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BACnet Stack access control bypass in RAMFS file size writes

CVE-2026-62977

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

6.9MediumCNA
Exploited
No known exploitation
Fix
Fixed in 1.5.1
Who is exposed
Not reported

Do first: Upgrade to a fixed version: 1.5.1.

Published
Last modified
Reference
Last checked
0.25% · 16th percentile
24/100 · provisional
None on Exploit-DB, Metasploit or GitHub

Summary

BACnet Stack, a BACnet protocol C library for embedded systems, does not check whether a stream-access FILE object is read-only before a remote client changes its size. When the file uses the RAMFS backend, an unauthenticated network client can enlarge a read-only file and then read the newly exposed bytes with AtomicReadFile, which may return uninitialized process memory, or shrink the file and truncate its content.

Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.

Original description

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.

cve.now Index

24Pcve.now Index 24 of 100 · methodology v3 · provisional
  • Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
  • Exposure17/25No login or user action needed per the CVSS vector, reachable over the network without a login · Unknown: No advisory context yet
  • Reach?/20Unknown. No tracked technology or package matched
  • Severity6.9/10CVSS 6.9
  • No patch0/5No signal

What would change the score

  • Would score about 69 if added to CISA KEV
  • Would score about 64 if CISA's SSVC rates exploitation active
  • Would score about 29 if advisories confirm default setups are exposed

How the score works · Rankings

What to do

  • Upgrade to a fixed version: 1.5.1.

Check your systems with an agent

Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.

Check whether my systems are affected by CVE-2026-62977 (BACnet Stack: `WriteProperty(File_Size)` can bypass read-only protection and expose uninitialized RAMFS tail bytes through `AtomicReadFile`), 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-2026-62977 (entry last updated 2026-11-13):
Affected:
- bacnet-stack bacnet-stack: fixed in 1.5.1
Fixed versions: 1.5.1.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-62977 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-62977 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-mwj7-2v5r-v934
- https://github.com/bacnet-stack/bacnet-stack/pull/1411
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-2026-62977
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-2026-62977"}.

Timeline

  1. CVE published
  2. First seen by cve.now

Affected versions

  • bacnet-stack bacnet-stack

    fixed in 1.5.1

Evidence 4

Vendor advisory · 1

Establishes: Affected and fixed versions.

  1. Advisory

CVE record · 2

Establishes: The identifier and description.

  1. Record
  2. Record

Other links · 1

  1. Repository

Corrections

None so far.

Something wrong on this page? Report an error. Every fix is listed here.

Questions about CVE-2026-62977

Is CVE-2026-62977 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: cve.now feed data

Which versions are affected by CVE-2026-62977?

bacnet-stack: before 1.5.1.Source: cve.now feed data

How do I fix CVE-2026-62977?

Upgrade to a fixed version: 1.5.1.Source: cve.now feed data

Sources

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