# CVE-2026-62977: BACnet Stack: `WriteProperty(File_Size)` can bypass read-only protection and expose uninitialized RAMFS tail bytes through `AtomicReadFile`

Status: published. Published: 2026-11-13. Updated: 2026-11-13.

BACnet Stack is a BACnet open source protocol stack C library for embedded systems. Prior to version 1.5.1, the BACnet FILE object implementation allows remote WriteProperty(PROP_FILE_SIZE) requests to change the size of a stream-access FILE object without checking whether that FILE is marked Read_Only. When the FILE backend is RAMFS, enlarging the file only performs realloc() and updates the stored size; the newly exposed tail region is not initialized before later reads. As a result, a remote client can enlarge a read-only RAMFS-backed stream-access FILE and then immediately retrieve the newly exposed region with AtomicReadFile(stream-access). Depending on the allocator and build configuration, the returned data may contain sanitizer fill bytes, stale heap contents, or other unintended process memory. Shrinking the file through the same path can also remotely truncate content that was intended to be read-only. This issue has been patched in versions 1.5.1 and 1.6.0.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- bacnet-stack / bacnet-stack: < 1.5.1 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 4.0: 6.9 (medium); CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-284, CWE-908

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-mwj7-2v5r-v934 (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/bacnet-stack/bacnet-stack/pull/1411 (CNA: GitHub_M; x_refsource_MISC)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-62977; retrieved 2026-11-13; source updated 2026-11-13

Revision: b2365dba35aa96f73b5bf702036ec47087fcbf7738245795f9bf8d5a9ecb3ba8

Canonical: https://y2.dev/cve/CVE-2026-62977

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