feat: add header integrity check and replay protection to control messages by markpmarton · Pull Request #1740 · agntcy/slim

markpmarton · 2026-06-12T08:16:24Z

Description

In the current implementation only publish message headers are e2e validated because it is built around the MLS payload encryption. To prevent malicious control message replay attacks, we need to add manual integrity check for pre- and post-session messages.

Changes

per sender monotonic increasing sequence_number added for replay protection
AAD signature e2e_header_sig added for header fields (signed by MLS key)
control message header fields signing and verification (with sequence number tracking)

Type of Change

Checklist

I have read the contributing guidelines
Existing issues have been referenced (where applicable)
I have verified this change is not present in other open pull requests
Functionality is documented
All code style checks pass
New code contribution is covered by automated tests
All new and existing tests pass

…l messages Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

Signed-off-by: Mark Marton <30534230+markpmarton@users.noreply.github.com>

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

codecov · 2026-06-12T15:36:17Z

Codecov Report

❌ Patch coverage is 92.63658% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
data-plane/core/session/src/session_controller.rs	80.80%	19 Missing ⚠️
data-plane/core/service/src/app.rs	64.28%	5 Missing ⚠️
data-plane/core/session/src/session_layer.rs	75.00%	2 Missing ⚠️
data-plane/core/session/src/session_moderator.rs	88.23%	2 Missing ⚠️
data-plane/core/session/src/session_participant.rs	84.61%	2 Missing ⚠️
data-plane/core/session/src/mls_state.rs	95.83%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

Signed-off-by: Mark Marton <30534230+markpmarton@users.noreply.github.com>

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

+                                session_ctx,
+                            ))) => return session_ctx,
+                            Some(Ok(_)) => continue,
+                            Some(Err(e)) => {


micpapal · 2026-06-18T07:46:59Z

+        );
+        // Require E2E verification only when the sender included a signature (matches pre-session path).
+        let e2e_required =
+            is_post_session_control && msg.get_slim_header().e2e_header_sig.is_some();


this should be always mandatory otherwise we can always send fake packets with e2e_header_sig set to None

micpapal · 2026-06-18T08:03:26Z

  optional uint64 incoming_conn = 9;
  optional bool error = 10;
  optional bytes header_mac = 11;
+  optional uint64 sequence_number = 12;


why do we need a new sequence_number here? isn't enough to use the message_id in session header?

AFAIK message_id is getting a random u32 for control messages, that is not viable for replay protection. Maybe it can be changed to a monotonic counter, I'm going to check it.

micpapal · 2026-06-18T08:12:05Z

+                let seen = seen_control_seqs.entry(sender.clone()).or_default();
+                if !seen.insert(s) {
+                    // Duplicate delivery (network retransmission or fanout). Handlers are idempotent.
+                    return Ok(());


I am not sure here, but should we drop the message here instead of return ok? If it is in seen it means that the message was already processed. Notice that if we use the message_id in the session header RTX message will have a different id with respect to the original message so the two messages will be handled as two different messages.

micpapal · 2026-06-18T08:14:21Z

                                }
                                Err(e) => {
-                                    debug!(
+                                    tracing::error!(


this should do back to debug

micpapal · 2026-06-18T09:19:49Z

+            custom_claims: Option<CustomClaims>,
+        }
+
+        let claims_res = verifier.get_claims(&identity).await;


here I think we can reuse the IdentityClaims in Auth. This part can be replaced with something like

let claims_json = verifier.get_claims(&identity).await?; let identity_claims = slim_auth::identity_claims::IdentityClaims::from_json(&claims_json)?; let pubkey = identity_claims.public_key;

micpapal · 2026-06-18T09:23:46Z

+
+                let aad = crate::mls_state::build_aad(m);
+                let private_key = identity_provider.get_signature_secret_key()?;
+                let public_key = identity_provider.get_signature_public_key()?;


This can be moved out of the loop

let private_key = identity_provider.get_signature_secret_key()?;
let public_key = identity_provider.get_signature_public_key()?;

micpapal · 2026-06-18T09:26:44Z


-            if let Err(e) =
-                crate::session_controller::verify_identity(&message, &layer.identity_verifier).await
+            let e2e_required = message.get_slim_header().e2e_header_sig.is_some();


here this also should always be mandatory

markpmarton and others added 5 commits June 11, 2026 16:07

feat: add header integrity validation and replay protection to contro…

24bb424

…l messages Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

feat: add header integrity validation and replay protection to contro…

d5e94d4

…l messages Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

feat: add header integrity validation and replay protection to contro…

6291e32

…l messages Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

Merge branch 'main' into control-message-header-integrity

6eb1d71

Signed-off-by: Mark Marton <30534230+markpmarton@users.noreply.github.com>

fix: fix linting

28a30be

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

github-advanced-security AI found potential problems Jun 12, 2026

View reviewed changes

Comment thread data-plane/core/session/src/session_layer.rs Fixed

markpmarton and others added 3 commits June 15, 2026 12:59

Merge branch 'main' into control-message-header-integrity

1c7e6aa

fix: fix for false positive CodeQL warning

0ca9333

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

fix: remove redundant AAD generator function

795a826

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

markpmarton marked this pull request as ready for review June 15, 2026 13:31

markpmarton requested a review from a team as a code owner June 15, 2026 13:31

markpmarton and others added 2 commits June 16, 2026 07:36

Merge branch 'main' into control-message-header-integrity

ae619b2

Signed-off-by: Mark Marton <30534230+markpmarton@users.noreply.github.com>

fix: add header signing for both cypher algo implementations

9c2ec9e

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

markpmarton force-pushed the control-message-header-integrity branch from 62f81dc to 9c2ec9e Compare June 16, 2026 13:04

markpmarton and others added 2 commits June 16, 2026 15:08

fix: fix linting

597d215

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

Merge branch 'main' into control-message-header-integrity

9d1a015

markpmarton marked this pull request as draft June 16, 2026 14:23

markpmarton added 4 commits June 17, 2026 07:15

fix: remove wasm32 target condition and generalize sign/verify functions

30985ca

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

fix: fix linting

a8df0b0

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

fix: increase coverage

706cc83

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

fix: fix multicast test race condition

3633e53

Signed-off-by: Mark Marton <mark.p.marton@gmail.com>

github-advanced-security AI found potential problems Jun 17, 2026

View reviewed changes

Comment thread data-plane/core/service/src/app.rs

session_ctx,

))) => return session_ctx,

Some(Ok(_)) => continue,

Some(Err(e)) => {

markpmarton marked this pull request as ready for review June 17, 2026 08:01

markpmarton added 3 commits June 17, 2026 14:34

Merge branch 'main' into control-message-header-integrity

f04bde7

Merge branch 'main' into control-message-header-integrity

2dc99ff

Merge branch 'main' into control-message-header-integrity

434e17f

micpapal requested changes Jun 18, 2026

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feat: add header integrity check and replay protection to control messages#1740

feat: add header integrity check and replay protection to control messages#1740
markpmarton wants to merge 19 commits into
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