SCP Server Verification Issues

2019.01.16
Risk: Medium
Local: Yes
Remote: No

scp client multiple vulnerabilities =================================== The latest version of this advisory is available at: https://sintonen.fi/advisories/scp-client-multiple-vulnerabilities.txt Overview -------- SCP clients from multiple vendors are susceptible to a malicious scp server performing unauthorized changes to target directory and/or client output manipulation. Description ----------- Many scp clients fail to verify if the objects returned by the scp server match those it asked for. This issue dates back to 1983 and rcp, on which scp is based. A separate flaw in the client allows the target directory attributes to be changed arbitrarily. Finally, two vulnerabilities in clients may allow server to spoof the client output. Impact ------ Malicious scp server can write arbitrary files to scp target directory, change the target directory permissions and to spoof the client output. Details ------- The discovered vulnerabilities, described in more detail below, enables the attack described here in brief. 1. The attacker controlled server or Man-in-the-Middle(*) attack drops .bash_aliases file to victim's home directory when the victim performs scp operation from the server. The transfer of extra files is hidden by sending ANSI control sequences via stderr. For example: user@local:~$ scp user@remote:readme.txt . readme.txt 100% 494 1.6KB/s 00:00 user@local:~$ 2. Once the victim launches a new shell, the malicious commands in .bash_aliases get executed. *) Man-in-the-Middle attack does require the victim to accept the wrong host fingerprint. Vulnerabilities --------------- 1. CWE-20: scp client improper directory name validation [CVE-2018-20685] The scp client allows server to modify permissions of the target directory by using empty ("D0777 0 \n") or dot ("D0777 0 .\n") directory name. 2. CWE-20: scp client missing received object name validation [CVE-2019-6111] Due to the scp implementation being derived from 1983 rcp [1], the server chooses which files/directories are sent to the client. However, scp client only perform cursory validation of the object name returned (only directory traversal attacks are prevented). A malicious scp server can overwrite arbitrary files in the scp client target directory. If recursive operation (-r) is performed, the server can manipulate subdirectories as well (for example overwrite .ssh/authorized_keys). The same vulnerability in WinSCP is known as CVE-2018-20684. 3. CWE-451: scp client spoofing via object name [CVE-2019-6109] Due to missing character encoding in the progress display, the object name can be used to manipulate the client output, for example to employ ANSI codes to hide additional files being transferred. 4. CWE-451: scp client spoofing via stderr [CVE-2019-6110] Due to accepting and displaying arbitrary stderr output from the scp server, a malicious server can manipulate the client output, for example to employ ANSI codes to hide additional files being transferred. Proof-of-Concept ---------------- Proof of concept malicious scp server will be released at a later date. Vulnerable versions ------------------- The following software packages have some or all vulnerabilities: ver #1 #2 #3 #4 OpenSSH scp <=7.9 x x x x PuTTY PSCP ? - - x x WinSCP scp mode <=5.13 - x - - Tectia SSH scpg3 is not affected since it exclusively uses sftp protocol. Mitigation ---------- 1. OpenSSH 1.1 Switch to sftp if possible 1.2 Alternatively apply the following patch to harden scp against most server-side manipulation attempts: https://sintonen.fi/advisories/scp-name-validator.patch NOTE: This patch may cause problems if the the remote and local shells don't agree on the way glob() pattern matching works. YMMV. 2. PuTTY 2.1 No fix is available yet 3. WinSCP 3.1. Upgrade to WinSCP 5.14 or later Similar or prior work --------------------- 1. CVE-2000-0992 - scp overwrites arbitrary files References ---------- 1. https://www.jeffgeerling.com/blog/brief-history-ssh-and-remote-access Credits ------- The vulnerability was discovered by Harry Sintonen / F-Secure Corporation. Timeline -------- 2018.08.08 initial discovery of vulnerabilities #1 and #2 2018.08.09 reported vulnerabilities #1 and #2 to OpenSSH 2018.08.10 OpenSSH acknowledged the vulnerabilities 2018.08.14 discovered & reported vulnerability #3 to OpenSSH 2018.08.15 discovered & reported vulnerability #4 to OpenSSH 2018.08.30 reported PSCP vulnerabilities (#3 and #4) to PuTTY developers 2018.08.31 reported WinSCP vulnerability (#2) to WinSCP developers 2018.09.04 WinSCP developers reported the vulnerability #2 fixed 2018.11.12 requested a status update from OpenSSH 2018.11.16 OpenSSH fixed vulnerability #1 2019.01.07 requested a status update from OpenSSH 2019.01.08 requested CVE assignments from MITRE 2019.01.10 received CVE assignments from MITRE 2019.01.11 public disclosure of the advisory 2019.01.14 added a warning about the potential issues caused by the patch


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