CVE-2026-109429 in Mistral Vibe
Summary
by MITRE • 10/13/2026
Mistral Vibe before 2.25.4 contains a command permission gate bypass vulnerability that allows attackers to execute arbitrary commands by hiding a command substitution inside a single-quoted variable value that tree-sitter-bash static parsing cannot detect. Attackers can craft input such as embedding a command substitution within a single-quoted variable and re-expanding it with the ${var@P} prompt-transform operator so bash's promptvars feature executes the hidden command at runtime while only the trailing allowlisted command is classified, resulting in silent arbitrary command execution with the privileges of the agent's terminal session.
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Analysis
by VulDB Data Team • 10/13/2026
The vulnerability identified in Mistral Vibe prior to version 2.25.4 represents a sophisticated bypass of command permission gates through the exploitation of shell parsing ambiguities and dynamic evaluation features inherent to Bash. This flaw allows attackers to execute arbitrary commands by concealing malicious code within single-quoted variable values, effectively evading static analysis tools such as tree-sitter-bash that rely on predictable syntactic structures for detection. The core technical mechanism involves embedding a command substitution inside a string that is initially treated as literal text due to the use of single quotes, which normally prevent expansion and interpretation by the shell during initial parsing phases.
The exploitation chain relies heavily on Bash's promptvars feature and the ${varP} parameter expansion operator. When an attacker crafts input where a variable contains hidden command substitution syntax within single quotes, this content remains inert until it is subjected to specific processing conditions. By re-expanding the variable using the P flag, which performs prompt-style expansion, the shell interprets the embedded commands as if they were part of the user's interactive session. This dynamic evaluation occurs at runtime rather than during static analysis, allowing the malicious payload to bypass security controls that only inspect the trailing allowlisted command or perform superficial syntactic checks without simulating full execution contexts.
From an operational perspective, this vulnerability results in silent arbitrary command execution with the privileges of the agent's terminal session. Because the attack leverages legitimate shell features and evades static detection mechanisms, it can execute commands indistinguishable from normal user activity until they are completed. This poses a severe risk to system integrity as attackers can read sensitive configuration files, exfiltrate data, install backdoors, or pivot further into connected systems without triggering traditional intrusion detection alerts that rely on pattern matching of known malicious signatures rather than behavioral analysis of shell dynamics.
This vulnerability aligns with CWE-78 Improper Neutralization of Special Elements used in an OS Command and CWE-94 Improper Control of Generation of Code, reflecting the failure to properly sanitize or validate dynamic input before execution. In terms of MITRE ATT&CK framework classification, this technique corresponds to T1059 Command and Scripting Interpreter with sub-techniques related to Bash scripting abuse and potentially T1620 Reflective Code Injection if memory manipulation is involved in more complex variants. The reliance on prompt expansion highlights the dangers of enabling interactive shell features like promptvars in automated or agent-based environments where strict command whitelisting is expected to provide security boundaries.
Mitigation strategies must focus on updating Mistral Vibe to version 2.25.4 or later, which addresses this parsing ambiguity and strengthens static analysis capabilities against such obfuscation techniques. Organizations should also enforce strict input validation that rejects any variable expansions involving command substitution syntax even when enclosed in single quotes if those variables are destined for dynamic evaluation contexts. Disabling promptvars in non-interactive shell sessions where possible reduces the attack surface significantly, as it prevents the runtime expansion mechanism exploited by this vulnerability. Additionally implementing behavioral monitoring tools capable of detecting anomalous patterns in shell variable expansions and command execution sequences can provide a secondary layer of defense against such sophisticated bypass attempts that evade static signature-based detection systems.