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CVE-2026-107717: Banks: User-controlled prompt input can be parsed as privileged chat messages

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-08 · updated 2026-10-13
• Weakness: CWE-20

Description

Banks generates meaningful LLM prompts using a simple template language. Prior to 2.5.0, Banks Prompt.chat_messages() attempts to parse every line of rendered template output as ChatMessage JSON. When an application renders untrusted data and passes the returned ChatMessage objects to an LLM provider, attacker-controlled JSON can cross the prompt boundary and become a system, assistant, or tool message because ChatMessage.role accepts arbitrary strings. This can override application instructions, alter the intended prompt structure, or confuse downstream tool and message handling. This issue is fixed in version 2.5.0.

Affected products

• masci banks: before 2.5.0

References

• https://github.com/masci/banks/commit/02172b816fb84f6a824cc09a8aca7416f53c12cb
• https://github.com/masci/banks/pull/78
• https://github.com/masci/banks/releases/tag/v2.5.0
• https://github.com/masci/banks/security/advisories/GHSA-hmq2-7hp6-7crh
• https://github.com/masci/banks/security/advisories/GHSA-hmq2-7hp6-7crh

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