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Disallow leading lines before the ident for server (#102)

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Per RFC4253 4.2 clients must be able to process other lines of data
before the version string, server behavior is not defined neither
with MUST/SHOULD nor with MAY.
If server process up to 50 lines too - it may cause too long hanging
session with invalid/evil client that consume host resources and
potentially may lead to DDoS on poor embedded boxes.
Let's require first line from client to be version string and fail
early if it's not - matches both RFC and real OpenSSH behavior.
This commit is contained in:
Vladislav Grishenko 2020-06-15 18:22:18 +05:00 committed by GitHub
parent a27e8b053e
commit 2301b6ac0b
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 5 additions and 2 deletions
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7
common-session.c
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@ -370,8 +370,11 @@ static void read_session_identification() {
int len = 0;
char done = 0;
int i;
/* If they send more than 50 lines, something is wrong */
for (i = 0; i < 50; i++) {
/* Servers may send other lines of data before sending the
* version string, client must be able to process such lines.
* If they send more than 50 lines, something is wrong */
for (i = IS_DROPBEAR_CLIENT ? 50 : 1; i > 0; i--) {
len = ident_readln(ses.sock_in, linebuf, sizeof(linebuf));
if (len < 0 && errno != EINTR) {