CVE-2026-62977: BACnet Stack: `WriteProperty(File_Size)` can bypass read-only protection and expose uninitialized RAMFS tail bytes through `AtomicReadFile`
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(PROPFILESIZE) requests to change the size of a stream-access FILE object without checking whether that FILE is marked ReadOnly. 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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 1.5.1
Event History
Frequently Asked Questions
Which deployments are exposed to unintended memory disclosure?
Deployments are exposed when they use a vulnerable version prior to 1.5.1, provide a stream-access FILE object backed by RAMFS, and allow remote BACnet clients to issue WriteProperty and AtomicReadFile requests. The issue affects FILE objects marked Read_Only because the affected resize path does not enforce that protection.
What does an attacker need to do to retrieve unintended data?
A remote client must send WriteProperty for PROP_FILE_SIZE to enlarge a read-only RAMFS-backed stream-access FILE object, then use AtomicReadFile with stream access to read the newly exposed region. The disclosed bytes can include sanitizer fill bytes, stale heap contents, or other unintended process memory depending on allocator and build configuration.
Can this issue also alter protected file content?
Yes. The same WriteProperty path can shrink a read-only file, remotely truncating content that was intended to be read-only.
Which versions contain the fix?
The issue is patched in versions 1.5.1 and 1.6.0.