Excelize: extractPart allocates attacker-controlled, unbounded and negative-sized buffers from CFB directory entries: remote panic / OOM DoS

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Excelize is a Go language library for reading and writing Microsoft Excel spreadsheets. From 2.3.1 to 2.11.0, extractPart allocates a byte slice directly from an attacker-controlled CFB directory-entry size before validating the sector chain or size domain. extractPart trusts the CFB directory entry streamSize for EncryptionInfo and EncryptedPackage allocations before validating the stream. When a crafted OLE compound file declares a negative or extremely large EncryptionInfo or EncryptedPackage stream size, the declared size reaches make with a negative length or forces a multi-gigabyte allocation, allowing an attacker to panic or exhaust process memory. No fixed version is available as of this review.

Source published

7 Oct 2026 17:48:03 UTC

Source modified

10 Oct 2026 02:38:31 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Excelize is a Go language library for reading and writing Microsoft Excel spreadsheets. From 2.3.1 to 2.11.0, extractPart allocates a byte slice directly from an attacker-controlled CFB directory-entry size before validating the sector chain or size domain. extractPart trusts the CFB directory entry streamSize for EncryptionInfo and EncryptedPackage allocations before validating the stream. When a crafted OLE compound file declares a negative or extremely large EncryptionInfo or EncryptedPackage stream size, the declared size reaches make with a negative length or forces a multi-gigabyte allocation, allowing an attacker to panic or exhaust process memory. No fixed version is available as of this review.

Source published

Oct 7, 2026, 5:48:03 PM (timezone not stated)

Source modified

Oct 10, 2026, 2:38:31 AM (timezone not stated)

Observed here

7 Nov 2026 12:38:38 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Excelize is a Go language library for reading and writing Microsoft Excel spreadsheets. From 2.3.1 to 2.11.0, extractPart allocates a byte slice directly from an attacker-controlled CFB directory-entry size before validating the sector chain or size domain. extractPart trusts the CFB directory entry streamSize for EncryptionInfo and EncryptedPackage allocations before validating the stream. When a crafted OLE compound file declares a negative or extremely large EncryptionInfo or EncryptedPackage stream size, the declared size reaches make with a negative length or forces a multi-gigabyte allocation, allowing an attacker to panic or exhaust process memory. No fixed version is available as of this review.

Source published

7 Oct 2026 18:17:19 UTC

Source modified

13 Oct 2026 14:23:50 UTC

Observed here

25 Oct 2026 04:17:41 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

### Summary `extractPart` allocates directly from the mscfb directory-entry size with no clamping (`crypt.go:198`, same missing bound at `crypt.go:204`): ```go buf := make([]byte, entry.Size) ``` `entry.Size` is the raw `streamSize` field from the attacker-controlled CFB directory entry (mscfb v1.0.8 `fixFile`, `file.go:109-120`: full uint64 → int64 for major version 4, low uint32 for v3). mscfb only detects short sector chains later, inside `File.Read` (`file.go:485-489` "emergency brake") — i.e. **after** the allocation. The zip path already enforces `UnzipSizeLimit`, and the KDF path bounds work with `maxSpinCount`; this path consults no limit at all. ### Details Two consequences, both execution-confirmed against pristine master (`ecd99d761fe0`, 2026-09-08): 1. **Hard crash**: a version-4 CFB declaring `streamSize = 0xFFFFFFFFFFFFFFFF` yields `entry.Size == -1` → `panic: runtime error: makeslice: len out of range`, which escapes `Decrypt` (`openReaderAt` converts errors, not panics, `excelize.go:210-214`) and kills the process. 2. **Memory exhaustion**: any CFB (v3 suffices) declaring `streamSize` up to `0xFFFFFFFF` forces up to 4 GiB of zeroed allocation from a ~12 KB file — an amplification factor of ~350,000×, trivially repeated per request in memory-limited deployments. Call path: `OpenFile`/`OpenReader`/`OpenBytes` → `openReaderAt` (`excelize.go:208-215`, OLE magic sniff) → `Decrypt` (`crypt.go:145-150`) → `extractPart` (`crypt.go:198`). ### PoC A standalone program (public API only) was provided to the maintainer by email (`2-extractpart-oom`): it builds a valid-enough v4 (4096-byte sector) CFB — header (signature, sector shift `0x0C`, `DIFAT[0]=1`), one directory sector with Root Entry (typeID 5, child=1) and a stream entry `EncryptionInfo` (objectType=2, startingSector=endOfChain, `streamSize=0xFFFFFFFFFFFFFFFF`). `mscfb.New` succeeds, `doc.Next()` returns the entry, and `extractPart` calls `make([]byte, -1)` before any chain validation. On master `ecd99d761fe0` the program prints `panic: runtime error: makeslice: len out of range`; with the proposed patch it prints BLOCKED. An `-oom` flag demonstrates the 4 GiB allocation variant with `streamSize = 0xFFFFFFFF`. ### Impact An attacker uploads any file starting with the OLE signature `D0 CF 11 E0 A1 B1 1A E1` (~12 KB is enough) to a service that calls `excelize.OpenFile`/`OpenReader`/`OpenBytes` on it (or calls `Decrypt` directly): remote, unauthenticated process crash (hard panic) or forced 4 GiB allocation per request (OOM) with a ~350,000× amplification factor. ### Proposed fix Bound the declared stream size before allocating in `extractPart`: reject negative sizes and sizes above a sane policy cap (1 GiB is far above any real encrypted-workbook part) with `ErrWorkbookFileFormat`, so malformed CFBs are rejected as errors instead of panicking or OOMing. A complete patch has been provided to the maintainer; verified that the patch blocks the PoC while legitimate `Encrypt()` output still opens.

Source published

8 Oct 2026 16:31:26 UTC

Source modified

19 Oct 2026 21:41:17 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH

Panic and memory exhaustion via unbounded CFB stream allocation in github.com/xuri/excelize

Source published

19 Oct 2026 21:08:21 UTC

Source modified

19 Oct 2026 21:31:52 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.