}
+
+-- ── Port rule ─────────────────────────────────────────────────────────────
+portrule = shortport.port_or_service(161, "snmp", "udp", {"open", "open|filtered"})
+
+-- ═════════════════════════════════════════════════════════════════════════
+-- BER / ASN.1 HELPERS
+-- ═════════════════════════════════════════════════════════════════════════
+
+local function encode_len(n)
+  if n < 128 then
+    return string.char(n)
+  elseif n < 256 then
+    return "\x81" .. string.char(n)
+  else
+    return "\x82" .. string.char(math.floor(n / 256)) .. string.char(n % 256)
+  end
+end
+
+-- INTEGER (tag 0x02) — handles any non-negative value correctly.
+-- A naive one-byte version breaks for values like 65507 (msgMaxSize).
+local function ber_int(n)
+  if n == 0 then return "\x02\x01\x00" end
+  local bytes = {}
+  local v = n
+  while v > 0 do
+    table.insert(bytes, 1, string.char(v % 256))
+    v = math.floor(v / 256)
+  end
+  -- BER integers are signed; prepend 0x00 if high bit would look negative.
+  if bytes[1]:byte() >= 0x80 then
+    table.insert(bytes, 1, "\x00")
+  end
+  local content = table.concat(bytes)
+  return "\x02" .. string.char(#content) .. content
+end
+
+-- OCTET STRING (tag 0x04)
+local function ber_octet(s)
+  s = s or ""
+  return "\x04" .. encode_len(#s) .. s
+end
+
+-- SEQUENCE (tag 0x30)
+local function ber_seq(payload)
+  return "\x30" .. encode_len(#payload) .. payload
+end
+
+-- Context-specific constructed tag (0xA0 + ctx)
+-- ctx=0 → GetRequest-PDU, ctx=2 → GetResponse-PDU
+local function ber_ctx(ctx, payload)
+  return string.char(0xA0 + ctx) .. encode_len(#payload) .. payload
+end
+
+-- ═════════════════════════════════════════════════════════════════════════
+-- SNMPv3 PACKET BUILDER
+-- ═════════════════════════════════════════════════════════════════════════
+--
+-- SNMPv3 wire format (RFC 3412 §6 + RFC 3414 §3):
+--
+--  SEQUENCE {
+--    INTEGER  msgVersion      (3)
+--    SEQUENCE msgGlobalData {
+--      INTEGER  msgID                  ← unique per probe (avoids dedup)
+--      INTEGER  msgMaxSize    (65507)
+--      OCTET STRING msgFlags  (0x04)   ← reportable | noAuthNoPriv
+--      INTEGER  msgSecurityModel (3)   ← USM
+--    }
+--    OCTET STRING msgSecurityParameters {
+--      SEQUENCE {
+--        OCTET STRING engineID   ("")  ← empty = engine-discovery probe
+--        INTEGER  engineBoots    (0)
+--        INTEGER  engineTime     (0)
+--        OCTET STRING userName         ← *** the username under test ***
+--        OCTET STRING authParams ("")
+--        OCTET STRING privParams ("")
+--      }
+--    }
+--    SEQUENCE scopedPDU {
+--      OCTET STRING contextEngineID ("")
+--      OCTET STRING contextName     ("")
+--      GetRequest-PDU [0] {
+--        INTEGER  requestID
+--        INTEGER  error-status  (0)
+--        INTEGER  error-index   (0)
+--        SEQUENCE varBindList {
+--          SEQUENCE varBind {
+--            OID  1.3.6.1.2.1.1.3.0   ← sysUpTime (harmless read-only OID)
+--            NULL
+--          }
+--        }
+--      }
+--    }
+--  }
+
+local msg_id_counter = 1000
+
+local function build_probe(username)
+  msg_id_counter = msg_id_counter + 1
+  local msg_id     = msg_id_counter
+  local request_id = msg_id_counter + 50000
+
+  local global = ber_seq(
+    ber_int(msg_id)   ..
+    ber_int(65507)    ..    -- msgMaxSize
+    ber_octet("\x04") ..    -- msgFlags: reportable, noAuthNoPriv
+    ber_int(3)              -- msgSecurityModel = USM
+  )
+
+  local usm = ber_seq(
+    ber_octet("")       ..  -- engineID (empty = discovery probe)
+    ber_int(0)          ..  -- engineBoots
+    ber_int(0)          ..  -- engineTime
+    ber_octet(username) ..  -- *** username under test ***
+    ber_octet("")       ..  -- authenticationParameters (none)
+    ber_octet("")           -- privacyParameters (none)
+  )
+  local sec_params = ber_octet(usm)
+
+  -- sysUpTime OID 1.3.6.1.2.1.1.3.0 pre-encoded in BER.
+  -- Tag=0x06, Length=0x08, Value=2b 06 01 02 01 01 03 00
+  local sysuptime_oid = "\x06\x08\x2b\x06\x01\x02\x01\x01\x03\x00"
+
+  local varbind = ber_seq(
+    sysuptime_oid ..
+    "\x05\x00"        -- NULL
+  )
+
+  local pdu = ber_ctx(0,
+    ber_int(request_id) ..
+    ber_int(0)          ..  -- error-status
+    ber_int(0)          ..  -- error-index
+    ber_seq(varbind)
+  )
+
+  local scoped = ber_seq(
+    ber_octet("") ..    -- contextEngineID
+    ber_octet("") ..    -- contextName
+    pdu
+  )
+
+  return ber_seq(
+    ber_int(3)   ..     -- msgVersion = 3
+    global       ..
+    sec_params   ..
+    scoped
+  )
+end
+
+-- ═════════════════════════════════════════════════════════════════════════
+-- RESPONSE CLASSIFIER
+-- ═════════════════════════════════════════════════════════════════════════
+--
+--   usmStatsUnknownUserNames  OID tail: 0f 01 01 03 00  → user NOT found
+--   usmStatsWrongDigests      OID tail: 0f 01 01 05 00  → user EXISTS
+--
+--   GetResponse-PDU fallback (tag 0xA2): some permissive agents skip the
+--   Report PDU entirely and return real data for noAuthNoPriv users.
+--   We also verify data:byte(1) == 0x30 (valid outer SEQUENCE tag) before
+--   trusting the 0xA2 match, reducing false positives from stray bytes.
+
+local OID_UNKNOWN = "\x0f\x01\x01\x03\x00"   -- usmStatsUnknownUserNames
+local OID_DIGEST  = "\x0f\x01\x01\x05\x00"   -- usmStatsWrongDigests
+
+local function classify(data)
+  if not data then return "unknown" end
+  if data:find(OID_DIGEST,  1, true) then return "valid"   end
+  if data:find(OID_UNKNOWN, 1, true) then return "invalid" end
+  -- Only treat 0xA2 as a GetResponse if the outer frame is a valid SEQUENCE.
+  if data:byte(1) == 0x30 and data:find("\xa2", 1, true) then
+    return "valid"
+  end
+  return "unknown"
+end
+
+-- ═════════════════════════════════════════════════════════════════════════
+-- USERNAME WORDLIST LOADER
+-- ═════════════════════════════════════════════════════════════════════════
+--
+-- Always returns (true, iterator) or (nil, error_string).
+-- The maxusers cap wraps the iterator so it stops early automatically.
+
+local function load_usernames()
+  local path     = stdnse.get_script_args("snmp-usernames.userdb")
+  local maxusers = tonumber(stdnse.get_script_args("snmp-usernames.maxusers"))
+
+  -- Validate maxusers — ignore non-positive or non-numeric values silently.
+  if maxusers and maxusers < 1 then
+    stdnse.debug1("snmp-usernames.maxusers invalid (%s), ignoring", tostring(maxusers))
+    maxusers = nil
+  end
+
+  local iter
+
+  if path then
+    -- Load from user-supplied file.
+    local f = io.open(path, "r")
+    if not f then
+      return nil, "Cannot open username file: " .. path
+    end
+    local list = {}
+    for line in f:lines() do
+      line = line:match("^%s*(.-)%s*$")            -- trim whitespace
+      if line ~= "" and not line:match("^#") then  -- skip blanks and comments
+        list[#list + 1] = line
+      end
+    end
+    f:close()
+    if #list == 0 then
+      return nil, "Username file is empty: " .. path
+    end
+    stdnse.debug1("Loaded %d usernames from %s", #list, path)
+    local i = 0
+    iter = function() i = i + 1; return list[i] end
+  else
+    -- Fall back to Nmap's built-in username list via unpwdb.
+    stdnse.debug1("No userdb specified — falling back to Nmap default username list")
+    local ok, raw_iter = unpwdb.usernames()
+    if not ok or not raw_iter then
+      return nil, "Failed to load Nmap default username list"
+    end
+    iter = raw_iter
+  end
+
+  -- Wrap iterator with maxusers cap if requested.
+  if maxusers then
+    stdnse.debug1("Username cap: %d", maxusers)
+    local count = 0
+    local base  = iter
+    iter = function()
+      if count >= maxusers then return nil end
+      count = count + 1
+      return base()
+    end
+  end
+
+  return true, iter
+end
+
+-- ═════════════════════════════════════════════════════════════════════════
+-- PROBE WITH RETRY
+-- ═════════════════════════════════════════════════════════════════════════
+--
+-- UDP is lossy — retry once before giving up on a username.
+
+local RETRIES = 2
+
+local function probe(sock, pkt)
+  for i = 1, RETRIES do
+    local sent, send_err = sock:send(pkt)
+    if not sent then
+      stdnse.debug1("Send error (attempt %d): %s", i, send_err or "")
+    else
+      local recv_ok, data = sock:receive()
+      if recv_ok then
+        return data
+      end
+      stdnse.debug2("No response on attempt %d", i)
+    end
+  end
+  return nil
+end
+
+-- ═════════════════════════════════════════════════════════════════════════
+-- MAIN ACTION
+-- ═════════════════════════════════════════════════════════════════════════
+
+action = function(host, port)
+
+  local ok, next_user = load_usernames()
+  if not ok then
+    return stdnse.format_output(false, next_user)
+  end
+
+  local sock = nmap.new_socket("udp")
+  sock:set_timeout(3000)
+
+  -- Using UDP connect so send()/receive() can be used instead of sendto().
+  local conn_ok, conn_err = sock:connect(host, port)
+  if not conn_ok then
+    return stdnse.format_output(false, "Connect failed: " .. (conn_err or "unknown"))
+  end
+
+  -- creds.State.USERNAME = username confirmed, password NOT tested.
+  -- This lets snmp-brute and other scripts chain attacks automatically.
+  local found = creds.Credentials:new(SCRIPT_NAME, host, port)
+  local total = 0
+  local hits  = 0
+
+  -- Preferred style: pull username at top of loop, break on nil.
+  while true do
+    local username = next_user()
+    if not username then break end
+
+    stdnse.debug2("Trying: %s", username)
+    total = total + 1
+
+    local data   = probe(sock, build_probe(username))
+    local result = classify(data)
+    stdnse.debug2("%-25s → %s", username, result)
+
+    if result == "valid" then
+      found:add(username, nil, creds.State.USERNAME)
+      hits = hits + 1
+      stdnse.debug1("FOUND: %s", username)
+    end
+
+    -- 50 ms pause — prevents flooding the agent and avoids triggering
+    -- rate-limiting or account lockout on sensitive devices.
+    stdnse.sleep(0.05)
+  end
+
+  sock:close()
+  stdnse.debug1("Done — probes: %d | found: %d", total, hits)
+
+  -- Returns nil when nothing found (Nmap convention = no output block shown).
+  return found:getTable()
+end