diff --git a/src/lib/gitea.ts b/src/lib/gitea.ts
index 475315c..87d4dd8 100644
--- a/src/lib/gitea.ts
+++ b/src/lib/gitea.ts
@@ -13,6 +13,7 @@ import { db, organizations, repositories } from "./db";
 import { eq, and, ne } from "drizzle-orm";
 import { decryptConfigTokens } from "./utils/config-encryption";
 import { formatDateShort } from "./utils";
+import { buildGithubSourceAuthPayload } from "./utils/mirror-source-auth";
 import {
   parseRepositoryMetadataState,
   serializeRepositoryMetadataState,
@@ -816,14 +817,22 @@ export const mirrorGithubRepoToGitea = async ({
 
     // Add authentication for private repositories
     if (repository.isPrivate) {
-      if (!config.githubConfig.token) {
-        throw new Error(
-          "GitHub token is required to mirror private repositories."
-        );
-      }
-      // Use separate auth fields (required for Forgejo 12+ compatibility)
-      migratePayload.auth_username = "oauth2"; // GitHub tokens work with any username
-      migratePayload.auth_token = decryptedConfig.githubConfig.token;
+      const githubOwner =
+        (
+          config.githubConfig as typeof config.githubConfig & {
+            owner?: string;
+          }
+        ).owner || "";
+
+      Object.assign(
+        migratePayload,
+        buildGithubSourceAuthPayload({
+          token: decryptedConfig.githubConfig.token,
+          githubOwner,
+          githubUsername: config.githubConfig.username,
+          repositoryOwner: repository.owner,
+        })
+      );
     }
 
     // Track whether the Gitea migrate call succeeded so the catch block
@@ -1496,14 +1505,22 @@ export async function mirrorGitHubRepoToGiteaOrg({
 
     // Add authentication for private repositories
     if (repository.isPrivate) {
-      if (!config.githubConfig?.token) {
-        throw new Error(
-          "GitHub token is required to mirror private repositories."
-        );
-      }
-      // Use separate auth fields (required for Forgejo 12+ compatibility)
-      migratePayload.auth_username = "oauth2"; // GitHub tokens work with any username
-      migratePayload.auth_token = decryptedConfig.githubConfig.token;
+      const githubOwner =
+        (
+          config.githubConfig as typeof config.githubConfig & {
+            owner?: string;
+          }
+        )?.owner || "";
+
+      Object.assign(
+        migratePayload,
+        buildGithubSourceAuthPayload({
+          token: decryptedConfig.githubConfig?.token,
+          githubOwner,
+          githubUsername: config.githubConfig?.username,
+          repositoryOwner: repository.owner,
+        })
+      );
     }
 
     let migrateSucceeded = false;
diff --git a/src/lib/utils/mirror-source-auth.test.ts b/src/lib/utils/mirror-source-auth.test.ts
new file mode 100644
index 0000000..49654be
--- /dev/null
+++ b/src/lib/utils/mirror-source-auth.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, test } from "bun:test";
+import { buildGithubSourceAuthPayload } from "./mirror-source-auth";
+
+describe("buildGithubSourceAuthPayload", () => {
+  test("uses configured owner when available", () => {
+    const auth = buildGithubSourceAuthPayload({
+      token: "ghp_test_token",
+      githubOwner: "ConfiguredOwner",
+      githubUsername: "fallback-user",
+      repositoryOwner: "repo-owner",
+    });
+
+    expect(auth).toEqual({
+      auth_username: "ConfiguredOwner",
+      auth_password: "ghp_test_token",
+      auth_token: "ghp_test_token",
+    });
+  });
+
+  test("falls back to configured username then repository owner", () => {
+    const authFromUsername = buildGithubSourceAuthPayload({
+      token: "token1",
+      githubUsername: "configured-user",
+      repositoryOwner: "repo-owner",
+    });
+
+    expect(authFromUsername.auth_username).toBe("configured-user");
+
+    const authFromRepoOwner = buildGithubSourceAuthPayload({
+      token: "token2",
+      repositoryOwner: "repo-owner",
+    });
+
+    expect(authFromRepoOwner.auth_username).toBe("repo-owner");
+  });
+
+  test("uses x-access-token as last-resort username", () => {
+    const auth = buildGithubSourceAuthPayload({
+      token: "ghp_test_token",
+    });
+
+    expect(auth.auth_username).toBe("x-access-token");
+  });
+
+  test("trims token whitespace", () => {
+    const auth = buildGithubSourceAuthPayload({
+      token: "  ghp_trimmed  ",
+      githubUsername: "user",
+    });
+
+    expect(auth.auth_password).toBe("ghp_trimmed");
+    expect(auth.auth_token).toBe("ghp_trimmed");
+  });
+
+  test("throws when token is missing", () => {
+    expect(() =>
+      buildGithubSourceAuthPayload({
+        token: "   ",
+        githubUsername: "user",
+      })
+    ).toThrow("GitHub token is required to mirror private repositories.");
+  });
+});
diff --git a/src/lib/utils/mirror-source-auth.ts b/src/lib/utils/mirror-source-auth.ts
new file mode 100644
index 0000000..293b5a4
--- /dev/null
+++ b/src/lib/utils/mirror-source-auth.ts
@@ -0,0 +1,46 @@
+interface BuildGithubSourceAuthPayloadParams {
+  token?: string | null;
+  githubOwner?: string | null;
+  githubUsername?: string | null;
+  repositoryOwner?: string | null;
+}
+
+export interface GithubSourceAuthPayload {
+  auth_username: string;
+  auth_password: string;
+  auth_token: string;
+}
+
+const DEFAULT_GITHUB_AUTH_USERNAME = "x-access-token";
+
+function normalize(value?: string | null): string {
+  return typeof value === "string" ? value.trim() : "";
+}
+
+/**
+ * Build source credentials for private GitHub repository mirroring.
+ * GitHub expects username + token-as-password over HTTPS (not the GitLab-style "oauth2" username).
+ */
+export function buildGithubSourceAuthPayload({
+  token,
+  githubOwner,
+  githubUsername,
+  repositoryOwner,
+}: BuildGithubSourceAuthPayloadParams): GithubSourceAuthPayload {
+  const normalizedToken = normalize(token);
+  if (!normalizedToken) {
+    throw new Error("GitHub token is required to mirror private repositories.");
+  }
+
+  const authUsername =
+    normalize(githubOwner) ||
+    normalize(githubUsername) ||
+    normalize(repositoryOwner) ||
+    DEFAULT_GITHUB_AUTH_USERNAME;
+
+  return {
+    auth_username: authUsername,
+    auth_password: normalizedToken,
+    auth_token: normalizedToken,
+  };
+}