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Properly store certificate exceptions in Java viewer
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Like the native viewer, the Java viewer didn't store certificate
exceptions properly. Whilst not as bad as the native viewer, it still
failed to check that a stored certificate wouldn't be maliciously used
for another server. In practice this can in most cases be used to
impersonate another server.

Handle this like the native viewer by storing exceptions for a specific
hostname/certificate combination.

(cherry picked from commit f029745)
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@bphinz @CendioOssman
bphinz authored and CendioOssman committed Sep 8, 2020
1 parent 7399eab commit 20dea80
Showing 1 changed file with 101 additions and 63 deletions.
164 changes: 101 additions & 63 deletions java/com/tigervnc/rfb/CSecurityTLS.java
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Original file line number Diff line number Diff line change
Expand Up @@ -107,12 +107,6 @@ public static void setDefaults()
X509CRL.setDefaultStr(getDefaultCRL());
}

// FIXME:
// Need to shutdown the connection cleanly

// FIXME?
// add a finalizer method that calls shutdown

public boolean processMsg(CConnection cc) {
is = (FdInStream)cc.getInStream();
os = (FdOutStream)cc.getOutStream();
Expand Down Expand Up @@ -269,8 +263,13 @@ public void checkServerTrusted(X509Certificate[] chain, String authType)
{
Collection<? extends Certificate> certs = null;
X509Certificate cert = chain[0];
String pk =
Base64.getEncoder().encodeToString(cert.getPublicKey().getEncoded());
try {
cert.checkValidity();
verifyHostname(cert);
} catch(CertificateParsingException e) {
throw new SystemException(e.getMessage());
} catch(CertificateNotYetValidException e) {
throw new AuthFailureException("server certificate has not been activated");
} catch(CertificateExpiredException e) {
Expand All @@ -279,73 +278,111 @@ public void checkServerTrusted(X509Certificate[] chain, String authType)
"do you want to continue?"))
throw new AuthFailureException("server certificate has expired");
}
String thumbprint = getThumbprint(cert);
File vncDir = new File(FileUtils.getVncHomeDir());
File certFile = new File(vncDir, "x509_savedcerts.pem");
CertificateFactory cf = CertificateFactory.getInstance("X.509");
if (vncDir.exists() && certFile.exists() && certFile.canRead()) {
InputStream certStream = new MyFileInputStream(certFile);
certs = cf.generateCertificates(certStream);
for (Certificate c : certs)
if (thumbprint.equals(getThumbprint((X509Certificate)c)))
return;
}
if (!vncDir.exists())
throw new AuthFailureException("Could not obtain VNC home directory "+
"path for known hosts storage");
File dbPath = new File(vncDir, "x509_known_hosts");
String info =
" Subject: "+cert.getSubjectX500Principal().getName()+"\n"+
" Issuer: "+cert.getIssuerX500Principal().getName()+"\n"+
" Serial Number: "+cert.getSerialNumber()+"\n"+
" Version: "+cert.getVersion()+"\n"+
" Signature Algorithm: "+cert.getPublicKey().getAlgorithm()+"\n"+
" Not Valid Before: "+cert.getNotBefore()+"\n"+
" Not Valid After: "+cert.getNotAfter()+"\n"+
" SHA-1 Fingerprint: "+getThumbprint(cert)+"\n";
try {
verifyHostname(cert);
if (dbPath.exists()) {
FileReader db = new FileReader(dbPath);
BufferedReader dbBuf = new BufferedReader(db);
String line;
String server = client.getServerName().toLowerCase();
while ((line = dbBuf.readLine())!=null) {
String fields[] = line.split("\\|");
if (fields.length==6) {
if (server.equals(fields[2]) && pk.equals(fields[5])) {
vlog.debug("Server certificate found in known hosts file");
dbBuf.close();
return;
} else if (server.equals(fields[2]) && !pk.equals(fields[5]) ||
!server.equals(fields[2]) && pk.equals(fields[5])) {
throw new CertStoreException();
}
}
}
dbBuf.close();
}
tm.checkServerTrusted(chain, authType);
} catch (IOException e) {
throw new AuthFailureException("Could not load known hosts database");
} catch (CertStoreException e) {
vlog.debug("Server host key mismatch");
vlog.debug(info);
String text =
"This host is previously known with a different "+
"certificate, and the new certificate has been "+
"signed by an unknown authority\n"+
"\n"+info+"\n"+
"Someone could be trying to impersonate the site and you should not continue.\n"+
"\n"+
"Do you want to make an exception for this server?";
if (!msg.showMsgBox(YES_NO_OPTION, "Unexpected certificate issuer", text))
throw new AuthFailureException("Unexpected certificate issuer");
store_pubkey(dbPath, client.getServerName().toLowerCase(), pk);
} catch (java.lang.Exception e) {
if (e.getCause() instanceof CertPathBuilderException) {
String certinfo =
vlog.debug("Server host not previously known");
vlog.debug(info);
String text =
"This certificate has been signed by an unknown authority\n"+
"\n"+info+"\n"+
"Someone could be trying to impersonate the site and you should not continue.\n"+
"\n"+
" Subject: "+cert.getSubjectX500Principal().getName()+"\n"+
" Issuer: "+cert.getIssuerX500Principal().getName()+"\n"+
" Serial Number: "+cert.getSerialNumber()+"\n"+
" Version: "+cert.getVersion()+"\n"+
" Signature Algorithm: "+cert.getPublicKey().getAlgorithm()+"\n"+
" Not Valid Before: "+cert.getNotBefore()+"\n"+
" Not Valid After: "+cert.getNotAfter()+"\n"+
" SHA1 Fingerprint: "+getThumbprint(cert)+"\n"+
"\n"+
"Do you want to save it and continue?";
if (!msg.showMsgBox(YES_NO_OPTION, "certificate issuer unknown",
certinfo)) {
throw new AuthFailureException("certificate issuer unknown");
}
if (certs == null || !certs.contains(cert)) {
byte[] der = cert.getEncoded();
String pem = Base64.getEncoder().encodeToString(der);
pem = pem.replaceAll("(.{64})", "$1\n");
FileWriter fw = null;
try {
if (!vncDir.exists())
vncDir.mkdir();
if (!certFile.exists() && !certFile.createNewFile()) {
vlog.error("Certificate save failed.");
} else {
fw = new FileWriter(certFile.getAbsolutePath(), true);
fw.write("-----BEGIN CERTIFICATE-----\n");
fw.write(pem+"\n");
fw.write("-----END CERTIFICATE-----\n");
}
} catch (IOException ioe) {
msg.showMsgBox(OK_OPTION, "certificate save failed",
"Could not save the certificate");
} finally {
try {
if (fw != null)
fw.close();
} catch(IOException ioe2) {
throw new Exception(ioe2.getMessage());
}
}
}
"Do you want to make an exception for this server?";
if (!msg.showMsgBox(YES_NO_OPTION, "Unknown certificate issuer", text))
throw new AuthFailureException("Unknown certificate issuer");
store_pubkey(dbPath, client.getServerName().toLowerCase(), pk);
} else {
throw new SystemException(e.getMessage());
}
}
}

private void store_pubkey(File dbPath, String serverName, String pk)
{
ArrayList<String> lines = new ArrayList<String>();
File vncDir = new File(FileUtils.getVncHomeDir());
try {
if (dbPath.exists()) {
FileReader db = new FileReader(dbPath);
BufferedReader dbBuf = new BufferedReader(db);
String line;
while ((line = dbBuf.readLine())!=null) {
String fields[] = line.split("\\|");
if (fields.length==6)
if (!serverName.equals(fields[2]) && !pk.equals(fields[5]))
lines.add(line);
}
dbBuf.close();
}
} catch (IOException e) {
throw new AuthFailureException("Could not load known hosts database");
}
try {
if (!dbPath.exists())
dbPath.createNewFile();
FileWriter fw = new FileWriter(dbPath.getAbsolutePath(), false);
Iterator i = lines.iterator();
while (i.hasNext())
fw.write((String)i.next()+"\n");
fw.write("|g0|"+serverName+"|*|0|"+pk+"\n");
fw.close();
} catch (IOException e) {
vlog.error("Failed to store server certificate to known hosts database");
}
}

public X509Certificate[] getAcceptedIssuers ()
{
return tm.getAcceptedIssuers();
Expand Down Expand Up @@ -399,12 +436,13 @@ private void verifyHostname(X509Certificate cert)
}
Object[] answer = {"YES", "NO"};
int ret = JOptionPane.showOptionDialog(null,
"Hostname verification failed. Do you want to continue?",
"Hostname Verification Failure",
"Hostname ("+client.getServerName()+") does not match the"+
" server certificate, do you want to continue?",
"Certificate hostname mismatch",
JOptionPane.YES_NO_OPTION, JOptionPane.WARNING_MESSAGE,
null, answer, answer[0]);
if (ret != JOptionPane.YES_OPTION)
throw new WarningException("Hostname verification failed.");
throw new WarningException("Certificate hostname mismatch.");
} catch (CertificateParsingException e) {
throw new SystemException(e.getMessage());
} catch (InvalidNameException e) {
Expand Down

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