Article from May 2013 from Azimuth Security on Exploiting Samsung Galaxy S4 secure boot.
Key quotes:

Examining the check_sig() function in more detail revealed that aboot uses the open-source mincrypt implementation of RSA for signature validation. The bootloader uses an RSA-2048 public key contained in aboot to decrypt a signature contained in the boot image itself, and compares the resulting plaintext against the SHA1 hash of the boot image. Since any modifications to the boot image would result in a different SHA1 hash, it is not possible to generate a valid signed boot image without breaking RSA-2048, generating a specific SHA1 collision, or obtaining Samsung’s private signing key.

The astute reader will have already noticed the design flaw present in the above program logic. Notice the order in which the steps are performed: first, aboot loads the kernel and ramdisk into memory at the addresses requested by the boot image header, and then signature validation is performed after this loading is complete. Because the boot image header is read straight from eMMC flash prior to any signature validation, it contains essentially untrusted data. As a result, it’s possible to flash a maliciously crafted boot image whose header values cause aboot to read the kernel or ramdisk into physical memory directly on top of aboot itself!

Exploitation of this flaw proved to be fairly straightforward. I prepare a specially crafted boot image that specifies a ramdisk load address equal to the address of the check_sig() function in aboot physical memory. In my malicious boot image, I place shellcode where the ramdisk is expected to reside. I flash this image by leveraging root access in the Android operating system to write to the boot block device. When aboot reads the supposed ramdisk from eMMC flash, it actually overwrites the check_sig() function with my shellcode, and then invokes it. The shellcode simply patches up the boot image header to contain sane values, copies the actual kernel and ramdisk into appropriate locations in memory, and returns zero, indicating the signature verification succeeded. At this point, aboot continues the boot process and finally boots my unsigned kernel and ramdisk. Victory!

Similar attack from Azimuth Security on Motorola phones from April 2013.
Key quotes:

At this point, the end was in sight, but I knew I would need a vulnerability in the TrustZone kernel in order to set this flag to zero, allowing my SMC call to blow the QFuse required to unlock the bootloader. Fortunately, I didn’t have to look long, since one of the other SMC commands in the same section of the TrustZone kernel contains a fairly obvious arbitrary memory write vulnerability…