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fix(av-surface): no runnable base64-to-shell line on disk

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PLAN § v8.1.3 tillegg a. The bash-evasion gallery's T8 comment held a
complete base64-decode-into-shell line whose 16-char blob decodes to a
home-directory deletion, under probe (b)'s 24-char floor. Probe (b2)
found two more of the same shape, fixed here too: the gallery README's
T8 row and tests/e2e/attack-chain.test.mjs (an /etc deletion). Chosen
over an exception list because the gate should have none.

The gallery and the e2e test now encode the blob at runtime from the
same fragments; the README shows a placeholder. Gallery: 10/10 blocked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Kjell Tore Guttormsen 2026-09-23 11:35:50 +02:00
commit 58d2e8ad8d
Signed by: ktg
SSH key fingerprint: SHA256:JakMjO6FTBBzN0Bhfj9saOoEjaFxlSdYuZQQpM/lF9Q
4 changed files with 12 additions and 8 deletions
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2
examples/bash-evasion-gallery/README.md
View file

@ -35,7 +35,7 @@ techniques below are caught.
| T5 | IFS word-splitting | `rm${IFS}-rf${IFS}$HOME` | `rm -rf $HOME` |
| T6 | ANSI-C hex quoting | `$'\x72\x6d' -rf $HOME` | `rm -rf $HOME` |
| T7 | process substitution | `cat <(echo rm) -rf $HOME` | `cat echo rm -rf $HOME` |
| T8 | base64-pipe-shell idiom | `echo cm0gLXJmICRIT01F \| base64 -d \| sh` | (separate BLOCK_RULE — not normalization) |
| T8 | base64-pipe-shell idiom | `echo <base64 of the baseline command> \| base64 -d \| sh` | (separate BLOCK_RULE — not normalization) |
| T9 | eval-via-variable (one-level forward flow) | `X=rm; $X -rf $HOME` | `X=rm; rm -rf $HOME` |
The canonical destructive target throughout the gallery is

8
examples/bash-evasion-gallery/expected-findings.md
View file

@ -56,7 +56,7 @@ Code 2.1.98+ harness-level checks.
in `expected-findings.md` for documentation; the run script
only checks exit code 2, so it continues to pass after a
rename)
- The base64 blob in T8 (`cm0gLXJmICRIT01F`) decodes to the
literal command. If you change the canonical destructive
target away from `$HOME`, regenerate the blob with
`printf '<new-cmd>' | base64`
- The base64 blob in T8 (`T8_BLOB`) is built at runtime from the
same fragments as the baseline command, so changing the
canonical destructive target updates it too. No runnable
base64-to-shell line is kept on disk (v8.1.3)

7
examples/bash-evasion-gallery/run-evasion-gallery.mjs
View file

@ -51,6 +51,9 @@ const RM = 'r' + 'm';
const RF = '-' + 'rf';
const TARGET = '$' + 'HOME';
const PIPE_SH = '| ' + 'sh';
// T8's blob is the canonical command base64-encoded at runtime, so no
// runnable base64-to-shell line sits in this file (v8.1.3).
const T8_BLOB = Buffer.from(`${RM} ${RF} ${TARGET}`).toString('base64');
const CASES = [
{
@ -108,8 +111,8 @@ const CASES = [
},
{
tag: 'T8 — base64-pipe-shell idiom',
// echo cm0gLXJmICRIT01F | base64 -d | sh — base64 of "rm -rf $HOME"
command: `echo cm0gLXJmICRIT01F | base64 -d ${PIPE_SH}`,
// echo <T8_BLOB>, decoded and piped into a shell
command: `echo ${T8_BLOB} | base64 -d ${PIPE_SH}`,
note: 'separate BLOCK_RULE — not a normalization, but the same shape',
expectBlock: true,
},