export SECRET=value; cmd "$SECRET"
export SECRET=value; cmd "$SECRET"
(exec -a "$(printf %-4096s cmd)" cmd "$secret")
(exec -a "$(printf %-4096s cmd)" cmd "$secret")
With LD_PRELOADLD_PRELOAD, for non-setuid/setgid dynamically linked executables on a GNU system:
/*
* replace ***** with secret read from fd 9
* gcc -Wall -fpic -shared -o inject_secret.so inject_secret.c -ldl
* LD_PRELOAD=/.../inject_secret.so cmd -p '*****' 9<<< secret
*/
#define _GNU_SOURCE
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dlfcn.h>
#define PLACEHOLDER "*****"
static char secret[1024];
int __libc_start_main(int (*main) (int, char**, char**),
int argc,
char **argv,
void (*init) (void),
void (*fini)(void),
void (*rtld_fini)(void),
void (*stack_end)){
static int (*real_libc_start_main)() = NULL;
int n;
if (!real_libc_start_main) {
real_libc_start_main = dlsym(RTLD_NEXT, "__libc_start_main");
if (!real_libc_start_main) abort();
}
n = read(9, secret, sizeof(secret));
if (n > 0) {
int i;
if (secret[n - 1] == '\n') secret[--n] = '\0';
for (i = 1; i < argc; i++)
if (strcmp(argv[i], PLACEHOLDER) == 0)
argv[i] = secret;
}
return real_libc_start_main(main, argc, argv, init, fini,
rtld_fini, stack_end);
}
/*
* replace ***** with secret read from fd 9
* gcc -Wall -fpic -shared -o inject_secret.so inject_secret.c -ldl
* LD_PRELOAD=/.../inject_secret.so cmd -p '*****' 9<<< secret
*/
#define _GNU_SOURCE
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dlfcn.h>
#define PLACEHOLDER "*****"
static char secret[1024];
int __libc_start_main(int (*main) (int, char**, char**),
int argc,
char **argv,
void (*init) (void),
void (*fini)(void),
void (*rtld_fini)(void),
void (*stack_end)){
static int (*real_libc_start_main)() = NULL;
int n;
if (!real_libc_start_main) {
real_libc_start_main = dlsym(RTLD_NEXT, "__libc_start_main");
if (!real_libc_start_main) abort();
}
n = read(9, secret, sizeof(secret));
if (n > 0) {
int i;
if (secret[n - 1] == '\n') secret[--n] = '\0';
for (i = 1; i < argc; i++)
if (strcmp(argv[i], PLACEHOLDER) == 0)
argv[i] = secret;
}
return real_libc_start_main(main, argc, argv, init, fini,
rtld_fini, stack_end);
}
tmp=$(mktemp) && cat << EOF > "$tmp" &&
break __libc_start_main
commands 1
set argv[1]="-opid,args"
continue
end
run
EOF
gdb --batch-silent --return-child-result -x "$tmp" --args ps '*****'
rm -f -- "$tmp"
tmp=$(mktemp) && cat << EOF > "$tmp" &&
break __libc_start_main
commands 1
set argv[1]="-opid,args"
continue
end
run
EOF
gdb --batch-silent --return-child-result -x "$tmp" --args ps '*****'
rm -f -- "$tmp"
Still with gdb, an approach that doesn't rely on executables being dynamically linked or have debug symbols and should work for any ELF exectuableexecutable on Linux at least could be:
#! /bin/sh -
# gdb+sh polyglot script to replace "*****" arguments with the content
# of the SECRET environment variable *after* execve and before calling
# the executable's main() function.
#
# Usage: SECRET=somesecret cmd --password '*****'
if ':' - ':'
then
# running in sh
# retrieve the start address for the executable
start=$(
LC_ALL=C objdump -f -- "$(command -v -- "$1")" |
sed -n 's/^start address //p'
)
[ -n "$start" ] || exit
# re-exec ourselve with gdb.
exec gdb --batch-silent --return-child-result -iex "set \$start = $start" -x "$0" --args "$@"
exit 1
fi
end
# running in gdb
break *$start
commands 1
# The stack on startup contains:
# argc argv[0]... argv[argc-1] 0 envp[0] envp[1]... 0 argv[] and envp[] strings
set $argc = *((int*)$sp)
set $argv = &((char**)$sp)[1]
set $envp = &($argv[$argc+1])
set $i = 0
while $envp[$i]
# look for an envp[] string starting with "SECRET=". We can't use strcmp()
# here as there's no guarantee that the debugged executable has such
# a function
set $e = $envp[$i]
if $e[0] == 'S' && \
$e[1] == 'E' && \
$e[2] == 'C' && \
$e[3] == 'R' && \
$e[4] == 'E' && \
$e[5] == 'T' && \
$e[6] == '='
set $secret = &($e[7])
# replace SECRET=xxx<NUL> with SECRE=<NUL>
set $e[5] = '='
set $e[6] = '\0'
# not calling loop_break as that causes a SEGV with my version of gdb
end
set $i = $i + 1
end
if $secret
# now looking for argv[] strings being "*****" and replace them with
# the secret identified earlier
set $i = 0
while $i < $argc
set $a = $argv[$i]
if $a[0] == '*' && \
$a[1] == '*' && \
$a[2] == '*' && \
$a[3] == '*' && \
$a[4] == '*' && \
$a[5] == '\0'
set $argv[$i] = $secret
end
set $i = $i + 1
end
end
# using "continue" as "detach" causes a SEGV with my version of gdb.
continue
end
run
#! /bin/sh -
# gdb+sh polyglot script to replace "*****" arguments with the content
# of the SECRET environment variable *after* execve and before calling
# the executable's main() function.
#
# Usage: SECRET=somesecret cmd --password '*****'
if ':' - ':'
then
# running in sh
# retrieve the start address for the executable
start=$(
LC_ALL=C objdump -f -- "$(command -v -- "$1")" |
sed -n 's/^start address //p'
)
[ -n "$start" ] || exit
# re-exec ourselve with gdb.
exec gdb --batch-silent --return-child-result -iex "set \$start = $start" -x "$0" --args "$@"
exit 1
fi
end
# running in gdb
break *$start
commands 1
# The stack on startup contains:
# argc argv[0]... argv[argc-1] 0 envp[0] envp[1]... 0 argv[] and envp[] strings
set $argc = *((int*)$sp)
set $argv = &((char**)$sp)[1]
set $envp = &($argv[$argc+1])
set $i = 0
while $envp[$i]
# look for an envp[] string starting with "SECRET=". We can't use strcmp()
# here as there's no guarantee that the debugged executable has such
# a function
set $e = $envp[$i]
if $e[0] == 'S' && \
$e[1] == 'E' && \
$e[2] == 'C' && \
$e[3] == 'R' && \
$e[4] == 'E' && \
$e[5] == 'T' && \
$e[6] == '='
set $secret = &($e[7])
# replace SECRET=xxx<NUL> with SECRE=<NUL>
set $e[5] = '='
set $e[6] = '\0'
# not calling loop_break as that causes a SEGV with my version of gdb
end
set $i = $i + 1
end
if $secret
# now looking for argv[] strings being "*****" and replace them with
# the secret identified earlier
set $i = 0
while $i < $argc
set $a = $argv[$i]
if $a[0] == '*' && \
$a[1] == '*' && \
$a[2] == '*' && \
$a[3] == '*' && \
$a[4] == '*' && \
$a[5] == '\0'
set $argv[$i] = $secret
end
set $i = $i + 1
end
end
# using "continue" as "detach" causes a SEGV with my version of gdb.
continue
end
run
set $argc = *((int*)($sp + 24))
set $argv = &((char**)$sp)[4]
