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Set a maximum packet size of 32768 bytes for version and cleartext packets #49

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Set a maximum packet size of 32768 bytes for version and cleartext packets #49

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97bef1b
Set a maximum packet size of 32768 bytes for version and cleartext pa…
Joannis Nov 20, 2020
3368ea9
Merge branch 'main' into jo-ssh-packet-limits
Joannis Feb 26, 2021
b73a357
Merge remote-tracking branch 'apple/main' into jo-ssh-packet-limits
Joannis Nov 11, 2021
13c7a3d
Make maximumPacketSize configurable and test if large packets are rej…
Joannis Nov 16, 2021
ffbf2df
Implement Cory's feedback
Nov 23, 2021
0f28c55
Merge remote-tracking branch 'apple/main' into jo-ssh-packet-limits
Nov 23, 2021
2379347
Define a default maximumPacketSize
Joannis Nov 25, 2021
8345d00
Update `testWeDealWithFlowControlProperly` to configurable maxPacketSize
Joannis Nov 25, 2021
10aa6ef
Run soundness script for code formatting.
gwynne Dec 4, 2021
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21 changes: 19 additions & 2 deletions Sources/NIOSSH/SSHPacketParser.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@ struct SSHPacketParser {

private var buffer: ByteBuffer
private var state: State
private let maximumPacketSize = 32768

/// Testing only: the number of bytes we can discard from this buffer.
internal var _discardableBytes: Int {
Expand Down Expand Up @@ -71,6 +72,10 @@ struct SSHPacketParser {
return nil
case .cleartextWaitingForLength:
if let length = self.buffer.getInteger(at: self.buffer.readerIndex, as: UInt32.self) {
if length >= self.maximumPacketSize {
throw NIOSSHError.invalidEncryptedPacketLength
}

if let message = try self.parsePlaintext(length: length) {
self.state = .cleartextWaitingForLength
return message
Expand Down Expand Up @@ -114,7 +119,13 @@ struct SSHPacketParser {
private mutating func readVersion() throws -> String? {
// Looking for a string ending with \r\n
let slice = self.buffer.readableBytesView
if let cr = slice.firstIndex(of: 13), cr.advanced(by: 1) < slice.endIndex, slice[cr.advanced(by: 1)] == 10 {

// Prevent the consumed bytes for a version from exceeding the maximum packet size
// In practice, if SSH version packets come anywhere near this it's already likely an attack
// More data cannot be blindly regarded as malicious though, since these might be multiple packets
let maximumVersionSize = min(slice.endIndex, self.maximumPacketSize)
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ByteBufferView is not necessarily zero-indexed, so self.maximumPacketSize has to be translated into index-space first. I think this should be let maxIndex = slice.index(slice.startIndex, offsetBy: min(slice.count, self.maximumPacketSize)).


if let cr = slice.firstIndex(of: 13), cr.advanced(by: 1) < maximumVersionSize, slice[cr.advanced(by: 1)] == 10 {
let version = String(decoding: slice[slice.startIndex ..< cr], as: UTF8.self)
// read \r\n
self.buffer.moveReaderIndex(forwardBy: slice.startIndex.distance(to: cr).advanced(by: 2))
Expand All @@ -132,7 +143,13 @@ struct SSHPacketParser {
try protection.decryptFirstBlock(&self.buffer)

// This force unwrap is safe because we must have a block size, and a block size is always going to be more than 4 bytes.
return self.buffer.getInteger(at: self.buffer.readerIndex)! + UInt32(protection.macBytes)
let packetLength = self.buffer.getInteger(at: self.buffer.readerIndex, as: UInt32.self)!

if packetLength >= self.maximumPacketSize {
throw NIOSSHError.invalidEncryptedPacketLength
}

return packetLength + UInt32(protection.macBytes)
}

private mutating func parsePlaintext(length: UInt32) throws -> SSHMessage? {
Expand Down