SNMPv3 auth fail with no auth

[macwilam]

Hi,
in my software that uses this crate i build a series of e2e tests. I think I found (with Claude) a small bug. Below is the report Claude generated for this.

SNMPv3 GET fails against RFC 3414-compliant agents: build() emits non-empty msgAuthenticationParameters for noAuthNoPriv

TL;DR

AsyncSession/SyncSession SNMPv3 requests built by v3::build() always reserve a
truncation_len-byte (zero-filled) msgAuthenticationParameters field, even when the
security level is noAuthNoPriv (no authentication). Per RFC 3414 §2.4 / §7.1, this field
must be empty (zero length) when the Auth flag is not set. Standards-compliant agents
reject the malformed message and reply with a usmStatsWrongDigests Report PDU, which the
client then surfaces as Error::AsnWrongType ("ASN.1 wrong type").

The discovery probe (build_init()) builds the field correctly (empty), so engine discovery
succeeds but the subsequent GET fails. net-snmp performing the same noAuthNoPriv GET against
the same agent succeeds, confirming the agent is compliant and the defect is client-side.

Affected versions

• Reproduced on the dobermai/snmp2 fork rev 7b339e8 (reported as version 0.5.1), built with
  default-features = false, features = ["tokio", "v3", "crypto-rust"].
• The defective code path (v3::build()) is unchanged from upstream roboplc/snmp2, so released
  0.5.0 is expected to be affected as well.
• SNMPv3 GET also fails on 0.4.x (observed as a hang/timeout rather than AsnWrongType), i.e.
  the v3 GET path has not been working for a while; this report pins down the 0.5.x mechanism.

Symptom

A noAuthNoPriv GET returns:

Error::AsnWrongType   // Display: "ASN.1 wrong type"

raised from AsyncSession::get() → Pdu::validate(MessageType::Response, ...) (the parsed
response is a Report, not a Response). Engine discovery (init()) completes normally just
before this.

Root cause

src/v3.rs, in build():

let truncation_len = security.auth_protocol.truncation_length();
...
sec_buf_seq.push_sequence(|buf| {
    buf.push_octet_string(&priv_params); // priv params
    let l0 = buf.len() - priv_params.len();
    buf.push_octet_string(&vec![0u8; truncation_len]); // auth params   <-- BUG
    let l1 = buf.len() - l0;
    buf.push_octet_string(security.username()); // user name
    buf.push_integer(security.engine_time()); // time
    buf.push_integer(security.engine_boots()); // boots
    buf.push_octet_string(security.engine_id()); // engine ID
    auth_pos = buf.len() - l1;
    sec_buf_len = buf.len();
});
...
auth_pos += buf.len() - inner_len;
if (auth_pos + truncation_len) > buf.len() {
    return Err(Error::ValueOutOfRange);
}
if security.need_auth() {
    let hmac = security.calculate_hmac(buf)?;
    buf[auth_pos..auth_pos + truncation_len].copy_from_slice(&hmac[..truncation_len]);
}

msgAuthenticationParameters is written as vec![0u8; truncation_len] unconditionally.
AuthProtocol has no "none" variant, so for a noAuthNoPriv session it takes its default
(Md5), whose truncation_length() is 12. The result is a 12-byte all-zero auth-params field
in a message whose msgFlags auth bit is 0 — which is malformed.

For contrast, build_init() builds the same field correctly:

sec.push_octet_string(&[]); // auth params  (empty — correct)

Evidence (captured wire bytes)

Captured by instrumenting build()/send_and_recv() and parse_v3() to dump hex.

Outgoing GET request (noAuthNoPriv), 136 bytes

308185020103300e020101020300ffe3040104020103043a3038041580004fb805656531
6634376534306539390001c700020102020200dc04087573722d6e6f6e65040c00000000
0000000000000000040030 3404 1580004fb8056565316634376534306539390001c700
0400a019020101020100020100300e300c06082b060102010101000500

Decoded (relevant parts):

30 0e                                  msgGlobalData SEQUENCE
   02 01 01                            msgID = 1
   02 03 00 ff e3                      msgMaxSize = 65507
   04 01 04                            msgFlags = 0x04  (REPORTABLE; AUTH=0, PRIV=0)
   02 01 03                            msgSecurityModel = 3 (USM)
04 3a                                  msgSecurityParameters OCTET STRING
   30 38                               USM SEQUENCE
      04 15 80004fb8...c700            msgAuthoritativeEngineID
      02 01 02                         msgAuthoritativeEngineBoots = 2
      02 02 00 dc                      msgAuthoritativeEngineTime = 220
      04 08 7573722d6e6f6e65           msgUserName = "usr-none"
      04 0c 00 00 00 00 00 00 00 00 00 00 00 00   msgAuthenticationParameters  <-- 12 zero bytes; should be 04 00
      04 00                            msgPrivacyParameters = empty
30 34                                  scopedPDU
   04 15 80004fb8...c700               contextEngineID
   04 00                               contextName = empty
   a0 19                               GetRequest
      02 01 01  request-id = 1
      02 01 00  error-status = 0
      02 01 00  error-index = 0
      30 0e 30 0c 06 08 2b060102010101 00 05 00   varbind: OID 1.3.6.1.2.1.1.1.0 = NULL

The msgFlags auth bit is 0, yet msgAuthenticationParameters carries 12 bytes.

Agent response: a Report PDU, 126 bytes

307c020103300e020101020300ffe3040104020103042e302c041580004fb80565653166
34376534306539390001c700020102020200dc04087573722d6e6f6e6504000400303704
1580004fb8056565316634376534306539390001c7000400 a81c 02010002010002010030
11300f060a2b060106030f01010500410103

Decoded scopedPDU:

a8 1c                                  Report-PDU                    <-- not Response (0xa2)
   02 01 00  request-id = 0
   02 01 00  error-status = 0
   02 01 00  error-index = 0
   30 11 30 0f
      06 0a 2b060106030f010105 00        OID 1.3.6.1.6.3.15.1.1.5.0  = usmStatsWrongDigests
      41 01 03                           Counter32 = 3

get() validates MessageType::Response, gets Report → Error::AsnWrongType.

Cross-check with net-snmp

The same agent (snmpsim) answers a net-snmp noAuthNoPriv GET correctly:

$ snmpget -v3 -l noAuthNoPriv -u usr-none 127.0.0.1:1162 1.3.6.1.2.1.1.1.0
SNMPv2-MIB::sysDescr.0 = STRING: <value>

net-snmp sends msgAuthenticationParameters as an empty OCTET STRING, so the agent accepts it.

Reproduction

1. Run an RFC-compliant SNMPv3 agent with a noAuthNoPriv user, e.g. snmpsim-lextudio:

   snmpsim-command-responder \
     --process-user nobody --process-group nogroup \
     --v3-engine-id auto \
     --v3-user usr-none \
     --data-dir ./recordings \
     --agent-udpv4-endpoint 0.0.0.0:1161
   

   (./recordings/.snmprec containing e.g. 1.3.6.1.2.1.1.1.0|4|test)

2. Client:

   use snmp2::{AsyncSession, Oid};
   use snmp2::v3::{Auth, Security};
   
   let oid = Oid::from(&[1,3,6,1,2,1,1,1,0]).unwrap();
   let security = Security::new(b"usr-none".as_slice(), &[]).with_auth(Auth::NoAuthNoPriv);
   let mut session = AsyncSession::new_v3(addr, 0, security).await?;
   session.init().await?;                  // succeeds (discovery)
   let pdu = session.get(&oid).await?;      // Err(AsnWrongType)

   init() succeeds; get() returns Err(Error::AsnWrongType).

Proposed fix

msgAuthenticationParameters must be empty when authentication is not in use, and the HMAC
back-fill (and its bounds check) must be skipped accordingly. In src/v3.rs::build():

     let truncation_len = security.auth_protocol.truncation_length();
+    let auth_len = if security.need_auth() { truncation_len } else { 0 };
     ...
     sec_buf_seq.push_sequence(|buf| {
         buf.push_octet_string(&priv_params); // priv params
         let l0 = buf.len() - priv_params.len();
-        buf.push_octet_string(&vec![0u8; truncation_len]); // auth params
+        buf.push_octet_string(&vec![0u8; auth_len]); // auth params (empty unless authenticating)
         let l1 = buf.len() - l0;
         ...
     });
     ...
     auth_pos += buf.len() - inner_len;
-    if (auth_pos + truncation_len) > buf.len() {
-        return Err(Error::ValueOutOfRange);
-    }
-
     if security.need_auth() {
+        if (auth_pos + truncation_len) > buf.len() {
+            return Err(Error::ValueOutOfRange);
+        }
         let hmac = security.calculate_hmac(buf)?;
         buf[auth_pos..auth_pos + truncation_len].copy_from_slice(&hmac[..truncation_len]);
     }

Rationale: with auth_len == 0 the field is encoded as an empty OCTET STRING (04 00), matching
build_init() and net-snmp. auth_pos is only used to back-fill the HMAC, which happens only
when need_auth() is true, so moving the bounds check inside that branch keeps the authenticated
path identical while making the noAuth path well-formed. (Proposed; not yet validated end-to-end.)

Notes

• This affects every request built via build() (GET / GETNEXT / GETBULK / SET / INFORM) at the
  noAuthNoPriv security level — not just GET.
• authNoPriv / authPriv are likely unaffected (the auth bit is set and the field length is
  expected), but were not separately reproduced here.
• Engine discovery is unaffected because it uses build_init(), which already emits an empty
  auth-params field.