The RSA-PSS implementation does not detect signature modification (prepending "0" bytes) to the signature

[adelapie]

The jsrsasign 8.0.16 RSASSA-PSS (RSA-PSS) implementation does not detect prepending 0s to the signature and accepts modifies signatures with prepended 0's as valid.

You can verify this using the following test vectors from Google Wycheproof:

{
  "algorithm" : "RSASSA-PSS",
  "generatorVersion" : "0.8r12",
  "numberOfTests" : 103,
  "header" : [
    "Test vectors of class RsassaPssVerify are intended for checking the",
    "verification of RSASSA-PSS signatures."
  ],
  "notes" : {
  },
  "schema" : "rsassa_pss_verify_schema.json",
  "testGroups" : [
    {
      "e" : "010001",
      "keyAsn" : "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",
      "keyDer" : "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",
      "keyPem" : "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAorRRoH0KpfluRVZxUTVQ\nUUqKW0YuvvcXCU+h/ugiJOY3+XRtP3yv0xh42AMltu9aFwD2WQO0aUKeidbqyIRQ\nl7WrOTGJ25JRLtincRoSU/rNIPecFegkfz0+QuRuSMmOJUov6XZTE6A+/48X4aAp\nOXofomqNzib0kO2BKZYV2YFMItphBCjgnH2WWFlCZvXAIdD87KCNlFoSvoLeTR7O\na0wDFFtdNJXU7VQR64eNrwX9evw+Ca2g8RJkIvWQl1oZaYFvSGmLy7obTZyuedRg\n2Pn4Xnl1AF2bwixOWsD3waRdElaaYoB9O5oC5aUw53MGb0U9H1tMLpz3ggKD90K5\n1QIDAQAB\n-----END PUBLIC KEY-----",
      "keysize" : 2048,
      "mgf" : "MGF1",
      "mgfSha" : "SHA-256",
      "n" : "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",
      "sLen" : 32,
      "sha" : "SHA-256",
      "type" : "RsassaPssVerify",
      "tests" : [
        {
          "tcId" : 99,
          "comment" : "prepending 0's to signature",
          "msg" : "313233343030",
          "sig" : "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",
          "result" : "invalid",
          "flags" : []
        },
        {
          "tcId" : 100,
          "comment" : "correct signature",
          "msg" : "313233343030",
          "sig" : "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",
          "result" : "valid",
          "flags" : []
        },
        {
          "tcId" : 101,
          "comment" : "appending 0's to signature",
          "msg" : "313233343030",
          "sig" : "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",
          "result" : "invalid",
          "flags" : []
        }
      ]
    }
  ]
}

in the following proof of concept:

var rs = require('jsrsasign');
var obj = require("./rsa_pss.json");

for (let testGroup of obj.testGroups) {

    var keyPem = testGroup.keyPem;
    
    for(let test of testGroup.tests) {
     console.log("[*] Test " + test.tcId + " result: " + test.result)
     
     try {
      var sig = new rs.Signature({alg: 'SHA256withRSAandMGF1'});
      sig.init(keyPem);

      sig.updateHex(test.msg);
      var result = sig.verify(test.sig);

     if (result == true) {
      if (test.result == "valid" || test.result == "acceptable")
       console.log("Result: PASS");
      else
       console.log("Result: FAIL")     
     }

     if (result == false) {
      if (test.result == "valid" || test.result == "acceptable")
       console.log("Result: FAIL");
      else
       console.log("Result: PASS")     
     }

     } catch (e) {
      console.log("ERROR - VERIFY: " + e)

      if (test.result == "valid" || test.result == "acceptable")
       console.log("Result: FAIL");
      else
       console.log("Result: PASS")     

     }

    }
}

with result:

[*] Test 99 result: invalid
Result: FAIL
[*] Test 100 result: valid
Result: PASS
[*] Test 101 result: invalid
Result: PASS

Best regards,
Antonio

[kjur]

Thank you for your report. The issue was fixed in 8.0.17 release today.

[adelapie]

CVE-2020-14968 is assigned to this issue with the following description: An issue was discovered in the jsrsasign package before 8.0.17 for Node.js. Its RSASSA-PSS (RSA-PSS) implementation does not detect signature manipulation/modification by prepending '\0' bytes to a signature (it accepts these modified signatures as valid). An attacker can abuse this behavior in an application by creating multiple valid signatures where only one signature should exist. Also, an attacker might prepend these bytes with the goal of triggering memory corruption issues.

[kjur]

jsrsasign security advisory (2020-Jun-24):
CVE-2020-14968
RSA-PSS signature validation vulnerability by prepending zeros
GHSA-q3gh-5r98-j4h3