Appsmith: Configuration-dependent origin validation bypass in password reset and email verification link generation

The resulting base URL is then used to construct token-bearing links:

• FORGOT_PASSWORD_CLIENT_URL_FORMAT at line 149
• reset URL generation at lines 282-289
• EMAIL_VERIFICATION_CLIENT_URL_FORMAT at line 152
• verification URL generation at lines 931-940

This means the base URL is not only used for branding or display purposes. It directly controls the clickable host of security-sensitive reset and verification links.

Note that the admin UI describes APPSMITH_BASE_URL as required for password reset and email verification links in:

• app/client/src/ce/pages/AdminSettings/config/configuration.tsx at lines 41-49

The reviewed self-host material indicates this protection is not fail-closed by default, which makes the vulnerable condition realistic on existing deployments where operators have not set APPSMITH_BASE_URL.

PoC

These steps were designed for validation on an Appsmith deployment that I own or am explicitly authorized to test.

1. Deploy a non-production Appsmith instance from current release, or any build containing the affected code.
2. Enable outbound email delivery.
3. Leave APPSMITH_BASE_URL unset or blank.
4. Use a test mailbox account you control, for example [email protected].
5. Send a password reset request with a forged Origin header:

curl -i -X POST 'https://YOUR-INSTANCE/api/v1/users/forgotPassword' \
  -H 'Content-Type: application/json' \
  -H 'Origin: https://attacker.example' \
  --data '{"email":"[email protected]"}'

6. Inspect the delivered email. If affected, the clickable reset link is generated under https://attacker.example/... instead of the legitimate Appsmith host.
7. Repeat the same validation for resend email verification using an unverified test account:

curl -i -X POST 'https://YOUR-INSTANCE/api/v1/users/resendEmailVerification' \
  -H 'Content-Type: application/json' \
  -H 'Origin: https://attacker.example' \
  --data '{"email":"[email protected]"}'

8. Inspect the delivered verification email. If affected, the clickable verification link is generated under https://attacker.example/....
9. Set APPSMITH_BASE_URL=https://YOUR-INSTANCE, restart the server, and repeat the same requests.
10. The expected secure behavior is that mismatched Origin is rejected, or the generated links no longer follow the forged request header.

Live tokens, third-party data, or unsafe exploitation material was not included in this report. The attached archive contains source excerpts, data-flow proof, safe validation notes, and supporting evidence collected from the reviewed current branch.

Impact

This is a trust-boundary failure in token-bearing email authentication flows.

Affected deployments are those where:

• APPSMITH_BASE_URL is unset or blank
• outbound email is enabled
• password reset and/or email verification flows are enabled

Attacker requirements are low:

• no prior authentication is required to reach the relevant endpoints
• the attacker only needs to trigger an email flow toward a target account

Security impact:

• Appsmith can generate password reset or verification emails whose clickable host is attacker-controlled
• victim interaction can expose security-sensitive token material
• this can plausibly result in account takeover
• at minimum, it enables trusted-email abuse and phishing through the application itself

mitigation

1. Remove the fallback behavior in resolveSecureBaseUrl(...) that accepts caller-supplied origin data when APPSMITH_BASE_URL is unset.
2. Fail closed for all token-bearing email flows when no trusted base URL is configured.
3. Use a single trusted-base-url helper across:
   • password reset
   • resend email verification
   • invite flows
   • instance admin invite flows
4. Add regression tests for the unset-configuration case.
5. Add startup or admin-level validation so security-sensitive email flows cannot operate until a trusted base URL is configured.

Attachment

appsmith-email-link-origin-validation-bypass-20260321.zip