Low Severity
CVE-2026-111497
Published: Nov 2, 2026
CVE-2026-111497: Node.js Brotli decoder OOB write — analysis, PoC, Docker lab
# CVE-2026-111497: Node.js Brotli Decoder Out-of-Bounds Write
> Chinese version above, English version below.
This repository documents our reproduction and analysis of CVE-2026-111497. All experiments were completed locally or on self-built test machines, with no access to any third-party systems.
## Summary of Findings
| Capability | Result | Conditions |
|---|---|---|
| Local command execution | ✅ 3/3 reproducible | Node + unpatched libbrotlidec + deterministic layout (`--predictable` or ASLR disabled) |
| Denial of service | ✅ 86-byte stream sufficient | Any unpatched version, no high traffic required |
| Remote overflow trigger | ✅ Demonstrated | Target process accepts two attacker-controlled writes |
| Remote process memory read | ✅ In inspector-exposed scenarios | Process running with `--inspect` |
| Remote blind RCE | ❌ Not achieved | ASLR enabled, no `--predictable`, wrap compensation unpredictable |
The last point is not due to lack of effort, but because the vulnerability itself only provides "16-byte write relative to ring," with no built-in address leak. Public PoCs also stop at the same point. Section 5 details the specific blockers.
## 1. Vulnerability Details
Node.js's built-in Brotli decoder in the streaming path of `BrotliDecoderDecompressStream` lacks length validation for tail copies to `BrotliDecoderState.buffer.u8`.
- Affected: Node 22.x < 22.23.4, 24.x < 24.22.1, 26.x < 26.12.1; EOL branches no longer receive fixes.
- Fix commit: `google/brotli@0ed4268a4401abcb4472096340477cf4af7d1a2b` ("Detect and report invalid stream earlier"), with Node.js simultaneously exposing `BROTLI_DECODER_ERROR_FORMAT_BLOCK_SWITCH` in `src/node_zlib.cc`.
- Oldest reproducible commit: `34f020fa9daba975747b29885314c2545b7aa040`.
Triggering requires approximately 23 MB of stream data: first a 16 MiB compressed metablock filled with 2^24 zero-output dictionary transform commands, exhausting `block_length[2]`; then the decoder returns `NEEDS_MORE_INPUT` but has consumed all input. Writing a small segment afterward copies attacker bytes into the 8-byte `buffer.u8[]`, crossing structure boundaries.
## 2. Environment Requirements
- Linux x86-64;
- Node.js 22/24/26 with `libbrotlidec.so.1` still at unpatched version (≤ 1.2.0 / 1.1.x). Ubuntu 24.04's included `nodejs` 22.22.1 dynamically links system brotli and can reproduce directly;
- `gcc`, `python3` (for stream generation and compilation);
- Ability to read `/proc/self/mem` (PoC uses it to locate decoder state and libc);
- Deterministic layout: `setarch x86_64 -R node ...` or `node --predictable`.
Check if target uses dynamic linking:
```console
$ ldd "$(command -v node)" | grep brotli
libbrotlidec.so.1 => /usr/lib/x86_64-linux-gnu/libbrotlidec.so.1
$ node -p "process.versions.brotli"
1.2.0
```
If `ldd` has no `libbrotlidec` (common in official static builds), the vulnerable code path usually cannot be reached; switch to distribution-packaged Node.
## 3. Reproduction
Generate trigger stream (approximately 23 MB, not stored in repository):
```console
$ python3 poc/gen_stream.py stAA536.br "$(python3 -c 'print("aa"*536)')"
wrote=stAA536.br bytes=23069229 sha256=...
```
Run local PoC:
```console
$ PWNCMD='printf CVE-2026-111497 > /tmp/pwn_marker.txt' PK=4 \
node --predictable poc/pwn_local.js
```