ImageSharp: CCITT fax decompression (T4/Modified Huffman): unbounded WriteBits overflows strip buffer — heap OOB write in SixLabors.ImageSharp

{
  "id": "2cca8940662d46257f254fe140490a6ea2ddb9537fbaff432a803723bf2ed31c",
  "url": "https://osv.dev/vulnerability/GHSA-jj3q-cwqj-842r",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "ImageSharp: CCITT fax decompression (T4/Modified Huffman): unbounded WriteBits overflows strip buffer — heap OOB write in SixLabors.ImageSharp",
    "summary": "## Summary When 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 > 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). Verified 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. ## Details Root cause: the bit-writing sink",
    "evidence": "osv:GHSA-jj3q-cwqj-842r",
    "products": [
      {
        "name": {
          "value": "ImageSharp",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "3.2.0",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      },
      {
        "name": {
          "value": "ImageSharp",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "4.1.1",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "GHSA-jj3q-cwqj-842r",
      "aliases": [
        "CVE-2026-106117",
        "GHSA-jj3q-cwqj-842r"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "3.0.0"
                },
                {
                  "fixed": "3.2.0"
                }
              ]
            }
          ],
          "package": {
            "name": "ImageSharp",
            "purl": "pkg:nuget/ImageSharp",
            "ecosystem": "NuGet"
          },
          "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"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "4.0.0"
                },
                {
                  "fixed": "4.1.1"
                }
              ]
            }
          ],
          "package": {
            "name": "ImageSharp",
            "purl": "pkg:nuget/ImageSharp",
            "ecosystem": "NuGet"
          },
          "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"
          }
        }
      ],
      "modified": "2026-10-12T14:45:06.701457452Z",
      "provider": "osv",
      "references": [
        "https://github.com/SixLabors/ImageSharp/security/advisories/GHSA-jj3q-cwqj-842r",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-106117",
        "https://github.com/SixLabors/ImageSharp/pull/3176",
        "https://github.com/SixLabors/ImageSharp/commit/e8892ecd8a95dee95c4480fda51cfe0b32f3b846",
        "https://github.com/SixLabors/ImageSharp",
        "https://github.com/SixLabors/ImageSharp/releases/tag/v3.2.0",
        "https://github.com/SixLabors/ImageSharp/releases/tag/v4.1.1"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-787"
        ],
        "severity": "HIGH",
        "github_reviewed": true,
        "nvd_published_at": "2026-10-06T18:16:53Z",
        "github_reviewed_at": "2026-10-07T16:18:50Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-106117",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 299963,
  "published_at": "2026-10-07T16:18:50+00:00"
}