Headline
CVE-2018-20685: upstream: disallow empty incoming filename or ones that refer to the · openssh/openssh-portable@6010c03
In OpenSSH 7.9, scp.c in the scp client allows remote SSH servers to bypass intended access restrictions via the filename of . or an empty filename. The impact is modifying the permissions of the target directory on the client side.
@@ -1,4 +1,4 @@
/* $OpenBSD: scp.c,v 1.197 2018/06/01 04:31:48 dtucker Exp $ */
/* $OpenBSD: scp.c,v 1.198 2018/11/16 03:03:10 djm Exp $ */
/*
* scp - secure remote copy. This is basically patched BSD rcp which
* uses ssh to do the data transfer (instead of using rcmd).
@@ -1106,7 +1106,8 @@ sink(int argc, char **argv)
SCREWUP(“size out of range”);
size = (off_t)ull;
if ((strchr(cp, ‘/’) != NULL) || (strcmp(cp, “…”) == 0)) {
if (*cp == ‘\0’ || strchr(cp, ‘/’) != NULL ||
strcmp(cp, “.”) == 0 || strcmp(cp, “…”) == 0) {
run_err("error: unexpected filename: %s", cp);
exit(1);
}
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An issue was discovered in OpenSSH 7.9. Due to the scp implementation being derived from 1983 rcp, the server chooses which files/directories are sent to the client. However, the scp client only performs cursory validation of the object name returned (only directory traversal attacks are prevented). A malicious scp server (or Man-in-The-Middle attacker) 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, to overwrite the .ssh/authorized_keys file).