diff --git a/app/api/auth/github/callback/route.ts b/app/api/auth/github/callback/route.ts
index 6a1ef73..435680a 100644
--- a/app/api/auth/github/callback/route.ts
+++ b/app/api/auth/github/callback/route.ts
@@ -4,6 +4,7 @@ import { cookies } from "next/headers"
 export async function GET(request: NextRequest) {
   const searchParams = request.nextUrl.searchParams
   const code = searchParams.get("code")
+  const state = searchParams.get("state")
   const error = searchParams.get("error")
 
   const origin = request.nextUrl.origin
@@ -13,6 +14,16 @@ export async function GET(request: NextRequest) {
     return NextResponse.redirect(new URL("/?github_error=" + error, origin))
   }
 
+  const cookieStore = await cookies()
+  const savedState = cookieStore.get("github_oauth_state")?.value
+
+  if (!state || state !== savedState) {
+    console.error("[GitHub] State mismatch")
+    return NextResponse.redirect(new URL("/?github_error=state_mismatch", origin))
+  }
+
+  cookieStore.delete("github_oauth_state")
+
   if (!code) {
     return NextResponse.redirect(new URL("/?github_error=no_code", origin))
   }
@@ -46,7 +57,6 @@ export async function GET(request: NextRequest) {
       return NextResponse.redirect(new URL("/?github_error=no_access_token", origin))
     }
 
-    const cookieStore = await cookies()
     cookieStore.set("github_token", accessToken, {
       httpOnly: true,
       secure: process.env.NODE_ENV === "production",
diff --git a/app/api/auth/github/route.ts b/app/api/auth/github/route.ts
index ce0175f..4359e8a 100644
--- a/app/api/auth/github/route.ts
+++ b/app/api/auth/github/route.ts
@@ -1,4 +1,5 @@
 import { NextRequest, NextResponse } from "next/server"
+import { cookies } from "next/headers"
 
 export async function GET(request: NextRequest) {
   const clientId = process.env.GITHUB_CLIENT_ID
@@ -9,11 +10,22 @@ export async function GET(request: NextRequest) {
 
   const redirectUri = `${request.nextUrl.origin}/api/auth/github/callback`
   const scope = "repo read:user"
+  const state = crypto.randomUUID()
+
+  const cookieStore = await cookies()
+  cookieStore.set("github_oauth_state", state, {
+    httpOnly: true,
+    secure: process.env.NODE_ENV === "production",
+    sameSite: "lax",
+    maxAge: 300,
+    path: "/",
+  })
 
   const githubAuthUrl = new URL("https://github.com/login/oauth/authorize")
   githubAuthUrl.searchParams.set("client_id", clientId)
   githubAuthUrl.searchParams.set("scope", scope)
   githubAuthUrl.searchParams.set("redirect_uri", redirectUri)
+  githubAuthUrl.searchParams.set("state", state)
 
   return NextResponse.redirect(githubAuthUrl.toString())
 }
diff --git a/app/api/auth/slack/callback/route.ts b/app/api/auth/slack/callback/route.ts
index dc82a77..7952162 100644
--- a/app/api/auth/slack/callback/route.ts
+++ b/app/api/auth/slack/callback/route.ts
@@ -4,6 +4,7 @@ import { cookies } from "next/headers"
 export async function GET(request: NextRequest) {
   const searchParams = request.nextUrl.searchParams
   const code = searchParams.get("code")
+  const state = searchParams.get("state")
   const error = searchParams.get("error")
 
   const origin = request.nextUrl.origin
@@ -13,6 +14,16 @@ export async function GET(request: NextRequest) {
     return NextResponse.redirect(new URL("/?slack_error=" + error, origin))
   }
 
+  const cookieStore = await cookies()
+  const savedState = cookieStore.get("slack_oauth_state")?.value
+
+  if (!state || state !== savedState) {
+    console.error("[Slack] State mismatch")
+    return NextResponse.redirect(new URL("/?slack_error=state_mismatch", origin))
+  }
+
+  cookieStore.delete("slack_oauth_state")
+
   if (!code) {
     return NextResponse.redirect(new URL("/?slack_error=no_code", origin))
   }
@@ -46,7 +57,6 @@ export async function GET(request: NextRequest) {
     }
 
     // Store token in HTTP-only cookie
-    const cookieStore = await cookies()
     cookieStore.set("slack_token", userToken, {
       httpOnly: true,
       secure: process.env.NODE_ENV === "production",
diff --git a/app/api/auth/slack/route.ts b/app/api/auth/slack/route.ts
index 5a62f6a..073aae8 100644
--- a/app/api/auth/slack/route.ts
+++ b/app/api/auth/slack/route.ts
@@ -1,4 +1,5 @@
 import { NextRequest, NextResponse } from "next/server"
+import { cookies } from "next/headers"
 
 export async function GET(request: NextRequest) {
   const clientId = process.env.SLACK_CLIENT_ID
@@ -9,11 +10,22 @@ export async function GET(request: NextRequest) {
 
   const redirectUri = `${request.nextUrl.origin}/api/auth/slack/callback`
   const scopes = ["search:read", "users:read", "im:read"].join(",")
+  const state = crypto.randomUUID()
+
+  const cookieStore = await cookies()
+  cookieStore.set("slack_oauth_state", state, {
+    httpOnly: true,
+    secure: process.env.NODE_ENV === "production",
+    sameSite: "lax",
+    maxAge: 300,
+    path: "/",
+  })
 
   const slackAuthUrl = new URL("https://slack.com/oauth/v2/authorize")
   slackAuthUrl.searchParams.set("client_id", clientId)
   slackAuthUrl.searchParams.set("user_scope", scopes)
   slackAuthUrl.searchParams.set("redirect_uri", redirectUri)
+  slackAuthUrl.searchParams.set("state", state)
 
   return NextResponse.redirect(slackAuthUrl.toString())
 }
diff --git a/package.json b/package.json
index 8e3cec4..c918776 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "worklog",
-  "version": "2.0.18",
+  "version": "2.0.19",
   "private": true,
   "scripts": {
     "dev": "next dev",