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OSEC-2026-23
Several issues have been discovered in the Marshal module:
ANT-2026-K7NTK0FT Stale obj_counter across unmarshal calls enables null/freed intern_obj_table indexing
ANT-2026-2NJYQWEX Custom block size wraparound in CODE_CUSTOM_LEN allocation
ANT-2026-PJC00M7X Unmarshaler accepts forged Closure_tag metadata without validation
This is a followup for OSEC-2026-01
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.
Am I affected?
If you use one of the affected bindings Marshal.from_channel, Marshal.from_bytes, Marshal.from_string, Stdlib.input_value (or a wrapper in a library, such as Lwt_io.read_value).
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).
It is recommended to use Marshal only for communication between trusted processes.
Timeline
- 2026-08-24: vulnerability was found
- 2026-08-27: forwarded report to security@ocaml.org
- 2026-09-29: fix merged to trunk
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The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a low impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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