diff --git a/apps/api/src/app.module.ts b/apps/api/src/app.module.ts
index 5aecc53..172b310 100644
--- a/apps/api/src/app.module.ts
+++ b/apps/api/src/app.module.ts
@@ -12,6 +12,7 @@ import { MailModule } from './mail/mail.module';
 import { CalendarModule } from './calendar/calendar.module';
 import { DashboardModule } from './dashboard/dashboard.module';
 import { DkvModule } from './dkv/dkv.module';
+import { CertManagerModule } from './cert-manager/cert-manager.module';
 import { DomaincheckModule } from './domaincheck/domaincheck.module';
 import { ModuleRegistryModule } from './module-registry/module-registry.module';
 import { PrismaModule } from './prisma/prisma.module';
@@ -33,6 +34,7 @@ import { UserModule } from './user/user.module';
     LdapModule,
     ModuleRegistryModule,
     DomaincheckModule,
+    CertManagerModule,
     DashboardModule,
     CalendarModule,
     SettingsModule,
diff --git a/apps/api/src/cert-manager/cert-manager.controller.ts b/apps/api/src/cert-manager/cert-manager.controller.ts
new file mode 100644
index 0000000..edfd7eb
--- /dev/null
+++ b/apps/api/src/cert-manager/cert-manager.controller.ts
@@ -0,0 +1,114 @@
+import {
+  BadRequestException,
+  Body,
+  Controller,
+  Post,
+  UploadedFile,
+  UploadedFiles,
+  UseInterceptors,
+} from '@nestjs/common';
+import { FileInterceptor, FilesInterceptor } from '@nestjs/platform-express';
+import { UseModule } from '../module-registry/module.guard';
+import { CertManagerService } from './cert-manager.service';
+
+/**
+ * CertManagerController — 4 POST endpoints for certificate operations.
+ *
+ * All routes are protected by:
+ * - Global JwtAuthGuard (authentication)
+ * - Global TenantGuard (tenant context)
+ * - @UseModule('cert-manager') ModuleGuard (module activation check)
+ *
+ * File size limit: 5 MB per file (T-09-03 — DoS mitigation).
+ * Password parameter is never passed to a logger (T-09-02 — InfoDisc mitigation).
+ */
+@Controller('modules/cert-manager')
+@UseModule('cert-manager')
+export class CertManagerController {
+  constructor(private readonly certManagerService: CertManagerService) {}
+
+  /**
+   * POST /modules/cert-manager/parse
+   * Inspect a single certificate: subject, issuer, validity, SANs, fingerprints.
+   * Accepts multipart file upload OR JSON body with pemText.
+   */
+  @Post('parse')
+  @UseInterceptors(
+    FileInterceptor('file', {
+      limits: { fileSize: 5 * 1024 * 1024 },
+    }),
+  )
+  async parseCert(
+    @UploadedFile() file: any,
+    @Body('password') password?: string,
+    @Body('pemText') pemText?: string,
+  ) {
+    if (!file && !pemText) {
+      throw new BadRequestException('No file or PEM text provided');
+    }
+    return this.certManagerService.parseCert({ file, pemText, password });
+  }
+
+  /**
+   * POST /modules/cert-manager/split
+   * Split a fullchain.pem or P7B bundle into individual certificates.
+   */
+  @Post('split')
+  @UseInterceptors(
+    FileInterceptor('file', {
+      limits: { fileSize: 5 * 1024 * 1024 },
+    }),
+  )
+  async splitCerts(
+    @UploadedFile() file: any,
+    @Body('password') password?: string,
+  ) {
+    if (!file) {
+      throw new BadRequestException('No file provided');
+    }
+    return this.certManagerService.splitCerts({ file, password });
+  }
+
+  /**
+   * POST /modules/cert-manager/merge
+   * Merge multiple certificates into a PEM chain or PFX bundle.
+   * Uses FilesInterceptor (plural) to accept multiple files with field name "files".
+   */
+  @Post('merge')
+  @UseInterceptors(
+    FilesInterceptor('files', 20, {
+      limits: { fileSize: 5 * 1024 * 1024 },
+    }),
+  )
+  async mergeCerts(
+    @UploadedFiles() files: any[],
+    @Body('outputFormat') outputFormat: string,
+    @Body('password') password?: string,
+  ) {
+    if (!files || files.length < 2) {
+      throw new BadRequestException('At least 2 files required for merge');
+    }
+    return this.certManagerService.mergeCerts({ files, outputFormat, password });
+  }
+
+  /**
+   * POST /modules/cert-manager/convert
+   * Convert a certificate between PEM, DER, PFX/P12, P7B, CRT/CER formats.
+   */
+  @Post('convert')
+  @UseInterceptors(
+    FileInterceptor('file', {
+      limits: { fileSize: 5 * 1024 * 1024 },
+    }),
+  )
+  async convertCert(
+    @UploadedFile() file: any,
+    @Body('targetFormat') targetFormat: string,
+    @Body('password') password?: string,
+  ) {
+    if (!file) {
+      throw new BadRequestException('No file provided');
+    }
+    return this.certManagerService.convertCert({ file, targetFormat, password });
+  }
+}
diff --git a/apps/api/src/cert-manager/cert-manager.module.ts b/apps/api/src/cert-manager/cert-manager.module.ts
new file mode 100644
index 0000000..f793e4a
--- /dev/null
+++ b/apps/api/src/cert-manager/cert-manager.module.ts
@@ -0,0 +1,41 @@
+import { Logger, Module, OnModuleInit } from '@nestjs/common';
+import { ModuleRegistryModule } from '../module-registry/module-registry.module';
+import { ModuleRegistryService } from '../module-registry/module-registry.service';
+import { CertManagerController } from './cert-manager.controller';
+import { seedCertManagerModule } from './cert-manager.seed';
+import { CertManagerService } from './cert-manager.service';
+
+/**
+ * NestJS module for the Cert Manager feature.
+ *
+ * Provides server-side certificate inspection, splitting, merging,
+ * and format conversion using node-forge (pure JS, no native bindings).
+ *
+ * Seeds itself into the module registry on application startup via
+ * OnModuleInit lifecycle hook (CERT-06).
+ *
+ * After seeding: an admin must activate the module per-tenant via the
+ * Marketplace UI before endpoints become accessible (ModuleGuard checks
+ * TenantModuleActivation, not isSystem flag — RESEARCH.md Pitfall 2).
+ */
+@Module({
+  imports: [ModuleRegistryModule],
+  controllers: [CertManagerController],
+  providers: [CertManagerService],
+})
+export class CertManagerModule implements OnModuleInit {
+  private readonly logger = new Logger(CertManagerModule.name);
+
+  constructor(
+    private readonly moduleRegistryService: ModuleRegistryService,
+  ) {}
+
+  async onModuleInit(): Promise<void> {
+    try {
+      await seedCertManagerModule(this.moduleRegistryService);
+      this.logger.log('Cert-Manager module seeded in registry');
+    } catch (error) {
+      this.logger.error('Failed to seed cert-manager module', error);
+    }
+  }
+}
diff --git a/apps/api/src/cert-manager/cert-manager.seed.ts b/apps/api/src/cert-manager/cert-manager.seed.ts
new file mode 100644
index 0000000..dbbf03e
--- /dev/null
+++ b/apps/api/src/cert-manager/cert-manager.seed.ts
@@ -0,0 +1,26 @@
+import { ModuleRegistryService } from '../module-registry/module-registry.service';
+
+/**
+ * Seeds the cert-manager module into the module registry.
+ *
+ * Called during CertManagerModule initialization to ensure the
+ * "cert-manager" module record exists in the database (CERT-06).
+ *
+ * isSystem: true registers the module in the registry but does NOT
+ * auto-activate it per tenant. Admin must activate via Marketplace UI.
+ */
+export async function seedCertManagerModule(
+  moduleRegistryService: ModuleRegistryService,
+): Promise<void> {
+  await moduleRegistryService.seedModule({
+    slug: 'cert-manager',
+    name: 'Cert Manager',
+    version: '1.0.0',
+    category: 'security-tools',
+    description: {
+      de: 'Zertifikate analysieren, konvertieren und verwalten',
+      en: 'Inspect, convert and manage certificates',
+    },
+    isSystem: true,
+  });
+}
diff --git a/apps/api/src/cert-manager/cert-manager.service.ts b/apps/api/src/cert-manager/cert-manager.service.ts
new file mode 100644
index 0000000..3ce7225
--- /dev/null
+++ b/apps/api/src/cert-manager/cert-manager.service.ts
@@ -0,0 +1,122 @@
+import { BadRequestException, Injectable, Logger, NotImplementedException } from '@nestjs/common';
+import * as forge from 'node-forge';
+
+/**
+ * CertManagerService — server-side certificate operations.
+ *
+ * All cryptographic processing is ephemeral (upload → process → return).
+ * No data is persisted to disk or database.
+ *
+ * SECURITY NOTES:
+ * - Binary buffers MUST use toString('binary') for forge (never 'utf-8' — Pitfall 1)
+ * - Password parameters are never passed to the logger
+ * - All forge operations wrapped in try/catch → BadRequestException
+ */
+@Injectable()
+export class CertManagerService {
+  private readonly logger = new Logger(CertManagerService.name);
+
+  // ---------------------------------------------------------------------------
+  // Shared helpers (used by all operation methods)
+  // ---------------------------------------------------------------------------
+
+  /**
+   * Detect the format of a certificate file from extension + content sniff.
+   * .cer is ambiguous — resolved by inspecting the first bytes of the buffer.
+   */
+  detectFormat(
+    filename: string,
+    buffer: Buffer,
+  ): 'pem' | 'der' | 'pfx' | 'p7b' {
+    const ext = filename.split('.').pop()?.toLowerCase() ?? '';
+    const isPemContent = buffer.slice(0, 27).toString('ascii').includes('-----BEGIN');
+
+    if (ext === 'pfx' || ext === 'p12') return 'pfx';
+    if (ext === 'p7b' || ext === 'p7c') return 'p7b';
+    if (ext === 'der') return 'der';
+    if (ext === 'pem' || ext === 'crt') return 'pem';
+    if (ext === 'cer') return isPemContent ? 'pem' : 'der'; // .cer is ambiguous
+    // Fallback: sniff content
+    return isPemContent ? 'pem' : 'der';
+  }
+
+  /**
+   * Convert a Node.js Buffer to a forge ByteStringBuffer using 'binary' encoding.
+   *
+   * CRITICAL: Always use 'binary' encoding — UTF-8 corrupts DER/PFX/P7B binary data.
+   * See RESEARCH.md Pitfall 1.
+   */
+  toForgeBuffer(buffer: Buffer): forge.util.ByteStringBuffer {
+    return forge.util.createBuffer(buffer.toString('binary'));
+  }
+
+  /**
+   * Compute SHA-1 or SHA-256 fingerprint of a certificate.
+   * Hash is computed over the DER-encoded bytes, returned as uppercase colon-joined hex.
+   */
+  getFingerprint(cert: forge.pki.Certificate, algorithm: 'sha1' | 'sha256'): string {
+    const md = algorithm === 'sha1' ? forge.md.sha1.create() : forge.md.sha256.create();
+    const der = forge.asn1.toDer(forge.pki.certificateToAsn1(cert)).getBytes();
+    md.update(der);
+    return md.digest().toHex().match(/.{2}/g)!.join(':').toUpperCase();
+  }
+
+  /**
+   * Split a PEM string containing one or more concatenated certificates.
+   * Returns an array of parsed forge Certificate objects.
+   */
+  parsePemChain(pem: string): forge.pki.Certificate[] {
+    const blocks =
+      pem.match(/-----BEGIN CERTIFICATE-----[\s\S]+?-----END CERTIFICATE-----/g) ?? [];
+    return blocks.map((b) => forge.pki.certificateFromPem(b));
+  }
+
+  // ---------------------------------------------------------------------------
+  // Operation method stubs (implemented in later plan slices)
+  // ---------------------------------------------------------------------------
+
+  async parseCert(_input: {
+    file?: any;
+    pemText?: string;
+    password?: string;
+  }): Promise<never> {
+    throw new NotImplementedException('parseCert is not yet implemented');
+  }
+
+  async splitCerts(_input: {
+    file?: any;
+    password?: string;
+  }): Promise<never> {
+    throw new NotImplementedException('splitCerts is not yet implemented');
+  }
+
+  async mergeCerts(_input: {
+    files?: any[];
+    outputFormat: string;
+    password?: string;
+  }): Promise<never> {
+    throw new NotImplementedException('mergeCerts is not yet implemented');
+  }
+
+  async convertCert(_input: {
+    file?: any;
+    targetFormat: string;
+    password?: string;
+  }): Promise<never> {
+    throw new NotImplementedException('convertCert is not yet implemented');
+  }
+
+  // ---------------------------------------------------------------------------
+  // Internal helpers for later slices
+  // ---------------------------------------------------------------------------
+
+  /** Wrap a node-forge operation and re-throw as BadRequestException on failure */
+  protected _parseOrThrow<T>(fn: () => T, errorMsg: string): T {
+    try {
+      return fn();
+    } catch (_err) {
+      this.logger.warn(`Cert parse failed: ${errorMsg}`);
+      throw new BadRequestException(errorMsg);
+    }
+  }
+}
diff --git a/apps/api/src/cert-manager/dto/convert-cert.dto.ts b/apps/api/src/cert-manager/dto/convert-cert.dto.ts
new file mode 100644
index 0000000..c157f36
--- /dev/null
+++ b/apps/api/src/cert-manager/dto/convert-cert.dto.ts
@@ -0,0 +1,8 @@
+/**
+ * DTO for the cert-manager convert endpoint body fields.
+ * Used alongside FileInterceptor for single-file upload.
+ */
+export class ConvertCertDto {
+  targetFormat!: 'pem' | 'der' | 'pfx' | 'p7b';
+  password?: string;
+}
diff --git a/apps/api/src/cert-manager/dto/merge-certs.dto.ts b/apps/api/src/cert-manager/dto/merge-certs.dto.ts
new file mode 100644
index 0000000..776ff41
--- /dev/null
+++ b/apps/api/src/cert-manager/dto/merge-certs.dto.ts
@@ -0,0 +1,8 @@
+/**
+ * DTO for the cert-manager merge endpoint body fields.
+ * Used alongside FilesInterceptor for multi-file upload.
+ */
+export class MergeCertsDto {
+  outputFormat!: 'pem' | 'pfx';
+  password?: string;
+}
diff --git a/apps/api/src/cert-manager/dto/parse-cert.dto.ts b/apps/api/src/cert-manager/dto/parse-cert.dto.ts
new file mode 100644
index 0000000..342511f
--- /dev/null
+++ b/apps/api/src/cert-manager/dto/parse-cert.dto.ts
@@ -0,0 +1,8 @@
+/**
+ * DTO for the cert-manager parse endpoint (text paste / JSON body path).
+ * For file uploads the body fields are extracted via @Body() in the controller.
+ */
+export class ParseCertDto {
+  pemText!: string;
+  password?: string;
+}