{"id":"914c9eba5375720c","repo":"labstack/echo","slug":"unexpected-error-happened-when-reading-from-bufio","errorCode":null,"errorMessage":"unexpected error happened when reading from bufio.NewReader(crypto/rand.Reader)","messagePattern":"unexpected error happened when reading from bufio\\.NewReader\\(crypto/rand\\.Reader\\)","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"middleware/util.go","lineNumber":93,"sourceCode":"\trlen := n + n/4 // perf: avoid read from rand.Reader many times\n\tif cap(sc.r) < rlen {\n\t\tsc.r = make([]byte, rlen)\n\t} else {\n\t\tsc.r = sc.r[:rlen]\n\t}\n\tb, r := sc.b, sc.r\n\tvar i uint8 = 0\n\n\t// security note:\n\t// we can't just simply do b[i]=randomStringCharset[rb%len(randomStringCharset)],\n\t// len(len(randomStringCharset)) is 52, and rb is [0, 255], 256 = 52 * 4 + 48.\n\t// make the first 48 characters more possibly to be generated then others.\n\t// So we have to skip bytes when rb > randomStringMaxByt\n\n\tfor {\n\t\t_, err := io.ReadFull(reader, r)\n\t\tif err != nil {\n\t\t\tpanic(\"unexpected error happened when reading from bufio.NewReader(crypto/rand.Reader)\")\n\t\t}\n\t\tfor _, rb := range r {\n\t\t\tif rb > randomStringMaxByte {\n\t\t\t\t// Skip this number to avoid bias.\n\t\t\t\tcontinue\n\t\t\t}\n\t\t\tb[i] = randomStringCharset[rb%randomStringCharsetLen]\n\t\t\ti++\n\t\t\tif i == length {\n\t\t\t\treturn string(b[:length])\n\t\t\t}\n\t\t}\n\t}\n}\n\nfunc validateOrigins(origins []string, what string) error {\n\tfor _, o := range origins {\n\t\tif err := validateOrigin(o, what); err != nil {","sourceCodeStart":75,"sourceCodeEnd":111,"githubUrl":"https://github.com/labstack/echo/blob/05489dc1730161df26b72d1ae2a3ba6fb8178fc7/middleware/util.go#L75-L111","documentation":"Panicked by randomString (util.go:93) when io.ReadFull(reader, r) fails while reading from a pooled bufio.Reader backed by crypto/rand.Reader. randomString generates request IDs (RequestID middleware), CSRF tokens (CSRF middleware), and is used in tests. A read failure from the system CSPRNG is treated as fatal because secure tokens cannot be produced.","triggerScenarios":"The OS cryptographic random source (/dev/urandom or getrandom syscall) fails or is unavailable. This is extremely rare and usually indicates a broken or highly constrained environment: a misconfigured container with /dev/urandom unavailable, a seccomp/AppArmor profile blocking getrandom, or a depleted entropy source on older kernels.","commonSituations":"Containers/chroots without /dev/urandom mounted; strict seccomp filters blocking the getrandom syscall; very early boot on embedded systems before RNG initialization; sandboxed runtimes (some gVisor/WSL2 edge cases).","solutions":["Ensure /dev/urandom is available and readable inside the deployment (mount it in containers).","Adjust seccomp/AppArmor profiles to permit the getrandom syscall.","On systems where the CSPRNG may be slow at boot, delay serving traffic until /dev/urandom is initialized.","Run on a kernel version with a reliable getrandom implementation (modern Linux is fine)."],"exampleFix":null,"handlingStrategy":"validation","validationCode":"// Verify the system CSPRNG is readable at startup.\nfunc checkEntropy() error {\n    b := make([]byte, 16)\n    _, err := io.ReadFull(rand.Reader, b)\n    return err\n}\n\nif err := checkEntropy(); err != nil {\n    log.Fatalf(\"crypto/rand unavailable: %v\", err)\n}","typeGuard":null,"tryCatchPattern":null,"preventionTips":["Ensure /dev/urandom is mounted and readable in containers.","Allow the getrandom syscall in seccomp/AppArmor profiles.","Run health checks on the CSPRNG at startup for security-sensitive services."],"tags":["crypto","random","csrf","request-id","panic","runtime","system"],"analyzedSha":"05489dc1730161df26b72d1ae2a3ba6fb8178fc7","analyzedAt":"2026-08-04T21:32:47.783Z","schemaVersion":2}