# CVE-2026-107729: SumatraPDF: Unsigned-to-signed hdrLen validation bypass in SumatraPDF MOBI parsing causes out-of-bounds read

> An attacker can cause a denial of service in SumatraPDF by tricking a user into opening a crafted MOBI file that triggers an out-of-bounds read via integer overflow.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 8 Oct 2026
- **Updated:** 9 Oct 2026
- **Also known as:** GHSA-86hq-m6hv-67h2

## What is CVE-2026-107729?

CVE-2026-107729 is a medium-severity vulnerability in sumatrapdfreader sumatrapdf, published on 8 Oct 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

SumatraPDF is a multi-format reader for Windows. In 3.7.0.22298, src/MobiDoc.cpp narrows the untrusted unsigned mobiHdr.hdrLen field to a signed integer for validation; values above INT_MAX become negative and bypass the upper-bound check. When the EXTH flag is set, the original unsigned value is reused as a pointer offset, causing DecodeExthHeader() to read beyond the record buffer. Opening a crafted MOBI file can reliably terminate the application with a native access violation; no code execution, information disclosure, or integrity impact has been demonstrated. No fixed version is available as of this review.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-107729?


## How do I fix CVE-2026-107729?

No fix has been published for CVE-2026-107729 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-107729 being exploited?

There is no known exploitation of CVE-2026-107729 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-107729 and fix it if it is affected.

What it is: CVE-2026-107729 is a medium-severity vulnerability in sumatrapdfreader sumatrapdf, published on 8 Oct 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-107729 in the wild and no public exploit code.

Affected product: sumatrapdfreader sumatrapdf. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-107729/ (Markdown: https://insights.sbomify.com/CVE-2026-107729/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-107729
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-107729
- OSV: https://osv.dev/vulnerability/CVE-2026-107729

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-107729/
Summaries are generated and may contain mistakes; the official description above is the record.