https://sympa.inria.fr/sympa/arc/ocsf-ocaml-security-announcements/2026-02/msg00000.html
announces:
From: Hannes Mehnert <[email protected]>
To: [email protected]
Subject: [ocsf-ocaml-security-announcements] OSEC-2026-01 in the OCaml runtime:
Buffer Over-Read in OCaml Marshal Deserialization
Date: Tue, 17 Feb 2026 15:16:54 +0100
Dear everyone,
it is my pleasure to announce the first security announcement of this year,
and the first on this mailing list.
It should any moment now also appear at https://osv.dev/list?q=OSEC-2026-01
Human link:
https://github.com/ocaml/security-advisories/tree/main/advisories/2026/OSEC-2026-01.md
```
id: OSEC-2026-01
modified: "2026-02-17T13:30:00Z"
published: "2026-01-24T13:30:00Z"
aliases: [ GHSA-j26j-m5xr-g23c GHSA-m34r-cgq7-jhfm ]
severity: "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N"
severity_score: "6.8"
affected: "ocaml" {< "4.14.3" | (>= "5" & < "5.4.1")}
events: [
[
git "https://github.com/ocaml/ocaml";; [
[fixed "b0a2614684a52acded784ec213f14ddfe085d146"]
]
]
[
git "https://github.com/ocaml/ocaml";; [
[fixed "e3919fef436f89271bc30bbe8592851f7289fb68"]
]
]
]
references: [
[report
"https://github.com/ocaml/security-advisories/security/advisories/GHSA-j26j-m5xr-g23c";;]
]
credits: [
[reporter "Justin Timperio"]
[remediation_developer "Nicolás Ojeda Bär"]
[remediation_developer "Xavier Leroy"]
[remediation_developer "Gabriel Scherer"]
[remediation_reviewer "Xavier Leroy"]
[remediation_reviewer "Olivier Nicole"]
[remediation_verifier "Mindy Preston"]
[remediation_verifier "Edwin Török"]
[coordinator "Hannes Mehnert"]
]
cwe: [ CWE-126 CWE-502 CWE-754 ]
```
# Buffer Over-Read in OCaml Marshal Deserialization
## Summary
A critical buffer over-read vulnerability in OCaml's Marshal deserialization
(runtime/intern.c) enables remote code execution through a multi-phase attack
chain. The vulnerability stems from missing bounds validation in the
readblock() function, which performs unbounded memcpy() operations using
attacker-controlled lengths from malicious Marshal data.
Please note that Marshal is not type safe, and you have to be careful if you
use the deserialization on untrusted input (due to type confusion, and remote
code execution by design - you can use Marshal for code).
Affected functions: `Marshal.from_channel`, `Marshal.from_bytes`,
`Marshal.from_string`, `Stdlib.input_value`, `Pervasives.input_value`
when reading data from an untrusted source.
## Vulnerability Attack Vector
Corrupted or malicious marshaled data that causes undefined behaviour in the
runtime system when unmarshaled.
`input_value` should either fail cleanly or produce a well-formed OCaml object,
without corrupting the runtime system.
Consequently, this excludes:
* well-formed marshaled data that produces an OCaml object that is not of the
type expected by the OCaml code and causes the Ocaml code to crash or
misbehave
* misuses of the OCaml runtime system by the program performing input_value,
such as setting `Debugger.function_placeholder` to the wrong function.
The former issue may be addressed at some point by validating the unmarshaled
OCaml value against the expected type, using the functions from module `Obj`
and some kind of run-time type description.
The latter issue is a bug in the program that unmarshals the data.
## Fix
### OCaml runtime
The OCaml runtime has been hardened with additional bounds checks. An exception
is raised on bad input.
### Third party libraries
Third party libraries that want to harden their custom Marshal deserialization
code can follow the example fix for bigarrays from the standard library.
There are new macros in `custom.h` called `Wsize_custom_data` and
`Bsize_custom_data` that return the size in words or bytes of the allocated
custom destination block. The deserializer needs to ensure it only writes data
within those bounds.
This only needs to be done if the library defines a custom type in a C binding,
and `struct custom_operations`'s `deserialize` field is not set to `NULL` or
`custom_deserialize_default`, and `struct custom_operations`'s `fixed_length`
field is set to `NULL` or `custom_fixed_length_default`
Since `Marshal.from*` and `input_value` remain unsafe to use, the fix for the
OCaml runtime is released, and we wouldn't attempt to coordinate updating all
deserialization functions in the ecosystem.
## Timeline
- Nov 4th 2025: Discovery Date: Discovered first in OxCaml
- Nov 5th 2025: First Disclosure Date (Jane Street Team): Emailed top
maintainers, no response.
- Nov 9th 2025: Second Disclosure Date (OCaml Team): Submitted to OCaml/ocaml
GitHub Repo as a Security Advisory.
- Nov 11th 2025: Emailed OCaml Security Mail List: Submitted to OCaml over
email, responded asking for details.
- Nov 11th 2025: Third Disclosure (OCaml Security Response Team): Submitted
to ocaml/security-advisories GitHub Repo as a Security Advisory.
- Dec 16th 2025: Initial patch is developed
- Dec 17th 2025: Fuzz testing found further issues
- Dec 24th 2025: Final patch for OCaml is developed
- Dec 25th 2025: Fuzz testing couldn't find any further issues
- Jan 2nd 2026: Patch got reviewed by OCaml maintainers
- Jan 4th 2026: Benchmarking of the patch with good results
- Jan 6th 2026: Reporter got contacted to confirm
- Jan 25th 2026: Further related issues discovered by fuzzing
- Feb 17th 2026: fixed OCaml releases are published, security advisory is
published
A followup note in
https://sympa.inria.fr/sympa/arc/ocsf-ocaml-security-announcements/2026-02/msg00001.html
notes that the "published" date in the JSON was incorrect and should be
2026-02-17.
--
-Alan Coopersmith- [email protected]
Oracle Solaris Engineering - https://blogs.oracle.com/solaris
