HackSkills is an organized knowledge base of installable skills that gives AI agents practical security knowledge across areas such as web security, privilege escalation, reverse engineering, and digital forensics. It is intended for bug bounty work, penetration testing, CTF competitions, and authorized security research. The catalogue entries are the project's own master, category, and topic skills.
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 agentmods add skills/yaklang/hack-skills/expression-language-injectionnpx skills add yaklang/hack-skills --skill expression-language-injectiongit clone --depth 1 https://github.com/yaklang/hack-skillsWrote 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/yaklang/hack-skills/expression-language-injection)<a href="https://agentmods.dev/skills/yaklang/hack-skills/expression-language-injection"><img src="https://agentmods.dev/badge/skills/yaklang/hack-skills/expression-language-injection.svg" alt="Measured on agentmods" 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 | $0.00045 | $0.02327 |
| Opus 5 | $0.00023 | $0.01163 |
| Sonnet 5 | $0.00009 | $0.00465 |
| Haiku 4.5 | $0.00005 | $0.00233 |
Grade A, and why
expression-language-injection scanned grade A with 0 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 5d 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.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
Copies of this mod
4 near-identical copies found in the catalogue:
- expression-language-injection — 100% identical, 0 lines differ
- expression-language-injection — 100% identical, 0 lines differ
- expression-language-injection — 100% identical, 0 lines differ
- expression-language-injection — 91% identical, 4 lines differ
How it starts
The opening of the file, as written. The whole thing — 244 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SKILL: Expression Language Injection — Expert Attack Playbook
AI LOAD INSTRUCTION: Expert EL injection techniques covering SpEL (Spring), OGNL (Struts2), and Java EL (JSP/JSF). Distinct from SSTI — EL injection targets expression evaluators in Java frameworks, not template engines. Covers sandbox bypass,
_memberAccessmanipulation, actuator abuse, and real-world CVE chains.
0. RELATED ROUTING
- ssti-server-side-template-injection for template engines (Jinja2, FreeMarker, Twig) — different attack surface
- jndi-injection when EL evaluation leads to JNDI lookup
Key distinction: SSTI targets template rendering engines; EL injection targets expression evaluators embedded in Java frameworks. They share detection probes (${7*7}) but diverge in exploitation.
1. DETECTION — POLYGLOT PROBES
${7*7} → 49 = SpEL, OGNL, or Java EL
#{7*7} → 49 = SpEL (alternative syntax) or JSF EL
%{7*7} → 49 = OGNL (Struts2)
${T(java.lang.Math).random()} → random float = SpEL confirmed
%{#context} → object dump = OGNL confirmed
Disambiguation
Response to ${7*7} |
Response to %{7*7} |
Engine |
|---|---|---|
| 49 | literal %{7*7} |
SpEL or Java EL |
literal ${7*7} |
49 | OGNL (Struts2) |
| 49 | 49 | Both may be active |
2. SpEL (SPRING EXPRESSION LANGUAGE)
Where SpEL Appears
@Value("${...}")annotations- Spring Security expressions (
@PreAuthorize) - Spring Cloud Gateway route predicates and filters
- Thymeleaf
th:text="${...}"(when combined with__${...}__preprocessing) - Spring Data
@Querywith SpEL
RCE via Runtime.exec
${T(java.lang.Runtime).getRuntime().exec("id")}
RCE with Output Capture (Commons IO)
${T(org.apache.commons.io.IOUtils).toString(T(java.lang.Runtime).getRuntime().exec("id").getInputStream())}
RCE with Output Capture (Spring StreamUtils)
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.
- 5d ago First seen · 244 lines · 45 tokens per session scan A dd96ba349b08
expression-language-injection is a skill published in the GitHub repository yaklang/hack-skills (2,064 stars, last pushed 2mo ago), licensed MIT. It adds 45 tokens to every session and 2,327 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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