CVE-2023-35956 is a high severity (7.8) heap overflow in GTKWave 3.3.115, reached by opening a crafted FST waveform file. This entry concerns the fastlz_decompress function. Debian's fixed packages are built from upstream 3.3.118, so update to that or your distribution's patched build.
What's affected
The record lists GTKWave 3.3.115, from Tony Bybell, as the affected version. It names no other release and gives no fixed version.
The Debian security tracker fills in the fix side. Its fixed packages are built from upstream 3.3.118:
- Debian 10 (buster): 3.3.98+really3.3.118-0+deb10u1, in advisory DLA-3785-1.
- Debian 11 (bullseye): 3.3.104+really3.3.118-0+deb11u1, in advisory DSA-5653-1.
- Debian 12 (bookworm): 3.3.118-0.1~deb12u1, in advisory DSA-5653-1.
- Debian testing and unstable carry 3.3.121 and 3.3.127.
So the record and the distribution disagree in how they describe the range. The record says 3.3.115. The fixed builds are 3.3.118. Releases between the two are not covered by either source. Treat 3.3.118 as the version to reach, and confirm against the Talos report, which I could not open because it blocked my request. Other distributions may use their own version strings, so read your vendor's advisory.
Talos groups this CVE with three others under TALOS-2023-1785. This one concerns the decompression function fastlz_decompress, which handles one of the compression schemes an FST file can use. The related entries cover the other schemes and a copy routine.
How to check if you're exposed
GTKWave is a desktop waveform viewer, so it usually sits on engineering workstations, lab machines and verification servers rather than production hosts. It also gets pulled into container images and build environments.
gtkwave --version
dpkg -s gtkwave | grep Version
brew list --versions gtkwave
Use the line that matches your system. On Debian or Ubuntu, compare the package version with the fixed builds listed above, and remember that distributions can backport fixes under older numbers, so read the package changelog for the CVE id if the number looks low.
Also look for shared directories where waveform files arrive, such as simulation output folders and vendor drops. The risk comes from opening a file, so knowing where untrusted files come from matters as much as the version number.
What to do now
- Find every machine, image and shared tool that has GTKWave installed, and record the version.
- Update to upstream 3.3.118 or later, or to your distribution's fixed package. On Debian, that means the update from DLA-3785-1 or DSA-5653-1.
- Until updated, do not open .fst files from sources you do not trust, and do not open them on a machine that holds credentials or source code.
- Treat waveform files that arrive from vendors, contractors or the internet as untrusted input, and open them on a disposable machine where you can.
- Re-check the version after the update, including inside container images that were built earlier.
Why it matters
The weakness is CWE-119, improper restriction of operations within the bounds of a memory buffer, in the heap memory area. The record says the problem sits in the VCDATA parsing of the FST reader, in the function fstReaderIterBlocks2. The text for this CVE says it concerns fastlz_decompress.
A heap overflow means a program writes past the end of a block of memory it allocated. The result can be a crash, or, with a carefully built file, control of the program. The record scores it 7.8 with a local vector, no privileges and one required user action, which is opening the file. It is not on CISA's Known Exploited Vulnerabilities list, and I found no claim of exploitation.
File format parsers are a steady source of this class of bug. Tools that open files from outside parties inherit the risk of every format they support, so treat what you open as untrusted input.
Quick check
Which part of GTKWave is involved?
The FST reader, in the VCDATA parsing function fstReaderIterBlocks2. This entry concerns the fastlz_decompress function.
Is a login or network access needed?
No. The record uses a local vector. The user has to open the crafted file.
Sources
- Debian Security Tracker entry for CVE-2023-35956
- Talos report TALOS-2023-1785 (cited by the record; blocked to automated fetch)
- Debian LTS announcement cited by the record (needs JavaScript; not read)
- Safeguard Gold entry for CVE-2023-35956
Last reviewed: 19 October 2026.
