{"id":"GHSA-jj3q-cwqj-842r","summary":"ImageSharp: CCITT fax decompression (T4/Modified Huffman): unbounded WriteBits overflows strip buffer — heap OOB write in SixLabors.ImageSharp","details":"## Summary\n\nWhen decoding a fax-compressed strip TIFF (Compression=3 / Group 3 1D, or Compression=2 / Modified Huffman), the CCITT decompressors write decoded runs through `BitWriterUtils.WriteBits/WriteBit/WriteZeroBit`, which advance and write bits via `Unsafe.Add` with read-modify-write semantics — **without ever comparing the write position against the target buffer length**. The strip buffer is sized `ImageWidth × RowsPerStrip` (8 bytes in the PoC), but two independent defects let an attacker write tens of millions of bits past it: (a) T4 only increments `rowsWritten` when an EOL code is read, and one `ReadNextRun` accumulates unlimited makeup codes (+2560 px per 12-bit code) into a single `RunLength` that `WritePixelRun` then writes in one shot; (b) Modified Huffman writes **before** validating (`pixelsWritten \u003e Width` is checked at :90-93, after the write at :56-63), so the overflow completes even though an exception is thrown later. One crafted ~90 KB file writes ~19.2 MB linearly past an 8-byte buffer and deterministically kills the process; the write offset and length are fully attacker-controlled (classic heap-corruption primitive on the managed heap).\n\nVerified at commit `5cd4d0d26a82a9549f297a237aea9cf665bddff8` (main; latest release v4.1.0, the supported major). A related tiled-path variant (tile-buffer width mismatch) was reported separately as GHSA-v76p-62qx-wwq2 — this report covers the distinct strip-path root cause.\n\n## Details\n\nRoot cause: the bit-writing sink has no bounds check, and neither decompressor constrains run lengths against the strip buffer.\n\n- Unchecked write primitive: [BitWriterUtils.cs#L51](https://github.com/SixLabors/ImageSharp/blob/5cd4d0d26a82a9549f297a237aea9cf665bddff8/src/ImageSharp/Formats/Tiff/Compression/BitWriterUtils.cs#L51) (`WriteBit`), :58 (`WriteZeroBit` — also read-modify-write, so even all-white runs really write), :11 (`WriteBits`)\n- T4 trigger: [T4TiffCompression.cs#L75](https://github.com/SixLabors/ImageSharp/blob/5cd4d0d26a82a9549f297a237aea9cf665bddff8/src/ImageSharp/Formats/Tiff/Compression/Decompressors/T4TiffCompression.cs#L75) and L109-L119 — `rowsWritten` only increments on EOL; consecutive makeup codes accumulate into one `RunLength`, then `WritePixelRun` writes it in full\n- MH trigger: [ModifiedHuffmanTiffCompression.cs#L56-L63](https://github.com/SixLabors/ImageSharp/blob/5cd4d0d26a82a9549f297a237aea9cf665bddff8/src/ImageSharp/Formats/Tiff/Compression/Decompressors/ModifiedHuffmanTiffCompression.cs#L56-L63) — write happens before the width check at L90-L93\n- Buffer size: [TiffDecoderCore.cs#L932-L967](https://github.com/SixLabors/ImageSharp/blob/5cd4d0d26a82a9549f297a237aea9cf665bddff8/src/ImageSharp/Formats/Tiff/TiffDecoderCore.cs#L932-L967) — `CalculateStripBufferSize` = width × bpp/8 × rowsPerStrip\n\nAttack surface: `Image.Load(stream)` on an attacker-supplied strip TIFF (`TiffDecoderCore.DecodeStripsChunky` → `TiffDecompressorsFactory.Create` → `Decompress`). Default configuration, no authentication, no user interaction — a plain strip TIFF (far more common than the tiled variant) with Compression=2 or 3 and a chain of CCITT makeup codes.\n\nRelationship to GHSA-v76p-62qx-wwq2: that report is the **tiled** variant — a caller-side width mismatch (`TiffDecompressorsFactory` drops tile parameters) that overflows with perfectly legal run codes. This report is the **strip** variant — the callee-side missing bounds check plus T4's EOL-only row accounting and MH's write-before-validate. Fixing the caller mismatch does not address this vector; bounding `BitWriterUtils` addresses both (see remediation).\n\n**Suggested remediation:**\n1. Bound `BitWriterUtils.WriteBits/WriteBit/WriteZeroBit` against `buffer.Length*8` (return bool / throw `ImageFormatException`) — do not rely on caller discipline.\n2. In the T4/MH decompress loops, validate `(bitsWritten + RunLength) \u003c= buffer.Length*8` **before** writing; abort T4 when accumulated rows exceed stripHeight instead of waiting for the loop to end naturally.\n3. In Modified Huffman, move the `pixelsWritten \u003e Width` check before the actual write.\n4. Regression fuzz cases: Compression=2/3, EOL-less oversized makeup chains, edge widths; strip and tiled paths share the same constraint.\n\n## PoC\n\nFull PoC posted as the first comment below: `Program.cs` (driver), `poc-tiff-t4.tif` + `poc-tiff-mh.tif` (crafted files, ~90 KB each, base64 inline), README.\n\n1. Build a console project referencing `src/ImageSharp/ImageSharp.csproj`, run it against either crafted file (or call `Image.Load` on it).\n2. PoC layout: TIFF (II, 42), ImageWidth=64, ImageLength=1, BitsPerSample=1, Photometric=WhiteIsZero, RowsPerStrip=1; strip data = `EOL(12bit)` + 60000× `white makeup 2560 (000000011111)` + `white terminating code (000111)` — declared run ≈ 153,600,001 px ≈ 19.2 MB into an 8-byte buffer.\n3. Observed (both variants):\n\n   ```\n   strip payload 90003 bytes, claimed run ~153,600,001 px = 19,200,000 bytes into 8-byte buffer\n   Fatal error. System.AccessViolationException: Attempted to read or write protected memory.\n      at SixLabors.ImageSharp.Formats.Tiff.Compression.BitWriterUtils.WriteBits(Span`1\u003cByte\u003e, IntPtr, IntPtr, Byte)\n      at ...ModifiedHuffmanTiffCompression.Decompress(...)\n      at SixLabors.ImageSharp.Formats.Tiff.TiffDecoderCore.DecodeStripsChunky[Rgba32](...)\n   Aborted (core dumped); exit=134\n   ```\n\n   The T4 variant crashes identically at `T4TiffCompression.WritePixelRun → BitWriterUtils.WriteBits`.\n4. Control: an equivalent file without the makeup chain (legal EOL-delimited rows) decodes normally — the crash comes from the oversized runs, not the container.\n\n## Impact\n\n- **What it is:** out-of-bounds write (CWE-787). For any service decoding untrusted TIFFs (image hosting/transcoding/thumbnails/CMS): reliable remote DoS (uncatchable fatal process crash), plus a heap OOB write with attacker-controlled offset and length — a realistic heap-corruption / potential code-execution surface. In scope of your SECURITY.md as a library vulnerability.\n- **Who is impacted:** applications decoding untrusted TIFF with SixLabors.ImageSharp at the current major (verified on main past v4.1.0); plain strip TIFFs with Compression=2/3 are the trigger, which ordinary TIFF writers can produce.\n\n---\n\n---\n\nReported by **Kimi Security Team** (bug-report@moonshot.ai).","aliases":["CVE-2026-106117"],"modified":"2026-10-07T16:30:04.883311002Z","published":"2026-10-07T16:18:50Z","database_specific":{"github_reviewed":true,"github_reviewed_at":"2026-10-07T16:18:50Z","nvd_published_at":"2026-10-06T18:16:53Z","cwe_ids":["CWE-787"],"severity":"HIGH"},"references":[{"type":"WEB","url":"https://github.com/SixLabors/ImageSharp/security/advisories/GHSA-jj3q-cwqj-842r"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-106117"},{"type":"WEB","url":"https://github.com/SixLabors/ImageSharp/pull/3176"},{"type":"WEB","url":"https://github.com/SixLabors/ImageSharp/commit/e8892ecd8a95dee95c4480fda51cfe0b32f3b846"},{"type":"PACKAGE","url":"https://github.com/SixLabors/ImageSharp"},{"type":"WEB","url":"https://github.com/SixLabors/ImageSharp/releases/tag/v4.1.1"}],"affected":[{"package":{"name":"ImageSharp","ecosystem":"NuGet","purl":"pkg:nuget/ImageSharp"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.0.0"},{"fixed":"4.1.1"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-jj3q-cwqj-842r/GHSA-jj3q-cwqj-842r.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}